diff --git a/Changelog.md b/Changelog.md
new file mode 100644
--- /dev/null
+++ b/Changelog.md
@@ -0,0 +1,8 @@
+# Changelog
+
+## 0.0.1
+  - raising upper bounds of optics and attoparsec
+
+## 0.0.0
+Initial version, providing PCA, DBScan and hierarchical clustering.
+Command line interface to CREST XYZ trajectories.
diff --git a/ConClusion.cabal b/ConClusion.cabal
new file mode 100644
--- /dev/null
+++ b/ConClusion.cabal
@@ -0,0 +1,119 @@
+cabal-version: 1.12
+
+-- This file has been generated from package.yaml by hpack version 0.34.4.
+--
+-- see: https://github.com/sol/hpack
+
+name:           ConClusion
+version:        0.0.1
+synopsis:       Cluster algorithms, PCA, and chemical conformere analysis
+description:    Please see the README on GitLab at <https://gitlab.com/theoretical-chemistry-jena/quantum-chemistry/ConfoCluster>
+category:       Statistics, Chemistry
+bug-reports:    https://gitlab.com/theoretical-chemistry-jena/quantum-chemistry/ConfoCluster/-/issues
+author:         Phillip Seeber
+maintainer:     phillip.seeber@googlemail.com
+copyright:      2021 Phillip Seeber
+license:        AGPL-3
+license-file:   LICENSE.md
+build-type:     Simple
+extra-source-files:
+    README.md
+    LICENSE.md
+    Changelog.md
+
+source-repository head
+  type: git
+  location: https://gitlab.com/theoretical-chemistry-jena/quantum-chemistry/ConfoCluster
+
+library
+  exposed-modules:
+      ConClusion.Chemistry.Topology
+      ConClusion.Numeric.Data
+      ConClusion.Numeric.Statistics
+  other-modules:
+      Paths_ConClusion
+  hs-source-dirs:
+      src
+  default-extensions:
+      BangPatterns
+      OverloadedStrings
+      NoImplicitPrelude
+      FlexibleContexts
+      ScopedTypeVariables
+      OverloadedLabels
+      DataKinds
+      FlexibleInstances
+      MultiParamTypeClasses
+      UndecidableInstances
+      TypeFamilies
+      DuplicateRecordFields
+      ScopedTypeVariables
+      DataKinds
+      DeriveAnyClass
+      DeriveDataTypeable
+      DeriveGeneric
+      DeriveTraversable
+      FlexibleInstances
+      GeneralizedNewtypeDeriving
+      TypeApplications
+      RecordWildCards
+      NamedFieldPuns
+  ghc-options: -Wall -Wno-unused-top-binds -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wpartial-fields -Wredundant-constraints
+  build-depends:
+      PSQueue >=1.1.0.1 && <1.2
+    , aeson ==1.5.*
+    , attoparsec >=0.13.0.0 && <0.15
+    , base >=4.7 && <4.15
+    , containers >=0.6.0.0 && <0.7
+    , formatting >=7.1.0 && <7.2
+    , hmatrix >=0.20.0 && <0.21
+    , massiv >=0.6.0.0 && <0.7
+    , rio >=0.1.13.0 && <0.2
+  default-language: Haskell2010
+
+executable conclusion
+  main-is: ConClusion.hs
+  other-modules:
+      Paths_ConClusion
+  hs-source-dirs:
+      app
+  default-extensions:
+      BangPatterns
+      OverloadedStrings
+      NoImplicitPrelude
+      FlexibleContexts
+      ScopedTypeVariables
+      OverloadedLabels
+      DataKinds
+      FlexibleInstances
+      MultiParamTypeClasses
+      UndecidableInstances
+      TypeFamilies
+      DuplicateRecordFields
+      ScopedTypeVariables
+      DataKinds
+      DeriveAnyClass
+      DeriveDataTypeable
+      DeriveGeneric
+      DeriveTraversable
+      FlexibleInstances
+      GeneralizedNewtypeDeriving
+      TypeApplications
+      RecordWildCards
+      NamedFieldPuns
+  ghc-options: -Wall -Wno-unused-top-binds -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wpartial-fields -Wredundant-constraints -threaded -rtsopts -with-rtsopts=-N
+  build-depends:
+      ConClusion
+    , PSQueue >=1.1.0.1 && <1.2
+    , aeson ==1.5.*
+    , attoparsec >=0.13.0.0 && <0.15
+    , base >=4.7 && <4.15
+    , cmdargs >=0.10.0 && <0.11
+    , containers >=0.6.0.0 && <0.7
+    , formatting >=7.1.0 && <7.2
+    , hmatrix >=0.20.0 && <0.21
+    , massiv >=0.6.0.0 && <0.7
+    , optics >=0.3 && <0.5
+    , rio >=0.1.13.0 && <0.2
+    , text >=1.2.0.0 && <1.3
+  default-language: Haskell2010
diff --git a/LICENSE.md b/LICENSE.md
new file mode 100644
--- /dev/null
+++ b/LICENSE.md
@@ -0,0 +1,662 @@
+
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diff --git a/README.md b/README.md
new file mode 100644
--- /dev/null
+++ b/README.md
@@ -0,0 +1,123 @@
+# ConClusion
+ConClusion provides principal component analysis, hierarchical clustering and DBScan in Haskell.
+There is also a command line interface for processing of [CREST](https://github.com/grimme-lab/crest) conformere trajectories.
+Hence the name: CONformere CLUStering.
+The procedure to analyse conformere data has three steps:
+
+  1. Read the trajectory and calculate a set of features for each conformere.
+     The features can include the energy, a set of bond lengths, a set of bond angles, and a set of dihedral angles in arbitrary combination.
+     Those descriptors form a feature matrix.
+  2. A [principal component analysis](https://en.wikipedia.org/wiki/Principal_component_analysis) of the feature matrix might be perfomed to reduce the number of dimensions and remove redundancies.
+  3. The (potentially PCA-processed) feature matrix is being clustered.
+     Different distance measures are available.
+     Either [DBScan](https://en.wikipedia.org/wiki/DBSCAN) or [hierarchical clustering](https://en.wikipedia.org/wiki/Hierarchical_clustering) can be used to group different conformeres.
+
+While the command line interface only fits the work flow described above, the underlying clustering algorithms and PCA are implemented in a general way and can be utilised independently as library.
+
+## Installation
+### Bundled Archive
+A self-contained executable archive is build for the main branch and for releases.
+This can be executed directly on any Linux and has just to be downloaded.
+Go to the [page of releaes](https://gitlab.com/theoretical-chemistry-jena/quantum-chemistry/ConfoCluster/-/releases) and download an archive.
+Make it executable (e.g. `chmod +x conclusion`) and you are done!
+
+### From Source
+If you have the Haskell toolchain intalled and therefore working Cabal and GHC, you may build ConClusion from source.
+This also requires working BLAS and LAPACK libraries on your system.
+
+```bash
+git clone https://gitlab.com/theoretical-chemistry-jena/quantum-chemistry/ConfoCluster.git ConClusion
+cd ConClusion
+cabal install --installdir=$(PREFIX)
+```
+Choose a `PREFIX` where to install the executable.
+`$HOME/.local/bin/` is often a good choice.
+
+If you would like to use ConClusion on systems where Nix is not available (Windows, BSD, ...) this is the way to go.
+
+
+### With Nix
+When you have Nix available on your system, everything can be build by Nix:
+
+```bash
+git clone https://gitlab.com/theoretical-chemistry-jena/quantum-chemistry/ConfoCluster.git ConClusion
+cd ConClusion/nix
+nix-build -A ConClusion.components.exes.conclusion
+```
+
+
+## Usage
+The command line interface to the `conclusion` executable offers full control about all three steps described above.
+
+  - `-x --xyz` takes the path to the XYZ trajectory that is to be processed.
+    It must contain the energy as comment line (which is the case for CREST trajectories).
+
+  - `--dim` specifies the characterising features; therefore a set of internal coordinates and maybe the energy.
+    A comma separated list of an arbitrary amount of features can be specified with the following syntax:
+
+    - `e` for the energy
+    - `b m n` for a bond length, where `m` and `n` are the 0-based atom indices
+    - `a m n o` for an angle, where `m`, `n` and `o` are 0-based atom indices.
+      Calculates the angle around `n`
+    - `d m n o p` for a dihedral angle, where `m`, `n`, `o` and `p` are 0-based atom indices.
+      Calculates the rotation of the bond between `n` and `o`.
+      Dihedrals use a metric, that respects periodicity and direction of the rotation.
+      For each dihedral there will be two rows in the feature matrix, therefore.
+      See [this paper](https://doi.org/10.1002/prot.20310).
+    - (indices are 0-based)
+
+  - `-p --pca` activates dimensionalty reduction by principal component analysis.
+    Give an integer to specify how many principal components are kept.
+    During the execution of ConClusion the error introduced by PCA will be printed.
+
+  - `-c --cluster` activates clustering of the results.
+    The clustering algorithm can be selected by giving either `dbscan` or `hca`
+
+  - `--measure` specifies the distance measure between the conformeres.
+    By default an euclidean distance is used, but Manhattan and Mahalanobis distances are also available, as well as a general L_p norm.
+    If Mahalanobis distances are used, it might be worth a try to disable PCA.
+
+  - `--joinstrat` controls how inter-cluster distances are calculated in hierarchical clustering.
+    `single` might be the best choice to get dense groups of conformeres.
+
+  - `--distance` gives the search radius in DBScan or the dendrogram cut distance in hierarchical clustering.
+
+  - `--minsize` is the minimum size of a cluster in DBScan.
+    If `--forcemin` is given, the clusters obtained by HCA are also filtered for a minimum size.
+
+  - `--forcemin` forces filtering of HCA clusters for their minimum size as given by `--minsize`.
+    Disabled by default.
+
+Each processing step will produce a Gnuplot compatible file (space separated columns). The pure feature matrix will be `features.dat`, the results of the PCA will be in `pca.dat` and the clustering results will be in `cluster.dat`. The first column in `cluster.dat` will be an integer giving the cluster number this point belongs to, that can be used for colour-coding in Gnuplot.
+
+## Example
+A perylene dye with four phenoxy groups has different conformeres, that have different spectral properties.
+For solubility the dye has also some alkyl groups.
+Crest finds about 1400 conformeres, most of them being different only in the alkyl side-chains, that do not influence spectral properties.
+Therefore, a much smaller group of different conformeres with respect to different positions of phenoxy groups exist.
+From each of those groups the lowest energy conformere shall be obtained.
+We therefore select eight dihedral angles and the energy as features; 2 dihedrals for each phenoxy group.
+One dihedral per phenoxy group describing the rotation of the perylene-O bond, the second one describing the rotation around the O-Ph bond.
+As the dihedral angles are not independent from each other, as some orientations of phenoxy groups are not possible, we use a PCA to reduce dimensionalty and remove redundancies.
+After the PCA, DBScan is used to obtain clusters of similar conformeres.
+The lowest index in each cluster is also the lowest energy conformere in each group, as CREST sorts conformeres by energy.
+
+```bash
+conclusion \
+  --xyz=crest_conformers.xyz \
+  --pca=3 \
+  --dim="e, d 19 18 2 56, d 18 2 56 65, d 16 15 1 45, d 15 1 45 46, d 11 13 0 34, d 13 0 34 43, d 29 31 3 67, d31 3 67 76" \
+  --measure=manhattan \
+  --cluster=dbscan \
+  --distance=0.3 \
+  --minsize=5
+```
+
+## Library/Haskell Package
+ConClusion provides principal components analysis and the clustering algorithms DBScan and hierarchical clustering.
+The algorithms are implemented in efficient parallel arrays and perform quite well.
+For the API see the haddock documentation, which can be generated by:
+
+```bash
+cabal haddock
+```
diff --git a/app/ConClusion.hs b/app/ConClusion.hs
new file mode 100644
--- /dev/null
+++ b/app/ConClusion.hs
@@ -0,0 +1,742 @@
+-- |
+-- Module      : Main
+-- Description : Command Line Interface to analyse CREST results.
+-- Copyright   : Phillip Seeber, 2021
+-- License     : AGPL-3
+-- Maintainer  : phillip.seeber@uni-jena.de
+-- Stability   : experimental
+-- Portability : POSIX, Windows
+module Main (main) where
+
+import ConClusion.Chemistry.Topology hiding (xyz)
+import qualified ConClusion.Numeric.Statistics as Statistics
+import Data.Aeson
+import Data.Attoparsec.Text hiding (D)
+import Data.Foldable
+import qualified Data.IntSet as IntSet
+import Data.Massiv.Array as Massiv hiding (B, D)
+import qualified Data.Massiv.Array as Massiv
+import Data.Text.Lazy (toStrict)
+import Data.Text.Lazy.Builder as TB
+import Data.Version (showVersion)
+import Formatting hiding (char, (%))
+import qualified Formatting as F
+import Optics hiding (view)
+import Paths_ConClusion (version)
+import RIO hiding (Lens', Vector, lens, view, (^.))
