diff --git a/LICENSE b/LICENSE
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--- /dev/null
+++ b/LICENSE
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+Copyright Matthieu Monsch (c) 2018
+
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+
+    * Neither the name of Matthieu Monsch nor the names of other
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/README.md b/README.md
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+++ b/README.md
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+# yx
diff --git a/Setup.hs b/Setup.hs
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--- /dev/null
+++ b/Setup.hs
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+import Distribution.Simple
+main = defaultMain
diff --git a/src/Data/Geometry/YX.hs b/src/Data/Geometry/YX.hs
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+++ b/src/Data/Geometry/YX.hs
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+{-# LANGUAGE OverloadedStrings #-}
+
+-- | A bitmap-friendly XY coordinate.
+--
+-- YX rather than XY since layout is row major (first row sorts before the
+-- second, etc.).
+module Data.Geometry.YX ( YX(..)
+                        , rows
+                        , up, left, right, down
+                        , steps4, steps8
+                        , byteStringToArray, arrayToByteString ) where
+
+import Data.Array.IArray (IArray)
+import qualified Data.Array.IArray as Array
+import Data.ByteString (ByteString)
+import qualified Data.ByteString.Char8 as BS
+import Data.Ix (Ix)
+import qualified Data.Ix as Ix
+import Data.List (groupBy)
+
+-- | A 2D coordinate.
+--
+-- YX implements 'Num'. Integers are converted to their diagonal equivalent
+-- (for example @2@ becomes @YX 2 2@).
+data YX = YX { x :: !Int, y :: !Int } deriving (Eq, Ord, Show)
+
+lift1 :: (Int -> Int) -> YX -> YX
+lift1 f (YX y1 x1) = YX (f y1) (f x1)
+
+lift2 :: (Int -> Int -> Int) -> YX -> YX -> YX
+lift2 f (YX y1 x1) (YX y2 x2) = YX (f y1 y2) (f x1 x2)
+
+instance Num YX where
+  (+) = lift2 (+)
+  (*) = lift2 (*)
+  abs = lift1 abs
+  signum = lift1 signum
+  fromInteger i = let i' = fromInteger i in YX i' i'
+  negate = lift1 negate
+
+instance Ix YX where
+  range (YX yl xl, YX yu xu) =
+    [ YX y x | y <- Ix.range (yl, yu), x <- Ix.range (xl, xu) ]
+  index (YX yl xl, YX yu xu) (YX y x) =
+    Ix.index (yl, yu) y * Ix.rangeSize (xl, xu) + Ix.index (xl, xu) x
+  inRange (YX yl xl, YX yu xu) (YX y x) =
+    Ix.inRange (yl, yu) y && Ix.inRange (xl, xu) x
+
+-- | All coordinates, grouped by row.
+rows :: (YX, YX) -> [[YX]]
+rows = groupBy (\(YX y1 _) (YX y2 _) -> y1 == y2) . Ix.range
+
+-- | Basic steps.
+up, left, right, down :: YX
+up = YX (-1) 0
+left = YX 0 (-1)
+right = YX 0 1
+down = YX 1 0
+
+-- | Ordered steps arrays.
+steps4, steps8 :: [YX]
+steps4 = [up, left, right, down]
+steps8 = [up + left, up, up + right, left, right, down + left, down, down + right]
+
+-- Parse newline delimited bytestring into an array.
+byteStringToArray :: (IArray a e) => (Char -> Maybe e) -> ByteString -> Either String (a YX e)
+byteStringToArray f bs = shape (BS.split '\n' bs) (-1) >>= materialize bs where
+  shape [] (YX y x) = Right (YX y (max x 0))
+  shape (row : rows) yx@(YX y x0)
+    | null rows && BS.null row = shape [] yx -- Empty last row.
+    | otherwise = let x = BS.length row - 1
+                  in if x /= x0 && x0 >= 0
+                    then Left $ "bad row lengths: " ++ show x ++ ", " ++ show x0
+                    else shape rows (YX (y + 1) x)
+  materialize bs yx = Array.listArray (0, yx) <$> elems bs
+  elems = sequenceA . fmap parse . filter (/= '\n') . BS.unpack
+  parse c = case f c of
+    Just e -> Right e
+    Nothing -> Left $ "unknown char: " ++ show c
+
+-- | Reverse of `byteStringToArray`
+arrayToByteString :: (IArray a e) => (e -> Char) -> a YX e -> ByteString
+arrayToByteString f arr = BS.intercalate "\n" lines where
+  lines = fmap (BS.pack . fmap (f . (arr Array.!))) . rows . Array.bounds $ arr
diff --git a/test/Spec.hs b/test/Spec.hs
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+++ b/test/Spec.hs
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+{-# LANGUAGE OverloadedStrings #-}
+
+import qualified Data.Array.IArray as Array
+import Data.Char (digitToInt)
+import Data.Either (isLeft)
+import Data.Geometry.YX (YX)
+import qualified Data.Geometry.YX as YX
+import Test.Hspec
+import Test.Hspec.QuickCheck
+
+main :: IO ()
+main = hspec $ do
+  describe "bytestring conversions" $ do
+    it "should parse a simple square case" $ do
+      let
+        got = YX.byteStringToArray (Just . digitToInt) "12\n34\n"
+        want = Array.listArray (0, 1) [1..4] :: Array.Array YX Int
+      got `shouldBe` Right want
+    it "fails on uneven row" $ do
+      let got = YX.byteStringToArray Just "12\n3\n" :: Either String (Array.Array YX Char)
+      isLeft got `shouldBe` True
+    it "round trips" $ do
+      let
+        bs = "12\n34\n56"
+        Right arr = YX.byteStringToArray Just bs :: Either String (Array.Array YX Char)
+        bs' = YX.arrayToByteString id arr
+      bs' `shouldBe` bs
diff --git a/yx.cabal b/yx.cabal
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--- /dev/null
+++ b/yx.cabal
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+name:                yx
+version:             0.0.1.0
+synopsis:            Row-major coordinates
+description:         https://github.com/mtth/yx
+homepage:            https://github.com/mtth/yx
+license:             BSD3
+license-file:        LICENSE
+author:              Matthieu Monsch
+maintainer:          matthieu.monsch@gmail.com
+copyright:           2018 Matthieu Monsch
+category:            Web
+build-type:          Simple
+extra-source-files:  README.md
+cabal-version:       >=1.10
+
+library
+  hs-source-dirs:      src
+  exposed-modules:     Data.Geometry.YX
+  build-depends:       base >= 4.7 && < 5
+                     , array >= 0.5
+                     , bytestring >= 0.10
+  default-language:    Haskell2010
+
+test-suite yx-test
+  type: exitcode-stdio-1.0
+  main-is: Spec.hs
+  hs-source-dirs:
+      test
+  ghc-options: -threaded -rtsopts -with-rtsopts=-N
+  build-depends:
+      base >=4.7 && <5
+    , array >= 0.5
+    , bytestring >= 0.10
+    , hspec >=2.5 && <2.6
+    , yx
+  default-language: Haskell2010
+
+source-repository head
+  type:     git
+  location: https://github.com/mtth/yx