+import qualified RIO.Text as Text
+import System.Console.CmdArgs
+import System.IO.Unsafe (unsafePerformIO)
+
+main :: IO ()
+main = do
+  app <- cmdArgs conClusionCmd >>= cmdToEnv
+  runRIO app $ do
+    logInfo $ "ConClusion, version " <> (displayShow . showVersion $ version) <> "\n"
+
+    -- Processing.
+    (featureMat, _trj) <- processTrajectory
+    pcaMat <- dimReduction featureMat
+    _clusters <- clusterFeatures pcaMat
+
+    logInfo "Done. Good Luck with the results!"
+
+----------------------------------------------------------------------------------------------------
+-- Processing Logic
+
+-- | Read the trajectory and calculate the specified features.
+{-# SCC processTrajectory #-}
+processTrajectory :: (HasXYZ env, HasDim env, HasLogFunc env) => RIO env (Matrix DL Double, Trajectory)
+processTrajectory = do
+  -- Obtain environment
+  file <- view xyzL
+  dims <- view dimL
+  let outputFile = "features.dat"
+
+  -- Logging
+  {- ORMOLU_DISABLE -}
+  logInfo $
+    "+-----------------------+\n\
+    \| Trajectory Processing |\n\
+    \+-----------------------+\n\n\
+    \  Trajectory File     : "  <> displayShow file <> "\n\
+    \  Feature Matrix File : "  <> displayShow outputFile <> "\n\
+    \  Features            :\n" <> displayFeatures dims
+  {- ORMOLU_ENABLE -}
+
+  -- Processing.
+  raw <- readFileUtf8 file
+  trj <- handleFailure $ parse' trajectory raw
+  features <- handleFailure $ getFeatures dims trj
+
+  -- Output.
+  toGnuPlot Nothing (Just . Massiv.fromList Seq $ dims) (compute features)
+    >>= writeFileUtf8 outputFile
+
+  logInfo "\n\n\n"
+  return (features, trj)
+
+-- | Possibly perform a PCA.
+{-# SCC dimReduction #-}
+dimReduction :: (HasPrincipalComponentAnalysis env, HasLogFunc env) => Matrix DL Double -> RIO env (Matrix U Double)
+dimReduction mat = do
+  doPCA <- view pcaL
+  case doPCA of
+    Nothing -> return . compute $ featureMat
+    Just PrincipalComponentAnalysis {keep} -> do
+      let outputFile = "pca.dat"
+
+      -- Logging.
+      {- ORMOLU_DISABLE -}
+      logInfo
+        "+------------------------------+\n\
+        \| Principal Component Analysis |\n\
+        \+------------------------------+\n"
+      {- ORMOLU_ENABLE -}
+
+      -- Processing
+      pcaData <- Statistics.pca keep featureMat
+      let behaviourPercent = Statistics.remaining pcaData
+
+      -- Logging.
+      {- ORMOLU_DISABLE -}
+      logInfo $
+        "  Dimensions         : " <> display keep <> "\n\
+        \  Eigenvalues        : " <> (displayEigenvalues . Statistics.allEigenValues $ pcaData) <> "\n\
+        \  Mean Squared Error : " <> (displayMSE . Statistics.mse $ pcaData) <> "\n\
+        \  Behaviour Captured : " <> displayPercent behaviourPercent <> "\n\
+        \  PCA Matrix File    : " <> displayShow outputFile
+      {- ORMOLU_ENABLE -}
+
+      -- Output
+      toGnuPlot Nothing Nothing (Statistics.y pcaData) >>= writeFileUtf8 outputFile
+
+      logInfo "\n\n\n"
+      return . Statistics.y $ pcaData
+  where
+    featureMat = compute @U mat
+
+-- | Cluster analysis of the (dimensionalty reduced) feature matrix.
+{-# SCC clusterFeatures #-}
+clusterFeatures ::
+  (HasClustering env, HasLogFunc env) => Matrix U Double -> RIO env Statistics.Clusters
+clusterFeatures mat = do
+  doClustering <- view clusteringL
+  case doClustering of
+    Nothing -> let Sz (_ :. n) = size mat in return $ makeArray @Massiv.B @Ix1 @IntSet Par (Sz n) $ \i -> IntSet.singleton i
+    Just cl -> do
+      let outputFile = "cluster.dat"
+      -- Logging
+      {- ORMOLU_DISABLE -}
+      logInfo
+        "+------------+\n\
+        \| Clustering |\n\
+        \+------------+\n"
+      {- ORMOLU_ENABLE -}
+
+      clusters <- case cl of
+        DBScan {distance, measure, minSize} -> clusterDB distance measure minSize
+        HCA {distance, measure, joinstrat, forcemin} -> clusterHCA distance measure joinstrat forcemin
+
+      {- ORMOLU_DISABLE -}
+      logInfo $
+        "  Number of Clusters    : " <> (display . Massiv.elemsCount $ clusters) <> "\n\
+        \  Cluster Data File     : " <> displayShow outputFile <> "\n\
+        \  Cluster               : " <> displayClusters clusters
+      {- ORMOLU_ENABLE -}
+
+      -- Output
+      toGnuPlot (Just clusters) Nothing mat >>= writeFileUtf8 outputFile
+
+      return clusters
+  where
+    -- Make the distance function for clustering.
+    distFn m = case m of
+      Lr i -> Statistics.lpNorm i
+      Manhattan -> Statistics.manhattan
+      Euclidean -> Statistics.euclidean
+      Mahalanobis -> Statistics.mahalanobis
+
+    -- Cluster data with DBScan algorithm.
+    clusterDB d m s = do
+      clusters <- Statistics.dbscan (distFn m) s d mat
+      {- ORMOLU_DISABLE -}
+      logInfo $
+        "  Algorithm            : DBScan\n\
+        \  Distance Measure     : " <> displayShow m <> "\n\
+        \  Search Distance ε    : " <> display d <> "\n\
+        \  Minimal Cluster Size : " <> display s
+      {- ORMOLU_ENABLE -}
+      return clusters
+
+    -- Cluster data with Hierarchical Cluster Analysis.
+    clusterHCA d m j fm = do
+      dendrogram <- Statistics.hca (distFn m) j mat
+      let allClusters = Statistics.cutDendroAt dendrogram d
+          dendroFile = "dendrogram.json"
+          clusters = case fm of
+            Nothing -> allClusters
+            Just minSize -> compute . sfilter (\c -> IntSet.size c >= minSize) $ allClusters
+
+      -- Write dendrogram file to disk.
+      liftIO $ encodeFile dendroFile dendrogram
+
+      {- ORMOLU_DISABLE -}
+      logInfo $
+        "  Algorithm             : Hierarchical Cluster Analysis\n\
+        \  Distance Measure      : " <> displayShow m <> "\n\
+        \  Search Distance       : " <> display d <> "\n\
+        \  Cluster Join Strategy : " <> displayShow j <> "\n" <>
+        (case fm of
+          Nothing -> mempty
+          Just minSize -> "  Minimal Cluster Size  : " <> display minSize <> "\n") <>
+        "  Dendrogram File       : " <> displayShow dendroFile
+      {- ORMOLU_ENABLE -}
+      return clusters
+
+----------------------------------------------------------------------------------------------------
+-- Command line interfaces
+
+-- | Command line arguments for ConClusion.
+data ConClusionCmd = ConClusionCmd
+  { -- | Filepath to the CREST xyz trajectory file.
+    xyz :: FilePath,
+    -- | Dimensions to process.
+    dim :: String,
+    -- | If PCA should be used to reduce dimensionalty of the problem. If so, the number of
+    -- dimensions to keep.
+    pca :: Maybe Int,
+    -- | Cluster algorithm to be used for clustering, if any.
+    cluster :: Maybe String,
+    -- | Distance measure of points.
+    measure :: String,
+    -- | Cluster joining strategy for hierarchical clustering.
+    joinstrat :: String,
+    -- | Search distance in case of DBScan or distance for the Dendogram cut in case of hierarchical
+    -- clustering.
+    distance :: Double,
+    -- | Mininmal size of a cluster in case of DBScan.
+    minsize :: Int,
+    -- | If to force a minimum size of clusters in HCA.
+    forcemin :: Bool,
+    -- | Do verbose logging.
+    verbose :: Bool
+  }
+  deriving (Show, Data, Typeable)
+
+conClusionCmd :: ConClusionCmd
+conClusionCmd =
+  ConClusionCmd
+    { xyz =
+        "crest_conformers.xyz"
+          &= typFile
+          &= help "Path to CREST trajectory.",
+      dim =
+        "e"
+          &= help
+            "Expression of dimensions to analyse:\n\
+            \  - \"e\"         Energy\n\
+            \  - \"b m n\"     Bond length between the atoms with\n\
+            \                indices m and n\n\
+            \  - \"a m n o\"   Angle between the atoms with indices\n\
+            \                m, n and o\n\
+            \  - \"d m n o p\" Sinus of the dihedral between the atoms\n\
+            \                m, n, o and p\n\
+            \Multiple features may be combined by giving a comma separated list of them.\n\
+            \Default: e",
+      pca =
+        Nothing
+          &= typ "INT"
+          &= help
+            "Perform a principal component analysis on the features.\
+            \Optionally give the number of dimensions you want to keep.\n\
+            \Default: 2"
+          &= opt (2 :: Int),
+      cluster =
+        Nothing
+          &= help
+            "Cluster algorithm to cluster conformeres.\n\
+            \  - \"dbscan\" DBScan algorithm. Depends on \n\
+            \             \"measure\", \"distance\" and \"minSize\"\n\
+            \  - \"hca\"    Hierarchical Cluster Analysis. Depends on\n\
+            \             \"measure\", \"joinstrat\" and \"distance\"\n\
+            \Default: dbscan"
+          &= opt ("dbscan" :: String),
+      measure =
+        "mahalanobis"
+          &= help
+            "Distance measure for distance between conformeres.\n\
+            \  - \"lrX\"         General L_r norm, where X is a value\n\
+            \                  between 3 and 9\n\
+            \  - \"manhattan\"   Manhattan distances\n\
+            \  - \"euclidean\"   Euclidean distances\n\
+            \  - \"mahalanobis\" Mahalanobis distances\n\
+            \Default: mahalanobis",
+      joinstrat =
+        "ward"
+          &= help
+            "Strategy for inter-cluster distance in hierarchical clustering.\n\
+            \  - \"single\"     Single-Linkage (minimal distance of\n\
+            \                 points between clusters)\n\
+            \  - \"complete\"   Complete-Linkage (maximal distance of\n\
+            \                 points between clusters)\n\
+            \  - \"median\"      Median distance linkage between\n\
+            \                 all points between clusters)\n\
+            \  - \"upgma\"      Average-Group-Linkage (average distance\n\
+            \                 of all points of joined clusters)\n\
+            \  - \"wpgma\"      Average-Linkage (average distance\n\
+            \                 of all points between clusters)\n\
+            \  - \"centroid\" Distance of cluster centroids\n\
+            \  - \"ward\"     Ward distance of minimal residues\n\
+            \Default: ward",
+      distance =
+        0.1
+          &= typ "FLOAT"
+          &= help
+            "Search distance in DBScan or Dendogram cut distance in hierarchical clustering.\n\
+            \Default: 0.1",
+      minsize =
+        5
+          &= typ "INT"
+          &= help
+            "Minimal size of a cluster in DBScan.\n\
+            \Default: 5",
+      forcemin =
+        False
+          &= help
+            "Force a minimal cluster size in hierarchical clustering.\n\
+            \Default: False",
+      verbose =
+        False
+          &= help "Verbose logging."
+    }
+    &= summary
+      "Analysis of conformere ensembles.\n\
+      \Processes an annotated XYZ trajectory (as generated by CREST, for example), where the energy\
+      \is provided in the comment line. The analysis is a three-step procedure:\n\
+      \  1. Calculate selected features from the ensemble, that are meant to\n\
+      \     characterise the conformeres.\n\
+      \     Relevant settings: \"--dim\"\n\
+      \  2. (Optional) Perform a PCA analysis on the feature matrix to reduce the\n\
+      \     number of dimensions.\n\
+      \     Relevant settings: \"--pca\"\n\
+      \  3. (Optional) Cluster the feature matrix into groups of conformeres. If a PCA\n\
+      \     has been performed, use the feature matrix with reduced dimensionalty.\n\
+      \     Relevant settins: \"--cluster\", \"--measure\", \"joinstrat\", \"--distance\",\n\
+      \     \"--minsize\""
+
+----------------------------------------------------------------------------------------------------
+-- Runtime Environment.
+
+-- Reader classes
+class HasXYZ a where
+  xyzL :: Lens' a FilePath
+
+class HasDim a where
+  dimL :: Lens' a [Feature]
+
+class HasPrincipalComponentAnalysis a where
+  pcaL :: Lens' a (Maybe PrincipalComponentAnalysis)
+
+class HasClustering a where
+  clusteringL :: Lens' a (Maybe Clustering)
+
+-- | Full runtime environment of ConClusion with all settings combined.
+data App = App
+  { -- | Filepath to the CREST xyz trajectory file.
+    xyz :: FilePath,
+    -- | Features that are to be analysed.
+    dim :: [Feature],
+    -- | Settings for principal component analysis.
+    pca :: Maybe PrincipalComponentAnalysis,
+    -- | Settings for Clustering.
+    clustering :: Maybe Clustering,
+    -- | RIO's 'logFunc'.
+    logFunc :: LogFunc
+  }
+
+-- Lenses
+instance (k ~ A_Lens, a ~ FilePath, b ~ a) => LabelOptic "xyz" k App App a b where
+  labelOptic = lens (xyz :: App -> FilePath) $ \s b -> (s {xyz = b} :: App)
+
+instance (k ~ A_Lens, a ~ [Feature], b ~ a) => LabelOptic "dim" k App App a b where
+  labelOptic = lens (dim :: App -> [Feature]) $ \s b -> (s {dim = b} :: App)
+
+instance (k ~ A_Lens, a ~ Maybe PrincipalComponentAnalysis, b ~ a) => LabelOptic "pca" k App App a b where
+  labelOptic = lens (pca :: App -> Maybe PrincipalComponentAnalysis) $ \s b -> (s {pca = b} :: App)
+
+instance (k ~ A_Lens, a ~ Maybe Clustering, b ~ a) => LabelOptic "clustering" k App App a b where
+  labelOptic = lens clustering $ \s b -> s {clustering = b}
+
+instance (k ~ A_Lens, a ~ LogFunc, b ~ a) => LabelOptic "logFunc" k App App a b where
+  labelOptic = lens logFunc $ \s b -> s {logFunc = b}
+
+-- Reader Classes
+instance HasXYZ App where
+  xyzL = castOptic #xyz
+
+instance HasDim App where
+  dimL = castOptic #dim
+
+instance HasPrincipalComponentAnalysis App where
+  pcaL = castOptic #pca
+
+instance HasClustering App where
+  clusteringL = castOptic #clustering
+
+instance HasLogFunc App where
+  logFuncL = toLensVL #logFunc
+
+-- | Settings for principal component analysis.
+newtype PrincipalComponentAnalysis = PrincipalComponentAnalysis
+  { -- | Number of dimensions to keep from PCA.
+    keep :: Int
+  }
+
+-- Lenses
+instance (k ~ A_Lens, a ~ Int, b ~ a) => LabelOptic "keep" k PrincipalComponentAnalysis PrincipalComponentAnalysis a b where
+  labelOptic = lens keep $ \s b -> s {keep = b}
+
+-- | Settings for clustering.
+data Clustering
+  = -- | DBScan.
+    DBScan
+      { -- | Search distance around a point.
+        distance :: Double,
+        -- | Distance measure in clustering.
+        measure :: Measure,
+        -- | Minimal size of a cluster.
+        minSize :: Int
+      }
+  | -- | Hierarchical clustering.
+    HCA
+      { -- | Cut distance in the dendogram to select the number of clusters.
+        distance :: Double,
+        -- | Distance measure in clustering.
+        measure :: Measure,
+        -- | Join strategy for clusters.
+        joinstrat :: Statistics.JoinStrat Double,
+        -- | A minimum cluster size might be forced in post processing for HCA.
+        forcemin :: Maybe Int
+      }
+
+-- | Distance measure between clusters.
+data Measure
+  = Lr Int
+  | Manhattan
+  | Euclidean
+  | Mahalanobis
+  deriving (Show)
+
+----------------------------------------------------------------------------------------------------
+-- Utilities
+
+-- | Conversion of command line arguments into runtime environment.
+cmdToEnv :: ConClusionCmd -> IO App
+cmdToEnv ConClusionCmd {xyz, dim, pca, cluster, measure, joinstrat, distance, minsize, forcemin, verbose} = do
+  -- Dimensionalty/Feature construction.
+  dimApp <- case parseOnly dimParser (Text.pack dim) of
+    Left err -> throwM $ ArgumentException "dim" err "a comma separated list of dimension specifications"
+    Right res -> return res
+
+  -- PCA construction.
+  pcaSettings <- case pca of
+    Nothing -> return Nothing
+    Just x ->
+      if x >= 1
+        then return . Just $ PrincipalComponentAnalysis {keep = x}
+        else throwM $ ArgumentException "pca" (show x) "a positive integer smaller than the number of clusters."
+
+  -- Clustering construction.
+  clDistance <-
+    if distance <= 0
+      then throwM $ ArgumentException "distance" (show distance) "a positive number"
+      else return distance
+  clMeasure <- case measure of
+    "lr3" -> return $ Lr 3
+    "lr4" -> return $ Lr 4
+    "lr5" -> return $ Lr 5
+    "lr6" -> return $ Lr 6
+    "lr7" -> return $ Lr 7
+    "lr8" -> return $ Lr 8
+    "lr9" -> return $ Lr 9
+    "manhattan" -> return Manhattan
+    "euclidean" -> return Euclidean
+    "mahalanobis" -> return Mahalanobis
+    _ -> throwM $ ArgumentException "measure" measure "one of \"manhattan | euclidean | mahalanobis | lrX\" with X = [ 3 .. 9]"
+  clMinsize <-
+    if minsize <= 0
+      then throwM $ ArgumentException "minsize" (show minsize) ">= 1"
+      else return minsize
+  clJoinstrat <- case joinstrat of
+    "single" -> return Statistics.SingleLinkage
+    "complete" -> return Statistics.CompleteLinkage
+    "median" -> return Statistics.Median
+    "upgma" -> return Statistics.UPGMA
+    "wpgma" -> return Statistics.WPGMA
+    "centroid" -> return Statistics.Centroid
+    "ward" -> return Statistics.Ward
+    _ -> throwM $ ArgumentException "joinstrat" joinstrat "one of \"single | complete | upgma | wpgma | centroid | ward\""
+  clustering <- case cluster of
+    Just "dbscan" ->
+      return . Just $
+        DBScan
+          { distance = clDistance,
+            measure = clMeasure,
+            minSize = clMinsize
+          }
+    Just "hca" ->
+      return . Just $
+        HCA
+          { distance = clDistance,
+            measure = clMeasure,
+            joinstrat = clJoinstrat,
+            forcemin = if forcemin then Just clMinsize else Nothing
+          }
+    Nothing -> return Nothing
+    Just c -> throwM $ ArgumentException "cluster" c "one of \"dbscan | hca\""
+
+  -- Log function setup.
+  logOptions <-
+    id
+      . setLogUseTime False
+      . setLogUseColor True
+      . setLogVerboseFormat False
+      . setLogUseLoc False
+      <$> logOptionsHandle stdout verbose
+  (lf, _ :: IO ()) <- newLogFunc logOptions
+  return
+    App
+      { xyz = xyz,
+        dim = dimApp,
+        pca = pcaSettings,
+        clustering = clustering,
+        logFunc = lf
+      }
+
+-- | Parser for dimensionalty expressions.
+dimParser :: Parser [Feature]
+dimParser = many1 $ do
+  skipMany (char ' ')
+  fType <- eParser <|> bParser <|> aParser <|> dParser
+  skipMany (char ' ')
+  _ <- option ',' (char ',')
+  return fType
+  where
+    eParser = char 'e' *> return Energy
+    bParser = do
+      _ <- char 'b'
+      skipMany (char ' ')
+      a <- decimal
+      skipMany (char ' ')
+      b <- decimal
+      return . Bond $ B a b
+    aParser = do
+      _ <- char 'a'
+      skipMany (char ' ')
+      a <- decimal
+      skipMany (char ' ')
+      b <- decimal
+      skipMany (char ' ')
+      c <- decimal
+      return . Angle $ A a b c
+    dParser = do
+      _ <- char 'd'
+      skipMany (char ' ')
+      a <- decimal
+      skipMany (char ' ')
+      b <- decimal
+      skipMany (char ' ')
+      c <- decimal
+      skipMany (char ' ')
+      d <- decimal
+      return . Dihedral $ D a b c d
+
+-- | Substitution for RIO's @view@ for optics.
+view :: (Is k A_Getter, MonadReader s f) => Optic' k is s b -> f b
+view l = (^. l) <$> ask
+
+-- | Parsers lifted to 'MonadThrow'.
+parse' :: MonadThrow m => Parser x -> Text -> m x
+parse' p t = case parseOnly p t of
+  Left err -> throwM $ ParseException err
+  Right res -> return res
+
+-- | Logging a problem as an error and then rethrow the underlying exception.
+handleFailure :: (Exception e, HasLogFunc env) => Either e r -> RIO env r
+handleFailure (Right r) = return r
+handleFailure (Left e) = do
+  logError . displayShow $ e
+  throwM e
+
+-- | Display features for a trajectory in a nice way.
+displayFeatures :: Foldable f => f Feature -> Utf8Builder
+displayFeatures features =
+  foldl
+    ( \acc f ->
+        acc
+          <> ( case f of
+                 Energy -> "    - E"
+                 Bond (B a b) -> "    - B " <> display a <> " " <> display b
+                 Angle (A a b c) -> "    - A " <> display a <> " " <> display b <> " " <> display c
+                 Dihedral (D a b c d) -> "    - D " <> display a <> " " <> display b <> " " <> display c <> " " <> display d
+             )
+          <> "\n"
+    )
+    mempty
+    features
+
+-- | Display a percent value.
+displayPercent :: Double -> Utf8Builder
+displayPercent v = display $ sformat (fixed 1 F.% " %") v
+
+-- | Display a mean squared error.
+displayMSE :: Double -> Utf8Builder
+displayMSE v = display $ sformat (fixed 2) v
+
+-- | Display eigenvalues
+displayEigenvalues :: Vector U Double -> Utf8Builder
+displayEigenvalues vec = display . Massiv.fold . Massiv.map fmtD $ vec
+  where
+    fmtD d = format (right 6 ' ' F.%. fixed 2) d
+
+-- | Display best cluster members.
+displayBestMember :: Vector U Int -> Utf8Builder
+displayBestMember vec =
+  display . toLazyText . Massiv.fold . Massiv.map (\i -> bformat ("  - " F.% int F.% "\n") i) $ vec
+
+-- | Converts a matrix to a Gnuplot matrix. Prints a header line as a comment with the features, if
+-- available. Adds the assigned cluster number in the first column if available.
+{-# SCC toGnuPlot #-}
+toGnuPlot ::
+  ( MonadThrow m
+  ) =>
+  Maybe Statistics.Clusters ->
+  Maybe (Vector Massiv.B Feature) ->
+  Matrix U Double ->
+  m Text
+toGnuPlot clusters features mat = do
+  -- Build the lines representing the points. If cluster information is available, the first column
+  -- is the cluster, that this point is assigned to.
+  valueLines <- case clusters of
+    Nothing -> return . Massiv.fold . Massiv.map (writeLine . Left) . innerSlices $ mat
+    Just cl -> do
+      let points = innerSlices mat :: Vector Massiv.D (Vector M Double)
+          labeledCl = compute . Massiv.imap (,) $ cl :: Vector Massiv.B (Int, IntSet)
+
+      -- Anotate all points with their assignment to a cluster number. If a point is from DBScan, it
+      -- is not necessarily assigned to a cluster.
+      annoPoints <- Massiv.iforM @Massiv.B points $ \pointIx vec -> do
+        let filteredCl = Massiv.dropWhile (\(_, clPoints) -> not $ pointIx `IntSet.member` clPoints) labeledCl
+        let assignedCl = filteredCl !? 0 :: Maybe (Int, IntSet)
+            clNumber = fst <$> assignedCl :: Maybe Int
+        return (clNumber, vec)
+
+      -- return annoClusters
+      return . Massiv.fold . Massiv.map (writeLine . Right) $ annoPoints
+
+  featureLine <- case features of
+    Nothing -> return mempty
+    Just f -> do
+      let Sz nFeatureLabels = actualFeatureSize f
+          featureLine = writeFeatures f
+      if nFeatures == nFeatureLabels
+        then return featureLine
+        else throwM $ SizeMismatchException (Sz nFeatureLabels) (Sz nFeatures)
+  return . toStrict . TB.toLazyText $ featureLine <> valueLines
+  where
+    Sz (nFeatures :. _) = size mat
+
+    -- The column width in the Gnuplot file.
+    cw = 14
+
+    -- Formatter for doubles.
+    dForm a = bformat (left cw ' ' F.%. fixed 4) a
+
+    -- Formatter for Integers.
+    iForm = fitLeft 3 F.%. left 3 ' ' F.%. int
+
+    -- Formatter for Features
+    fForm f = case f of
+      Energy -> bformat (left cw ' ' F.%. builder) "E"
+      Bond (B a b) -> bformat (left cw ' ' F.%. (builder F.% iForm F.% "," F.% iForm)) "B" a b
+      Angle (A a b c) -> bformat (left cw ' ' F.%. (builder F.% iForm F.% "," F.% iForm F.% "," F.% iForm)) "A" a b c
+      Dihedral (D a b c d) ->
+        bformat (left cw ' ' F.%. (builder F.% iForm F.% "," F.% iForm F.% "," F.% iForm F.% "," F.% iForm)) "A sin" a b c d
+          <> bformat (left cw ' ' F.%. (builder F.% iForm F.% "," F.% iForm F.% "," F.% iForm F.% "," F.% iForm)) "A cos" a b c d
+
+    -- Print a vector of numeric values.
+    doubleVecB :: Source r Ix1 Double => Vector r Double -> TB.Builder
+    doubleVecB v = Massiv.fold . Massiv.map dForm $ v
+
+    -- Writer for a value line.
+    writeLine ::
+      (Source r Ix1 Double) => Either (Vector r Double) (Maybe Int, Vector r Double) -> TB.Builder
+    writeLine (Left vec) = doubleVecB vec <> "\n"
+    writeLine (Right (cl, vec)) = bformat iForm (fromMaybe (-1) cl) <> doubleVecB vec <> "\n"
+
+    -- Writer for the Feature line.
+    writeFeatures :: Vector Massiv.B Feature -> TB.Builder
+    writeFeatures vec = "#" <> (Massiv.fold . Massiv.map fForm $ vec) <> "\n"
+
+    -- Actual feature size.
+    actualFeatureSize vec =
+      unsafePerformIO $
+        Massiv.foldlP
+          ( \acc f -> case f of
+              Dihedral _ -> 2 + acc
+              _ -> 1 + acc
+          )
+          0
+          (+)
+          0
+          vec
+
+-- | Pretty printer for clusters.
+displayClusters :: Statistics.Clusters -> Utf8Builder
+displayClusters clusters = display . toLazyText . Massiv.fold . Massiv.map printCl $ clusters
+  where
+    -- Print clusters as list of indices.
+    iForm = fitLeft 5 F.%. left 5 ' ' F.%. int
+
+    -- Printer of a single cluster.
+    printCl :: IntSet -> TB.Builder
+    printCl cl =
+      fst $
+        IntSet.foldl
+          ( \(bAcc, counter) i ->
+              let this = bformat iForm i <> if counter `mod` (16 :: Int) == 0 then "\n      " else mempty
+               in (bAcc <> this, counter + 1)
+          )
+          ("\n\n    - ", 1)
+          cl
+
+----------------------------------------------------------------------------------------------------
+-- Exceptions
+
+-- | Exception thrown on unexcpected command line arguments.
+data ArgumentException = ArgumentException
+  { argName :: String,
+    is :: String,
+    should :: String
+  }
+
+instance Show ArgumentException where
+  show ArgumentException {argName, is, should} =
+    "ArgumentException: Field \""
+      <> argName
+      <> "\" got \""
+      <> is
+      <> "\" but must be "
+      <> should
+      <> "."
+
+instance Exception ArgumentException
+
+-- | Exceptions thrown on failed parsing.
+newtype ParseException = ParseException String
+
+instance Show ParseException where
+  show (ParseException err) = "ParseException: \"" <> err <> "\""
+
+instance Exception ParseException
diff --git a/src/ConClusion/Chemistry/Topology.hs b/src/ConClusion/Chemistry/Topology.hs
new file mode 100644
--- /dev/null
+++ b/src/ConClusion/Chemistry/Topology.hs
@@ -0,0 +1,210 @@
+-- |
+-- Module      : ConClusion.Chemistry.Topology
+-- Description : Principal Component Analysis
+-- Copyright   : Phillip Seeber, 2021
+-- License     : AGPL-3
+-- Maintainer  : phillip.seeber@googlemail.com
+-- Stability   : experimental
+-- Portability : POSIX, Windows
+--
+-- This module implements routines to work with simple molden style XYZ trajectories. This includes
+-- parsers as well as functions to obtain structural features in internal coordinates.
+--
+-- For an introduction into PCA see <https://www.cs.cmu.edu/~elaw/papers/pca.pdf>.
+--
+-- Diherdrals require a special metric, see <https://onlinelibrary.wiley.com/doi/full/10.1002/prot.20310)>.
+module ConClusion.Chemistry.Topology
+  ( Molecule,
+    Trajectory,
+    xyz,
+    trajectory,
+    B (..),
+    A (..),
+    D (..),
+    Feature (..),
+    getFeatures,
+  )
+where
+
+import ConClusion.Numeric.Data hiding (angle)
+import qualified ConClusion.Numeric.Data as ND
+import Data.Attoparsec.Text hiding (D)
+import Data.Foldable
+import Data.Massiv.Array as Massiv hiding (B, D, forM)
+import qualified Numeric.LinearAlgebra as LA
+import RIO
+import qualified RIO.Seq as Seq
+import qualified RIO.Text as Text
+import qualified RIO.Vector.Boxed as VectorB
+
+-- | A Molecule in cartesian coordinates.
+data Molecule = Molecule
+  { -- | The energy of the molecule.
+    energy :: !Double,
+    -- | Chemical symbols or names of the atoms. \(N\) vector.
+    atoms :: !(VectorB.Vector Text),
+    -- | Cartesian coordinates. Atoms as rows, xyz as columns. \(N \times 3\) matrix.
+    coordinates :: !(Massiv.Matrix S Double)
+  }
+
+type Trajectory = Seq Molecule
+
+-- | Parser for molecules in Molden XYZ format.
+xyz :: Parser Molecule
+xyz = do
+  -- Starting line with number of atoms.
+  nAtoms <- do
+    _ <- skipMany . char $ ' '
+    n <- decimal
+    _ <- skipMany . char $ ' '
+    endOfLine
+    return n
+
+  -- Comment line. Must contain the energy.
+  energy <- do
+    _ <- skipMany . char $ ' '
+    e <- double
+    _ <- skipMany . char $ ' '
+    endOfLine
+    return e
+
+  -- Multiple lines of atoms.
+  atoms <- count nAtoms $ do
+    _ <- skipMany . char $ ' '
+    name <- many1 $ letter <|> digit
+    _ <- skipMany1 . char $ ' '
+    x <- double
+    _ <- skipMany1 . char $ ' '
+    y <- double
+    _ <- skipMany1 . char $ ' '
+    z <- double
+    _ <- skipMany . char $ ' '
+    endOfLine
+    let row = Massiv.expandOuter (Sz 1) const . Massiv.fromList @S Seq $ [x, y, z]
+    return (Text.pack name, row)
+
+  return
+    Molecule
+      { energy = energy,
+        atoms = VectorB.fromListN nAtoms . fmap fst $ atoms,
+        coordinates = Massiv.compute . Massiv.concat' (Dim 2) . fmap snd $ atoms
+      }
+
+-- | Parser for trajectories in XYZ format as produced by CREST.
+{-# SCC trajectory #-}
+trajectory :: Parser Trajectory
+trajectory = do
+  mols <- Seq.fromList <$> many1 xyz
+  skipSpace
+  endOfInput
+  return mols
+
+-- | Selection of a bond between two atoms.
+data B = B Int Int
+
+-- | Selection of an angle between three atoms.
+data A = A Int Int Int
+
+-- | Selection of a dihedral angle between four atoms. Rotation around the central two.
+data D = D Int Int Int Int
+
+-- | Selections
+data Feature
+  = Energy
+  | Bond B
+  | Angle A
+  | Dihedral D
+
+-- | Calculate a bond distance.
+bond :: MonadThrow m => B -> Molecule -> m Double
+bond (B a b) Molecule {coordinates}
+  | a == b = throwM . IndexException $ "selected atoms are identical"
+  | otherwise = do
+    coordA <- compute @U <$> (coordinates !?> a)
+    coordB <- compute @U <$> (coordinates !?> b)
+    vecAB <- coordA .-. coordB
+    return . sqrt . Massiv.sum . Massiv.map (^ (2 :: Int)) $ vecAB
+
+-- | Calculates the sinus of an angle defined by three atoms.
+angle :: MonadThrow m => A -> Molecule -> m Double
+angle (A a b c) Molecule {coordinates}
+  | a == b || b == c || a == c = throwM . IndexException $ "selected atoms are identical"
+  | otherwise = do
+    coordA <- compute @U <$> (coordinates !?> a)
+    coordB <- compute @U <$> (coordinates !?> b)
+    coordC <- compute @U <$> (coordinates !?> c)
+    vecAB <- coordA .-. coordB
+    vecCB <- coordC .-. coordB
+    return $ ND.angle vecAB vecCB
+
+-- | Calculates the dihedral angle defined by four atoms. Respects rotation direction. Obtains the
+-- result in radian.
+dihedral' :: MonadThrow m => D -> Molecule -> m Double
+dihedral' (D a b c d) Molecule {coordinates}
+  | a == b || a == c || a == d || b == c || b == d || c == d = throwM . IndexException $ "selected atoms are indentical"
+  | otherwise = do
+    coordA <- compute @U <$> (coordinates !?> a)
+    coordB <- compute @U <$> (coordinates !?> b)
+    coordC <- compute @U <$> (coordinates !?> c)
+    coordD <- compute @U <$> (coordinates !?> d)
+    vecAB <- coordA .-. coordB
+    vecBC <- coordB .-. coordC
+    vecCD <- coordC .-. coordD
+    let planeABC = vecH2M $ LA.cross (vecM2H vecAB) (vecM2H vecBC) :: Massiv.Vector U Double
+        planeBCD = vecH2M $ LA.cross (vecM2H vecBC) (vecM2H vecCD) :: Massiv.Vector U Double
+        normVecRot = LA.cross (vecM2H vecCD) (vecM2H vecBC) :: LA.Vector Double
+        rotDir =
+          if vecH2M normVecRot !.! vecAB < 0
+            then -1
+            else 1
+    return $ rotDir * ND.angle planeABC planeBCD
+
+-- | Calculates the a metric value of the dihedral angle defined by four atoms. This must create 2
+-- values in the feature matrix, instead of one.
+-- See <https://onlinelibrary.wiley.com/doi/full/10.1002/prot.20310)>
+dihedral :: MonadThrow m => D -> Molecule -> m (Double, Double)
+dihedral d mol = do
+  dihedRad <- dihedral' d mol
+  return (sin dihedRad, cos dihedRad)
+
+-- | Get all selected features from a molecule.
+getFeature ::
+  ( Traversable t,
+    MonadThrow m
+  ) =>
+  -- | Selection of multiple features.
+  t Feature ->
+  -- | A Molecule where to apply to.
+  Molecule ->
+  m (Massiv.Vector Massiv.DL Double)
+getFeature sel mol = do
+  features <- for sel $ \s -> case s of
+    Energy -> return . Left . energy $ mol
+    Bond b -> Left <$> bond b mol
+    Angle a -> Left <$> angle a mol
+    Dihedral d -> Right <$> dihedral d mol
+
+  let featureVec =
+        foldl
+          ( \acc val -> case val of
+              Left s -> acc `Massiv.snoc` s
+              Right (a, b) -> acc `Massiv.snoc` a `Massiv.snoc` b
+          )
+          mempty
+          features
+
+  return featureVec
+
+-- | Obtains the feature matrix \(\mathbf{X}\) for a principal component analysis. Given \(m\)
+-- features to analyse in \(n\) measurements, \(\mathbf{X}\) will be a \(m \times n\) matrix.
+{-# SCC getFeatures #-}
+getFeatures ::
+  ( MonadThrow m,
+    Traversable f
+  ) =>
+  f Feature ->
+  Trajectory ->
+  m (Massiv.Matrix DL Double)
+getFeatures sel trj = traverse toCols trj >>= concatM (Dim 1)
+  where
+    toCols v = expandInner @Ix2 (Sz 1) const . compute @U <$> getFeature sel v
diff --git a/src/ConClusion/Numeric/Data.hs b/src/ConClusion/Numeric/Data.hs
new file mode 100644
--- /dev/null
+++ b/src/ConClusion/Numeric/Data.hs
@@ -0,0 +1,197 @@
+-- |
+-- Module      : ConClusion.Numeric.Data
+-- Description : Type castings, conversions and utilities
+-- Copyright   : Phillip Seeber, 2021
+-- License     : AGPL-3
+-- Maintainer  : phillip.seeber@googlemail.com
+-- Stability   : experimental
+-- Portability : POSIX, Windows
+module ConClusion.Numeric.Data
+  ( -- * Conversion of array types.
+
+    -- Conversion between HMatrix and Massiv's array types
+    IndexException (..),
+    vecH2M,
+    vecM2H,
+    matH2M,
+    matM2H,
+
+    -- * Array Processing
+    magnitude,
+    normalise,
+    angle,
+    minDistAt,
+    minDistAtVec,
+    iMinimumM,
+
+    -- * Utilities
+    printMat,
+
+    -- * Binary Trees
+    BinTree (..),
+    root,
+    takeBranchesWhile,
+    takeLeafyBranchesWhile,
+  )
+where
+
+import Data.Aeson hiding (Array)
+import Data.Massiv.Array as Massiv hiding (IndexException)
+import Data.Massiv.Array.Manifest.Vector as Massiv
+import Formatting
+import Numeric.LinearAlgebra as LA hiding (magnitude, (<>))
+import RIO
+import qualified RIO.Vector.Storable as VectorS
+import System.IO.Unsafe (unsafePerformIO)
+
+-- | Exception regarding indexing in some kind of aaray.
+newtype IndexException = IndexException String deriving (Show)
+
+instance Exception IndexException
+
+-- | Converts a vector from the HMatrix package to the Massiv representation.
+{-# SCC vecH2M #-}
+vecH2M :: (Element e, Mutable r Ix1 e) => VectorS.Vector e -> Massiv.Vector r e
+vecH2M hVec = fromVector' Seq (Sz $ VectorS.length hVec) hVec
+
+-- | Converts a vector from the Massiv representation to the HMatrix representation.
+{-# SCC vecM2H #-}
+vecM2H :: (Manifest r Ix1 e, Element e) => Massiv.Vector r e -> LA.Vector e
+vecM2H = Massiv.toVector
+
+-- | Converts a matrix from the HMatrix representation to the Massiv representation.
+{-# SCC matH2M #-}
+matH2M :: (Mutable r Ix1 e, Element e) => LA.Matrix e -> Massiv.Matrix r e
+matH2M hMat = Massiv.resize' (Sz $ nRows :. nCols) . vecH2M . LA.flatten $ hMat
+  where
+    nRows = LA.rows hMat
+    nCols = LA.cols hMat
+
+-- | Converts a matrix from Massiv to HMatrix representation.
+{-# SCC matM2H #-}
+matM2H :: (Manifest r Ix1 e, Element e, Resize r Ix2, Load r Ix2 e) => Massiv.Matrix r e -> LA.Matrix e
+matM2H mMat = LA.reshape nCols . vecM2H . Massiv.flatten $ mMat
+  where
+    Sz (_nRows :. nCols) = Massiv.size mMat
+
+-- | Magnitude of a vector (length).
+magnitude :: (Massiv.Numeric r e, Source r Ix1 e, Floating e) => Massiv.Vector r e -> e
+magnitude v = sqrt $ v !.! v
+
+-- | Normalise a vector.
+normalise :: (Massiv.Numeric r e, Source r Ix1 e, Floating e) => Massiv.Vector r e -> Massiv.Vector r e
+normalise v = v .* (1 / magnitude v)
+
+-- | Angle between two vectors.
+angle :: (Massiv.Numeric r e, Source r Ix1 e, Floating e) => Massiv.Vector r e -> Massiv.Vector r e -> e
+angle a b = acos $ a !.! b / (magnitude a * magnitude b)
+
+-- | Find the minimal distance in a distance matrix, which is not the main diagonal.
+{-# SCC minDistAt #-}
+minDistAt ::
+  ( Manifest r Ix2 e,
+    MonadThrow m,
+    Ord e
+  ) =>
+  Massiv.Matrix r e ->
+  m (e, Ix2)
+minDistAt arr
+  | isEmpty arr = throwM $ SizeEmptyException (Massiv.size arr)
+  | otherwise = return . unsafePerformIO $ ifoldlP minFold start chFold start arr
+  where
+    ix0 = pureIndex 0
+    e0 = arr Massiv.! ix0
+    start = (e0, ix0)
+    minFold acc@(eA, _) ix@(m :. n) e = if e < eA && m > n then (e, ix) else acc
+    chFold acc@(eA, _) ch@(e, _) = if e < eA then ch else acc
+
+-- | Find the minimal element of a vector, which is at a larger than the supplied index.
+minDistAtVec ::
+  ( Manifest r Ix1 e,
+    MonadThrow m,
+    Ord e
+  ) =>
+  Ix1 ->
+  Massiv.Vector r e ->
+  m (e, Ix1)
+minDistAtVec ixStart vec
+  | isEmpty vec = throwM $ SizeEmptyException (Massiv.size vec)
+  | ixStart >= nElems = throwM $ IndexOutOfBoundsException (Sz nElems) ixStart
+  | otherwise = do
+    let (minE, minIx) = unsafePerformIO $ ifoldlP minFold startAcc chFold startAcc searchVec
+    return (minE, minIx + ixStart + 1)
+  where
+    Sz nElems = Massiv.size vec
+    searchVec = Massiv.drop (Sz $ ixStart + 1) vec
+    ix0 = 0
+    e0 = searchVec Massiv.! ix0
+    startAcc = (e0, ix0)
+    minFold acc@(eA, _) ix e = if e < eA then (e, ix) else acc
+    chFold acc ch = min acc ch
+
+-- | Like 'Massiv.minimumM' but also returns the index of the minimal element.
+iMinimumM ::
+  ( Manifest r ix a,
+    MonadThrow m,
+    Ord a
+  ) =>
+  Array r ix a ->
+  m (a, ix)
+iMinimumM arr
+  | isEmpty arr = throwM $ SizeEmptyException (Massiv.size arr)
+  | otherwise = return . unsafePerformIO $ ifoldlP minFold start chFold start arr
+  where
+    ix0 = pureIndex 0
+    e0 = arr Massiv.! ix0
+    start = (e0, ix0)
+
+    minFold acc@(eA, _) ix e = if e < eA then (e, ix) else acc
+    chFold acc@(eA, _) ch@(e, _) = if e < eA then ch else acc
+
+-- | Quickly print a matrix with numerical values
+printMat :: (Source r Ix2 e, Real e) => Massiv.Matrix r e -> Massiv.Matrix D Text
+printMat mat = Massiv.map (sformat (left 4 ' ' %. fixed 2)) mat
+
+----------------------------------------------------------------------------------------------------
+-- Binary Trees.
+
+-- | A binary tree.
+data BinTree e = Leaf e | Node e (BinTree e) (BinTree e)
+  deriving (Eq, Show, Generic)
+
+instance (FromJSON e) => FromJSON (BinTree e)
+
+instance (ToJSON e) => ToJSON (BinTree e)
+
+instance Functor BinTree where
+  fmap f (Leaf a) = Leaf (f a)
+  fmap f (Node a l r) = Node (f a) (fmap f l) (fmap f r)
+
+-- | Look at the root of a binary tree.
+root :: BinTree e -> e
+root (Leaf e) = e
+root (Node e _ _) = e
+
+-- | Steps down each branch of a tree until some criterion is satisfied or the end of the branch is
+-- reached. Each end of the branch is added to a result.
+takeBranchesWhile :: (a -> Bool) -> BinTree a -> Massiv.Vector DL a
+takeBranchesWhile chk tree = go tree (Massiv.empty @DL)
+  where
+    go (Leaf v) acc = if chk v then acc `snoc` v else acc
+    go (Node v l r) acc =
+      let vAcc = if chk v then acc `snoc` v else acc
+          lAcc = go l vAcc
+          rAcc = go r lAcc
+       in if chk v then rAcc else vAcc
+
+-- | Takes the first value in each branch, that does not fullfill the criterion anymore and adds it
+-- to the result. Terminal leafes of the branches are always taken.
+takeLeafyBranchesWhile :: (a -> Bool) -> BinTree a -> Massiv.Vector DL a
+takeLeafyBranchesWhile chk tree = go tree (Massiv.empty @DL)
+  where
+    go (Leaf v) acc = acc `snoc` v
+    go (Node v l r) acc =
+      let vAcc = if chk v then acc else acc `snoc` v
+          lAcc = go l vAcc
+          rAcc = go r lAcc
+       in if chk v then rAcc else vAcc
diff --git a/src/ConClusion/Numeric/Statistics.hs b/src/ConClusion/Numeric/Statistics.hs
new file mode 100644
--- /dev/null
+++ b/src/ConClusion/Numeric/Statistics.hs
@@ -0,0 +1,947 @@
+-- |
+-- Module      : ConClusion.Numeric.Statistics
+-- Description : Statistical Functions
+-- Copyright   : Phillip Seeber, 2021
+-- License     : AGPL-3
+-- Maintainer  : phillip.seeber@googlemail.com
+-- Stability   : experimental
+-- Portability : POSIX, Windows
+module ConClusion.Numeric.Statistics
+  ( -- * PCA
+    PCA (..),
+    pca,
+
+    -- * Variance
+    normalise,
+    meanDeviation,
+    covariance,
+
+    -- * Distance Metrics
+    DistFn,
+    lpNorm,
+    manhattan,
+    euclidean,
+    mahalanobis,
+
+    -- * Cluster Algorithms
+    Clusters,
+
+    -- ** DBScan
+    DistanceInvalidException (..),
+    dbscan,
+
+    -- ** Hierarchical Cluster Analysis
+    Dendrogram,
+    JoinStrat (..),
+    hca,
+    cutDendroAt,
+  )
+where
+
+import ConClusion.Numeric.Data hiding (normalise)
+import Data.Aeson hiding (Array)
+import Data.Complex
+import qualified Data.IntSet as IntSet
+import Data.Massiv.Array as Massiv
+import Data.Massiv.Array.Unsafe as Massiv
+import qualified Data.PSQueue as PQ
+import qualified Numeric.LinearAlgebra as LA
+import RIO hiding (Vector)
+import System.IO.Unsafe (unsafePerformIO)
+
+----------------------------------------------------------------------------------------------------
+-- Others/Helpers
+
+-- | Solves eigenvalue problem of a square matrix and obtains its eigenvalues and eigenvectors.
+{-# SCC eig #-}
+eig ::
+  ( Mutable r1 Ix1 (Complex Double),
+    Mutable r2 Ix1 (Complex Double),
+    LA.Field e,
+    Manifest r3 Ix1 e,
+    Resize r3 Ix2,
+    Load r3 Ix2 e,
+    MonadThrow m
+  ) =>
+  Matrix r3 e ->
+  m (Vector r1 (Complex Double), Matrix r2 (Complex Double))
+eig covM
+  | m /= n = throwM $ IndexException "eigenvalue problems can only be solved for square matrix"
+  | otherwise = return . bimap vecH2M matH2M . LA.eig $ cov
+  where
+    Sz (m :. n) = size covM
+    cov = matM2H covM
+
+-- | Sort eigenvalues and eigenvectors by magnitude of the eigenvalues in descending order (largest
+-- eigenvalues first). Eigenvectors are the columns of the input matrix.
+{-# SCC eigSort #-}
+eigSort ::
+  ( Load r2 Ix2 e,
+    MonadThrow m,
+    Source r1 Ix1 e,
+    Source r2 Ix2 e,
+    Mutable r1 Ix1 e,
+    Mutable r2 Ix2 e,
+    Unbox e,
+    Ord e
+  ) =>
+  (Vector r1 e, Matrix r2 e) ->
+  m (Vector r1 e, Matrix r2 e)
+eigSort (vec, mat)
+  | m /= n = throwM $ IndexException "matrix of the eigenvectors is not a square matrix"
+  | n /= n' = throwM $ IndexException "different number of eigenvalues and eigenvectors"
+  | otherwise = do
+    let ixedEigenvalues = Massiv.zip vec ixVec
+        (eigenValSortAsc, ixSort) = (\a -> (get fst a, get snd a)) . quicksort . compute @U $ ixedEigenvalues
+        eigenVecSortAsc = backpermute' (Sz $ m :. n) (\(r :. c) -> r :. (ixSort ! c)) mat
+        eigenValSort = reverse' (Dim 1) eigenValSortAsc
+        eigenVecSort = reverse' (Dim 1) eigenVecSortAsc
+    return (compute eigenValSort, compute eigenVecSort)
+  where
+    Sz (m :. n) = size mat
+    Sz n' = size vec
+    ixVec = makeArrayLinear @D Seq (Sz n') id
+    get acc = compute @U . Massiv.map acc
+
+----------------------------------------------------------------------------------------------------
+-- Principal Component Analysis
+
+data PCA = PCA
+  { -- | Original feature matrix.
+    x :: Matrix U Double,
+    -- | Feature matrix in mean deviation form.
+    x' :: Matrix U Double,
+    -- | Transformed data.
+    y :: Matrix U Double,
+    -- | Transformation matrix to transform feature matrix into PCA result matrix.
+    a :: Matrix U Double,
+    -- | Mean squared error introduced by PCA.
+    mse :: Double,
+    -- | Percentage of the behaviour captured in the remaining dimensions.
+    remaining :: Double,
+    -- | All eigenvalues from the diagonalisation of the covariance matrix.
+    allEigenValues :: Vector U Double,
+    -- | Eigenvalues that were kept for PCA.
+    pcaEigenValues :: Vector U Double,
+    -- | All eigenvectors from the diagonalisation of the covariance matrix.
+    allEigenVecs :: Matrix U Double,
+    -- | Eigenvectors that were kept for PCA.
+    pcaEigenVecs :: Matrix U Double
+  }
+
+-- | Transform the input values with a transformation matrix \(\mathbf{A}\), where \(\mathbf{A}\) is
+-- constructed from the eigenvectors associated to the largest eigenvalues.
+{-# SCC transformToPCABasis #-}
+transformToPCABasis ::
+  ( Source (R r) Ix2 e,
+    Extract r Ix2 e,
+    Mutable r Ix2 e,
+    Numeric r e,
+    MonadThrow m
+  ) =>
+  -- | Number of dimensions to keep from PCA.
+  Int ->
+  -- | Matrix of the eigenvectors, sorted descendingly by eigenvalues, where the eigenvectors are
+  -- the columns of the matrix.
+  Matrix r e ->
+  -- | Feature matrix in mean deviation form.
+  Matrix r e ->
+  -- | Input data transformed by PCA to lower dimensions, and the transformation matrix
+  -- \(\mathbf{A}\).
+  m (Matrix r e, Matrix r e)
+transformToPCABasis nDim eigenVecMat featureMat
+  | mE /= nE = throwM $ IndexException "the matrix of the eigenvectors must be a quadratic matrix"
+  | nDim <= 0 = throwM $ IndexException "the number of dimensions of the PCA is smaller than or zero"
+  | nDim >= nE = throwM $ IndexException "more than the possible amount of dimensions has been selected"
+  | mE /= mF = throwM $ IndexException "eigenvector matrix and feature matrix have mismatching dimensions"
+  | otherwise = do
+    matA <- compute . transpose <$> extractM (0 :. 0) (Sz $ mE :. nDim) eigenVecMat
+    pcaData <- matA .><. featureMat
+    return (pcaData, matA)
+  where
+    Sz (mE :. nE) = size eigenVecMat
+    Sz (mF :. _nF) = size featureMat
+
+-- | Performs a PCA on the feature matrix \(\mathbf{X}\) by solving the eigenproblem of the
+-- covariance matrix. The function takes the feature matrix directly and perfoms the conversion
+-- to mean deviation form, the calculation of the covariance matrix and the eigenvalue problem
+-- automatically.
+{-# SCC pca #-}
+pca ::
+  ( Numeric r Double,
+    Mutable r Ix2 Double,
+    Manifest r Ix1 Double,
+    Source (R r) Ix2 Double,
+    Extract r Ix2 Double,
+    MonadThrow m
+  ) =>
+  -- | Dimensionalty after PCA transformation.
+  Int ->
+  -- | \(m \times n\) Feaute matrix \(\mathbf{X}\), with \(m\) different measurements (rows) in
+  -- \(n\) different trials (columns).
+  Matrix r Double ->
+  m PCA
+pca dim x = do
+  -- Calculate the mean deviation form of the feature matrix and the covariance matrix from it.
+  let x' = normalise . meanDeviation $ x
+      cov = covariance x'
+
+  -- Obtain eigenvalues and eigenvectors of the covariance matrix and sort them.
+  (eigValsC :: Vector U (Complex Double), eigVecsC :: Matrix U (Complex Double)) <- eig cov
+  let eigValsR = compute @U . Massiv.map realPart $ eigValsC
+      eigVecsR = compute . Massiv.map realPart $ eigVecsC
+  (eValS, eVecS) <- eigSort (eigValsR, eigVecsR)
+
+  -- Use the subset of the eigenvectors with the largest eigenvalues to transform the features in
+  -- mean deviation form into the result matrix Y.
+  (pcaData, matA) <- transformToPCABasis dim eVecS x'
+
+  -- Reconstuct the original data from lower dimensions and calculate the mean squared deviation.
+  reconstructX <- (compute . transpose $ matA) .><. pcaData
+  mse <- (/ fromIntegral n) . Massiv.sum . Massiv.map (** 2) <$> (x' .-. reconstructX)
+
+  -- For output give the eigenvalues and eigenvectors that were kept.
+  pcaEigenValues <- extractM 0 (Sz dim) eValS
+  pcaEigenVecs <- extractM (0 :. 0) (Sz $ m :. dim) eVecS
+
+  -- Calculate the amount of behaviour that could be kept.
+  let remaining = (Massiv.sum pcaEigenValues / Massiv.sum eValS) * 100
+
+  return
+    PCA
+      { x = compute x,
+        x' = compute x',
+        y = compute pcaData,
+        a = compute matA,
+        mse = mse,
+        remaining = remaining,
+        allEigenValues = eValS,
+        pcaEigenValues = compute pcaEigenValues,
+        allEigenVecs = compute eVecS,
+        pcaEigenVecs = compute pcaEigenVecs
+      }
+  where
+    Sz (m :. n) = size x
+
+----------------------------------------------------------------------------------------------------
+-- Variance
+
+-- | Subtract the mean value of all columns from the feature matrix. Brings the feature matrix to
+-- mean deviation form.
+{-# SCC meanDeviation #-}
+meanDeviation ::
+  ( Source r Ix2 e,
+    Fractional e,
+    Unbox e,
+    Numeric r e,
+    Mutable r Ix2 e
+  ) =>
+  Matrix r e ->
+  Matrix r e
+meanDeviation mat = mat !-! compute meanMat
+  where
+    Sz (_ :. n) = Massiv.size mat
+    featueMean = Massiv.foldlInner (+) 0 mat .* (1 / fromIntegral n)
+    meanMat = expandInner (Sz n) const . compute @U $ featueMean
+
+-- | Obtains the covariance matrix \(\mathbf{C_X}\) from the feature matrix \(\mathbf{X}\).
+-- \[
+--   \mathbf{C_X} \equiv \frac{1}{n - 1} \mathbf{X} \mathbf{X}^T
+-- \]
+-- where \(n\) is the number of columns in the matrix.
+--
+-- The feature matrix should be in mean deviation form, see 'meanDeviation'.
+{-# SCC covariance #-}
+covariance :: (Numeric r e, Mutable r Ix2 e, Fractional e) => Matrix r e -> Matrix r e
+covariance x = (1 / (fromIntegral n - 1)) *. (x !><! (compute . transpose $ x))
+  where
+    Sz (_ :. n) = size x
+
+-- | Normalise each value so that the maximum absolute value in each row becomes one.
+normalise ::
+  ( Ord e,
+    Unbox e,
+    Numeric r e,
+    Fractional e,
+    Source r Ix2 e,
+    Mutable r Ix2 e
+  ) =>
+  Array r Ix2 e ->
+  Array r Ix2 e
+normalise mat =
+  let absMat = Massiv.map abs mat
+      maxPerRow = compute @U . foldlInner max 0 $ absMat
+      divMat = compute . Massiv.map (1 /) . expandInner @Ix2 (Sz n) const $ maxPerRow
+   in divMat !*! mat
+  where
+    Sz (_ :. n) = size mat
+
+----------------------------------------------------------------------------------------------------
+-- Distance Measures
+
+-- | Distance matrix generator functions.
+type DistFn r e = Matrix r e -> Matrix r e
+
+-- | Builds the distance measures in a permutation matrix/distance matrix.
+buildDistMat ::
+  (Mutable r Ix2 e) =>
+  -- | Zip function to combine the elements of vectors \(\mathbf{a}\) \(\mathbf{b}\). Usually @(-)@.
+  -- \( f(\mathbf{a}_i, \mathbf{b}_i) = \mathbf{c} \)
+  (e -> e -> a) ->
+  -- | Fold the vector \(\mathbf{c}\) elementwise to a distance \(d\).
+  (a -> a -> a) ->
+  -- | Accumulator of the fold function.
+  a ->
+  -- | \(m \times n\) matrix, with \(n\) \(m\)-dimensional points (column vectors of the matrix).
+  Matrix r e ->
+  -- | Resulting distance matrix.
+  Matrix D a
+buildDistMat zipFn foldFn acc mat =
+  let a = transposeOuter . expandOuter @Ix3 (Sz n) const $ mat
+      b = transposeInner a
+      ab = Massiv.zipWith zipFn a b
+      d = foldlInner foldFn acc ab
+   in d
+  where
+    Sz (_ :. n) = size mat
+
+-- | The \(L_p\) norm between two vectors. Generalisation of Manhattan and Euclidean distances.
+-- \[
+--   d(\mathbf{a}, \mathbf{b}) = \left( \sum \limits_{i=1}^n \lvert \mathbf{a}_i - \mathbf{b}_i \rvert ^p \right) ^ \frac{1}{p}
+-- \]
+{-# SCC lpNorm #-}
+lpNorm :: (Mutable r Ix2 e, Floating e) => Int -> DistFn r e
+lpNorm p = compute . buildDistMat zipFn foldFn 0
+  where
+    zipFn a b = (^ p) . abs $ a - b
+    foldFn a b = (** (1 / fromIntegral p)) $ a + b
+
+-- | The Manhattan distance between two vectors. Specialisation of the \(L_p\) norm for \(p = 1\).
+-- \[
+--   d(\mathbf{a}, \mathbf{b}) = \sum \limits_{i=1}^n \lvert \mathbf{a}_i - \mathbf{b}_i \rvert
+-- \]
+{-# SCC manhattan #-}
+manhattan :: (Mutable r Ix2 e, Floating e) => DistFn r e
+manhattan = lpNorm 1
+
+-- | The Euclidean distance between two vectors. Specialisation of the \(L_p\) norm for \(p = 2\).
+-- \[
+--   d(\mathbf{a}, \mathbf{b}) = \sqrt{\sum \limits_{i=1}^n (\mathbf{a}_i - \mathbf{b}_i)^2}
+-- \]
+{-# SCC euclidean #-}
+euclidean :: (Mutable r Ix2 e, Floating e) => DistFn r e
+euclidean = lpNorm 2
+
+-- | Mahalanobis distance between points. Suitable for non correlated axes.
+-- \[
+--   d(\mathbf{a}, \mathbf{b}) = \sqrt{(\mathbf{a} - \mathbf{b})^T \mathbf{S}^{-1} (\mathbf{a} - \mathbf{b})}
+-- \]
+-- where \(\mathbf{S}\) is the covariance matrix.
+{-# SCC mahalanobis #-}
+mahalanobis :: (Unbox e, Numeric r e, Mutable r Ix2 e, Mutable r Ix1 e, LA.Field e) => DistFn r e
+mahalanobis points =
+  let a = transposeOuter . expandOuter @Ix3 (Sz n) const $ points
+      b = transposeInner a
+      abDiff = compute @U $ a !-! b
+      ixArray = makeArray @U @Ix2 @Ix2 Par (Sz $ n :. n) id
+      distMat =
+        Massiv.map
+          ( \(x :. y) ->
+              let ab = compute @U $ abDiff !> x !> y
+               in ab ><! covInv !.! ab
+          )
+          ixArray
+   in compute . Massiv.map sqrt $ distMat
+  where
+    Sz (_ :. n) = size points
+    cov = covariance . meanDeviation $ points
+    covInv = matH2M . LA.inv . matM2H $ cov
+
+----------------------------------------------------------------------------------------------------
+-- DBScan
+
+-- | Exception for invalid search distances.
+newtype DistanceInvalidException e = DistanceInvalidException e deriving (Show, Eq)
+
+instance (Typeable e, Show e) => Exception (DistanceInvalidException e)
+
+-- | Representation of clusters.
+type Clusters = Vector B IntSet
+
+-- | DBScan algorithm.
+{-# SCC dbscan #-}
+dbscan ::
+  ( MonadThrow m,
+    Ord e,
+    Num e,
+    Typeable e,
+    Show e,
+    Source r Ix2 e
+  ) =>
+  -- | Distance measure to build the distance matrix of all points.
+  DistFn r e ->
+  -- | Minimal number of members in a cluster.
+  Int ->
+  -- | Search radius \(\epsilon\)
+  e ->
+  -- | \(n\) \(m\)-dimensional data points as column vectors of a \(m \times n\) matrix.
+  Matrix r e ->
+  -- | Resulting clusters.
+  m Clusters
+dbscan distFn nPoints epsilon points
+  | isEmpty points = throwM $ SizeEmptyException (Sz 0 :: Sz1)
+  | nPoints < 1 = throwM $ SizeNegativeException (Sz nPoints)
+  | epsilon <= 0 = throwM $ DistanceInvalidException epsilon
+  | otherwise =
+    let pointNeighbours = ifoldlInner collectNeighbours mempty distMat
+        allClusters = joinOverlapping . compute @B $ pointNeighbours
+        largeClusters = sfilter (\s -> IntSet.size s >= nPoints) allClusters
+     in return $ compute largeClusters
+  where
+    -- The distance matrix in the measure chosen by the distance function.
+    distMat = distFn points
+
+    -- Function to collect the neighbours of a point within the search radius epsilon.
+    {-# SCC collectNeighbours #-}
+    collectNeighbours (_ :. n) acc d = if d <= epsilon then IntSet.insert n acc else acc
+
+    -- Construct the overlap matrix of all clusters.
+    compareSets :: (IntSet -> IntSet -> Bool) -> Vector B IntSet -> Matrix D Bool
+    compareSets fn clVec =
+      let a = expandOuter @Ix2 sz const clVec
+          b = transpose a
+          compareMat = Massiv.zipWith fn a b
+       in compareMat
+      where
+        sz = size clVec
+
+    -- Overlap matrix. Checks if two sets have any overlap. Sets do overlap with themself.
+    overlap :: Vector B IntSet -> Matrix D Bool
+    overlap = compareSets (\a b -> not $ IntSet.disjoint a b)
+
+    -- Check if any set overlaps wiht **any** other set.
+    anyOtherOverlap :: Vector B IntSet -> Bool
+    anyOtherOverlap = Massiv.any (== True) . imap (\(m :. n) v -> if m == n then False else v) . overlap
+
+    -- Check if two sets are identical. Sets are identical to themself.
+    same :: Vector B IntSet -> Matrix D Bool
+    same = compareSets (==)
+
+    -- Join all overlapping clusters recursively.
+    {-# SCC joinOverlapping #-}
+    joinOverlapping :: Vector B IntSet -> Vector B IntSet
+    joinOverlapping clVec =
+      let -- The overlap matrix of the clusters.
+          ovlpMat = compute @U . overlap $ clVec
+          anyOvlp = anyOtherOverlap clVec
+
+          -- Join all sets that have overlap but keep them redundantly (no reduction of the amount
+          -- of clusters).
+          joined =
+            ifoldlInner
+              (\(_ :. n) acc ovlp -> if ovlp then (clVec ! n) <> acc else acc)
+              mempty
+              ovlpMat
+
+          -- Find all sets at different indices that are the same. This is an upper triangular
+          -- matrix with the main diagonal being False.
+          sameMat =
+            compute @U
+              . imap (\(m :. n) v -> if m >= n then False else v)
+              . same
+              . compute @B
+              $ joined
+
+          -- Remove all sets that are redundant. Redundancy is checked by two criteria:
+          --   1. Is this cluster the same set of points as any other cluster? If yes, it is
+          --      redundant.
+          --   2. Is this cluster isolated it is not redundant.
+          nonRed =
+            compute @B
+              . sifilter
+                ( \ix _ ->
+                    let sameAsAnyOther = Massiv.any (== True) $ sameMat !> ix
+                     in not sameAsAnyOther
+                )
+              $ joined
+       in if anyOvlp then joinOverlapping nonRed else clVec
+
+----------------------------------------------------------------------------------------------------
+-- Hierarchical Cluster Analysis
+
+-- | Nodes of a dendrogram.
+data DendroNode e = DendroNode
+  { distance :: e,
+    cluster :: IntSet
+  }
+  deriving (Eq, Show, Generic)
+
+instance (FromJSON e) => FromJSON (DendroNode e)
+
+instance (ToJSON e) => ToJSON (DendroNode e)
+
+-- | A dendrogram as a binary tree.
+newtype Dendrogram e = Dendrogram {unDendro :: BinTree (DendroNode e)}
+  deriving (Show, Eq, Generic)
+
+instance ToJSON e => ToJSON (Dendrogram e)
+
+instance FromJSON e => FromJSON (Dendrogram e)
+
+-- | An accumulator to finally build a dendrogram by a bottom-up algorithm. Not to be exposed in the
+-- API.
+type DendroAcc e = Vector B (Dendrogram e)
+
+-- | Mutable version of the dendrogram accumulator.
+type DendroAccM m e = MArray (PrimState m) B Ix1 (Dendrogram e)
+
+-- | Cut a 'Dendrogram' at a given distance and obtain all clusters from it.
+cutDendroAt :: Ord e => Dendrogram e -> e -> Clusters
+cutDendroAt dendro dist =
+  let nodes = takeLeafyBranchesWhile (\DendroNode {distance} -> distance >= dist) . unDendro $ dendro
+      clusters = compute @B . Massiv.map cluster . compute @B $ nodes
+   in clusters
+
+-- | A strategy/distance measure for clusters.
+data JoinStrat e
+  = SingleLinkage
+  | CompleteLinkage
+  | Median
+  | UPGMA
+  | WPGMA
+  | Centroid
+  | Ward
+  | LWFB e
+  | LW e e e e
+  deriving (Eq, Show)
+
+-- | Lance Williams formula to update distances.
+{-# SCC lanceWilliams #-}
+lanceWilliams ::
+  Fractional e =>
+  -- | How to calculate distance between clusters of points.
+  JoinStrat e ->
+  -- | Number of points in cluster \(A\).
+  Int ->
+  -- | Number of points in cluster \(B\)
+  Int ->
+  -- | Number of points in cluster \(C\)
+  Int ->
+  -- | \(d(A, B)\)
+  e ->
+  -- | \(d(A, C)\)
+  e ->
+  -- | \(d(B, C)\)
+  e ->
+  -- | Updated distance \(D \(A \cup B, C\)
+  e
+lanceWilliams js nA nB nC dAB dAC dBC = alpha1 * dAC + alpha2 * dBC + beta * dAB + gamma * abs (dAC - dBC)
+  where
+    nA' = fromIntegral nA
+    nB' = fromIntegral nB
+    nC' = fromIntegral nC
+    (alpha1, alpha2, beta, gamma) = case js of
+      SingleLinkage -> (1 / 2, 1 / 2, 0, - 1 / 2)
+      CompleteLinkage -> (1 / 2, 1 / 2, 0, 1 / 2)
+      Median -> (1 / 2, 1 / 2, - 1 / 4, 0)
+      UPGMA -> (nA' / (nA' + nB'), nB' / (nA' + nB'), 0, 0)
+      WPGMA -> (1 / 2, 1 / 2, 0, 0)
+      Centroid -> (nA' / (nA' + nB'), nB' / (nA' + nB'), - (nA' * nB') / ((nA' + nB') ^ (2 :: Int)), 0)
+      Ward -> ((nA' + nC') / (nA' + nB' + nC'), (nA' + nC') / (nA' + nB' + nC'), - (nA' + nC') / (nA' + nB' + nC'), 0)
+      LWFB b -> ((1 - b) / 2, (1 - b) / 2, b, 0)
+      LW a b c d -> (a, b, c, d)
+
+----------------------------------------------------------------------------------------------------
+-- Müllner Generic Hierarchical Clustering
+
+-- | A neighbourlist. At index @i@ of the vector it contains a tuple with the minimal distance of
+-- this cluster to any other cluster and the index of the other cluster.
+type Neighbourlist r e = Vector r (e, Ix1)
+
+-- | A distance matrix.
+type DistanceMatrix r e = Matrix r e
+
+-- | Performance improved hierarchical clustering algorithm. @GENERIC_LINKAGE@ from figure 3,
+-- <https://arxiv.org/pdf/1109.2378.pdf>.
+{-# SCC hca #-}
+hca ::
+  ( MonadThrow m,
+    Mutable r Ix1 e,
+    Mutable r Ix2 e,
+    Mutable r Ix1 (e, Ix1),
+    Manifest (R r) Ix1 e,
+    OuterSlice r Ix2 e,
+    Ord e,
+    Unbox e,
+    Fractional e
+  ) =>
+  DistFn r e ->
+  JoinStrat e ->
+  Matrix r e ->
+  m (Dendrogram e)
+hca distFn joinStrat points
+  | Massiv.isEmpty points = throw $ SizeEmptyException (Sz nPoints)
+  | otherwise = do
+    let -- The distance matrix from the points.
+        distMat = distFn points
+
+    -- Initial vector of nearest neighbour to each point.
+    nNghbr <- nearestNeighbours distMat
+
+    let -- Initial priority queue of points. Has the minimum distance of all points.
+        pq = PQ.fromList . Massiv.toList . Massiv.imap (\k (d, _) -> k PQ.:-> d) $ nNghbr
+        -- Set of points not joined yet. Initially all points.
+        s = IntSet.fromDistinctAscList [0 .. nPoints - 1]
+        -- Initial dendrogram accumulator. The vector of all points as their own cluster.
+        dendroAcc =
+          makeArray @B @Ix1
+            Par
+            (Sz nPoints)
+            (\p -> Dendrogram . Leaf $ DendroNode {distance = 0, cluster = IntSet.singleton p})
+
+    distMatM <- return . unsafePerformIO . thaw $ distMat
+    nNghbrM <- return . unsafePerformIO . thaw $ nNghbr
+    dendroAccM <- return . unsafePerformIO . thaw $ dendroAcc
+
+    return . unsafePerformIO $ agglomerate joinStrat distMatM nNghbrM pq s dendroAccM
+  where
+    Sz (_mFeatures :. nPoints) = size points
+
+-- | Agglomerative clustering by the improved generic linkage algorithm. This is the main loop
+-- recursion L 10-43.
+{-# SCC agglomerate #-}
+agglomerate ::
+  ( MonadThrow m,
+    PrimMonad m,
+    MonadUnliftIO m,
+    PrimState m ~ RealWorld,
+    Mutable r Ix2 e,
+    OuterSlice r Ix2 e,
+    Manifest (R r) Ix1 e,
+    Mutable r Ix1 (e, Ix1),
+    Fractional e,
+    Ord e
+  ) =>
+  -- | Join strategy for clusters and therefore how to calculate cluster-cluster distances.
+  JoinStrat e ->
+  -- | Distance matrix.
+  MArray (PrimState m) r Ix2 e ->
+  -- | List of nearest neighbours for each point.
+  MArray (PrimState m) r Ix1 (e, Ix1) ->
+  -- | Priority queue with the distances as priorities and the cluster index as keys.
+  PQ.PSQ Ix1 e ->
+  -- | A set \(S\), that keeps track which clusters have already been joined.
+  IntSet ->
+  -- | Accumulator of the dendrogram. Should collapse to a singleton vector.
+  DendroAccM m e ->
+  -- | The final dendrogram, after all clusters have been joined.
+  m (Dendrogram e)
+agglomerate joinStrat distMat nNghbr pq s dendroAcc
+  | IntSet.null s = throw $ IndexException "No clusters left. This must never happen."
+  | otherwise = do
+    -- Obtain candidates for the two clusters to join and the minimal distance in the priority queue.
+    candidates <- getJoinCandidates nNghbr pq
+
+    -- If the distance between a b is not the minimal distance that the priority queue has found, the
+    -- neighbour list must be wrong and recalculated.
+    (a, b, delta, nNghbrU1, pqU1) <- recalculateNghbr candidates s distMat nNghbr pq
+
+    -- Remove the minimal element from the priority queue and join clusters a and b. The cluster
+    -- accumulator is reduced in its size: a is removed and b is updated with the joined cluster.
+    (newS, pqU2, newAcc) <- joinClusters a b delta s pqU1 dendroAcc
+
+    -- Update the distance matrix in the row and column of b but not at (b,b) and not at (a,b) and
+    -- (b,a).
+    newDistMat <- updateDistMat joinStrat a b newS distMat newAcc
+
+    -- Redirect neighbours to b, if they previously pointed to a.
+    nNghbrU2 <- redirectNeighbours a b newS newDistMat nNghbrU1
+
+    -- Update the neighbourlist and priority queue with the new distances to b.
+    (newNNghbr, newPQ) <- updateBNeighbour b s newDistMat nNghbrU2 pqU2
+
+    -- If the problem has been reduced to a single cluster the algorithm is done and the final
+    -- dendrogram can be obtained from the accumulator at index b. Otherwise join further.
+    if IntSet.size newS == 1
+      then newAcc `readM` b
+      else agglomerate joinStrat newDistMat newNNghbr newPQ newS newAcc
+
+-- | Obtain candidates for the clusters to join by looking at the minimal distance in the priority
+-- queue and the neighbourlist. L 11-13
+{-# SCC getJoinCandidates #-}
+getJoinCandidates ::
+  ( MonadThrow m,
+    PrimMonad m,
+    Mutable r Ix1 (e, Ix1),
+    Ord e
+  ) =>
+  MArray (PrimState m) r Ix1 (e, Ix1) ->
+  PQ.PSQ Ix1 e ->
+  m (Ix1, Ix1, e)
+getJoinCandidates nNghbr pq = do
+  (a PQ.:-> d) <- case PQ.findMin pq of
+    Nothing -> throwM $ IndexException "Empty priority queue"
+    Just v -> return v
+  (_, b) <- nNghbr `readM` a
+  return (a, b, d)
+
+-- | If the minimal distance @d@ found is not the distance between @a@ and @b@ recalculate the
+-- neighbour list, update the priority queue and obtain a new set of a,b and a distance between them.
+-- L 14-20.
+{-# SCC recalculateNghbr #-}
+recalculateNghbr ::
+  ( MonadThrow m,
+    PrimMonad m,
+    MonadUnliftIO m,
+    PrimState m ~ RealWorld,
+    OuterSlice r Ix2 e,
+    Manifest (R r) Ix1 e,
+    Mutable r Ix1 (e, Ix1),
+    Mutable r Ix2 e,
+    Ord e
+  ) =>
+  (Ix1, Ix1, e) ->
+  IntSet ->
+  MArray (PrimState m) r Ix2 e ->
+  MArray (PrimState m) r Ix1 (e, Ix1) ->
+  PQ.PSQ Ix1 e ->
+  m (Ix1, Ix1, e, MArray (PrimState m) r Ix1 (e, Ix1), PQ.PSQ Ix1 e)
+recalculateNghbr (cA, cB, d) s distMat nNghbr pq = do
+  dAB <- distMat `readM` (cA :. cB)
+  if d == dAB
+    then return (cA, cB, d, nNghbr, pq)
+    else do
+      -- Recalculate the nearest neighbours just on index cA. Consider only clusters, that were not
+      -- merged yet.
+      dmRowA <- searchRow cA s distMat >>= unsafeFreeze Par
+      newNeighbourA@(minDistA, _) <- minimumM dmRowA
+      writeM nNghbr cA newNeighbourA
+
+      -- Update the priority queue at key cA with the new distance.
+      let newPQ = PQ.adjust (const minDistA) cA pq
+
+      -- Determine new a, b and d from the updated neighbour list and priority queue.
+      (a PQ.:-> newD) <- case PQ.findMin newPQ of
+        Nothing -> throwM $ IndexException "Empty priority queue"
+        Just v -> return v
+      (_, b) <- nNghbr `readM` a
+      recalculateNghbr (a, b, newD) s distMat nNghbr newPQ
+
+-- | Joins the selected clusters \(A\) and \(B\) and updates the dendrogram accumulator at index b.
+-- A will not be removed so that the accumulator never shrinks.
+-- L 21-24
+{-# SCC joinClusters #-}
+joinClusters ::
+  ( MonadThrow m,
+    PrimMonad m,
+    Ord e
+  ) =>
+  Ix1 ->
+  Ix1 ->
+  e ->
+  IntSet ->
+  PQ.PSQ Ix1 e ->
+  DendroAccM m e ->
+  m (IntSet, PQ.PSQ Ix1 e, DendroAccM m e)
+joinClusters a b d s pq acc = do
+  clA <- acc `readM` a
+  let newPQ = PQ.deleteMin pq
+  modifyM_
+    acc
+    ( \clB ->
+        return
+          . Dendrogram
+          $ Node
+            ( DendroNode
+                { distance = d,
+                  cluster = (cluster . root . unDendro $ clA) <> (cluster . root . unDendro $ clB)
+                }
+            )
+            (unDendro clA)
+            (unDendro clB)
+    )
+    b
+  let newS = IntSet.delete a s
+  return (newS, newPQ, acc)
+
+-- | Update the distance matrix with a Lance-Williams update in the rows and columns of cluster b.
+-- L 25-27
+{-# SCC updateDistMat #-}
+updateDistMat ::
+  ( MonadThrow m,
+    PrimMonad m,
+    MonadUnliftIO m,
+    Mutable r Ix2 e,
+    Fractional e
+  ) =>
+  JoinStrat e ->
+  Ix1 ->
+  Ix1 ->
+  IntSet ->
+  MArray (PrimState m) r Ix2 e ->
+  DendroAccM m e ->
+  m (MArray (PrimState m) r Ix2 e)
+updateDistMat js a b s distMat dendroAcc
+  | nDM /= nDM = throwM $ SizeMismatchException (Sz nDM) (Sz nCl)
+  | mDM /= nDM = throwM $ SizeMismatchException (Sz mDM) (Sz nDM)
+  | otherwise = do
+    dAB <- distMat `readM` (a :. b)
+    nA <- clSize a
+    nB <- clSize b
+    forIO_ ixV $ \ix -> do
+      dAX <- distMat `readM` (a :. ix)
+      nX <- clSize ix
+      modifyM_ distMat (\dBX -> return $ lanceWilliams js nA nB nX dAB dAX dBX) (ix :. b)
+      modifyM_ distMat (\dBX -> return $ lanceWilliams js nA nB nX dAB dAX dBX) (b :. ix)
+    return distMat
+  where
+    Sz (mDM :. nDM) = msize distMat
+    Sz nCl = msize dendroAcc
+    ixV = Massiv.fromList @U Par . IntSet.toAscList . IntSet.delete b $ s
+    clSize i = IntSet.size . cluster . root . unDendro <$> dendroAcc `readM` i
+
+-- | Updates the neighbourlist. All elements with a smaller index than a, that had a as a nearest
+-- neighbour are blindly redirected to the union of a and b, now at index b.
+-- L 28-32
+{-# SCC redirectNeighbours #-}
+redirectNeighbours ::
+  ( MonadThrow m,
+    PrimMonad m,
+    MonadUnliftIO m,
+    Mutable r Ix1 (e, Ix1),
+    Mutable r Ix2 e
+  ) =>
+  Ix1 ->
+  Ix1 ->
+  IntSet ->
+  MArray (PrimState m) r Ix2 e ->
+  MArray (PrimState m) r Ix1 (e, Ix1) ->
+  m (MArray (PrimState m) r Ix1 (e, Ix1))
+redirectNeighbours a b s distMat nNghbr = do
+  forIO_ ixV $ \ix ->
+    modifyM_
+      nNghbr
+      ( \old@(_, nghbrX) ->
+          if nghbrX == a
+            then distMat `readM` (ix :. b) >>= \dXB -> return (dXB, b)
+            else return old
+      )
+      ix
+  return nNghbr
+  where
+    ixV = compute @U . sfilter (< a) . Massiv.fromList @U Par . IntSet.toAscList $ s
+
+-- | Updates the list of nearest neighbours for all combinations that might have changed by
+-- recalculation with the joined cluster AB at index b.
+-- L
+{-# SCC updateWithNewBDists #-}
+updateWithNewBDists ::
+  ( MonadThrow m,
+    MonadUnliftIO m,
+    PrimMonad m,
+    Mutable r Ix2 e,
+    Mutable r Ix1 (e, Ix1),
+    Ord e
+  ) =>
+  Ix1 ->
+  IntSet ->
+  MArray (PrimState m) r Ix2 e ->
+  MArray (PrimState m) r Ix1 (e, Ix1) ->
+  PQ.PSQ Ix1 e ->
+  m (MArray (PrimState m) r Ix1 (e, Ix1), PQ.PSQ Ix1 e)
+updateWithNewBDists b s distMat nNghbr pq = do
+  pqT <- newTVarIO pq
+  forIO_ ixV $ \ix -> do
+    dBX <- distMat `readM` (ix :. b)
+    currentPQ <- readTVarIO pqT
+    minDistX <- case PQ.lookup ix currentPQ of
+      Nothing -> throwM $ IndexException "Empty priority queue."
+      Just v -> return v
+    if dBX < minDistX
+      then do
+        writeM nNghbr ix (dBX, b)
+        atomically . writeTVar pqT . PQ.adjust (const dBX) ix $ currentPQ
+      else atomically . writeTVar pqT $ currentPQ
+
+  newPQ <- readTVarIO pqT
+  return (nNghbr, newPQ)
+  where
+    ixV = compute @U . Massiv.sfilter (< b) . Massiv.fromList @U Par . IntSet.toAscList $ s
+
+--  | Updates the list of nearest neighbours and the priority queue at key b.
+-- L 39-40
+{-# SCC updateBNeighbour #-}
+updateBNeighbour ::
+  ( MonadThrow m,
+    PrimMonad m,
+    MonadUnliftIO m,
+    Mutable r Ix1 (e, Ix1),
+    Mutable r Ix2 e,
+    Ord e
+  ) =>
+  Ix1 ->
+  IntSet ->
+  MArray (PrimState m) r Ix2 e ->
+  MArray (PrimState m) r Ix1 (e, Ix1) ->
+  PQ.PSQ Ix1 e ->
+  m (MArray (PrimState m) r Ix1 (e, Ix1), PQ.PSQ Ix1 e)
+updateBNeighbour b s distMat nNghbr pq =
+  if b >= nNeighbours
+    then return (nNghbr, pq)
+    else do
+      rowAB <- searchRow b s distMat >>= unsafeFreeze Par
+      newNeighbourB@(distB, neighbourB) <- minimumM rowAB
+      writeM nNghbr b newNeighbourB
+      let newPQ = PQ.adjust (const distB) neighbourB pq
+      return (nNghbr, newPQ)
+  where
+    Sz nNeighbours = msize nNghbr
+
+-- | Find the nearest neighbour for each point from a distance matrix. For each point it stores the
+-- minimum distance and the index of the other point, that is the nearest neighbour but at a higher
+-- index.
+{-# SCC nearestNeighbours #-}
+nearestNeighbours ::
+  ( MonadThrow m,
+    Mutable r Ix1 e,
+    Mutable r Ix1 (e, Ix1),
+    OuterSlice r Ix2 e,
+    Source (R r) Ix1 e,
+    Ord e,
+    Unbox e
+  ) =>
+  Matrix r e ->
+  m (Vector r (e, Ix1))
+nearestNeighbours distMat
+  | m /= n = throwM $ IndexException "Distance matrix is not square"
+  | m == 0 = throwM $ IndexException "Distance matrix is empty"
+  | otherwise =
+    let rows = compute @B . outerSlices $ distMat
+        minDistIx =
+          Massiv.imap (\i v -> unsafePerformIO . minDistAtVec i . compute @U $ v) . init $ rows
+     in return . compute $ minDistIx
+  where
+    Sz (m :. n) = size distMat
+
+-- Make a search row for distances. Takes row x from a distance matrix and zips them with their
+-- column index. Then keeps only the valid elements of the row, that are still part of the available
+-- points. A minimum or maximum search can be performed on the resulting vector and a valid pair of
+-- distance and index can be obtained.
+searchRow ::
+  ( PrimMonad m,
+    MonadThrow m,
+    MonadUnliftIO m,
+    Mutable r Ix2 e,
+    Mutable r Ix1 (e, Ix1)
+  ) =>
+  Ix1 ->
+  IntSet ->
+  MArray (PrimState m) r Ix2 e ->
+  m (MArray (PrimState m) r Ix1 (e, Ix1))
+searchRow x s dm =
+  makeMArray Par (size ixV) $ \ix -> do
+    dmIx <- ixV !? ix
+    (dm `readM` (x :. dmIx)) >>= \dist -> return (dist, dmIx)
+  where
+    ixV :: Vector U Ix1
+    ixV = compute @U . sfilter (> x) . Massiv.fromList @U Par . IntSet.toAscList $ s
