diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,3 @@
+# 0
+
+* Split off from `codex`
diff --git a/LICENSE.md b/LICENSE.md
new file mode 100644
--- /dev/null
+++ b/LICENSE.md
@@ -0,0 +1,230 @@
+# License
+
+Licensed under either of
+ * Apache License, Version 2.0 (http://www.apache.org/licenses/LICENSE-2.0)
+ * BSD 2-Clause license (https://opensource.org/licenses/BSD-2-Clause)
+at your option.
+
+## BSD 2-Clause License
+
+- Copyright 2019 Edward Kmett and Sean Chalmers
+
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions
+are met:
+
+1. Redistributions of source code must retain the above copyright
+ notice, this list of conditions and the following disclaimer.
+
+2. Redistributions in binary form must reproduce the above copyright
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+ documentation and/or other materials provided with the distribution.
+
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+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.
+
+## Apache License
+
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diff --git a/README.md b/README.md
new file mode 100644
--- /dev/null
+++ b/README.md
@@ -0,0 +1,16 @@
+ptrdiff
+=======
+
+[](https://hackage.haskell.org/package/ptrdiff)
+
+Type "safe" pointer differences. Mostly makes it easier to get things right when working with `plusPtr`.
+
+Contact Information
+-------------------
+
+Contributions and bug reports are welcome!
+
+Please feel free to contact me through github or on the #haskell IRC channel on irc.freenode.net.
+
+-Edward Kmett
+
diff --git a/ptrdiff.cabal b/ptrdiff.cabal
new file mode 100644
--- /dev/null
+++ b/ptrdiff.cabal
@@ -0,0 +1,32 @@
+cabal-version: 2.2
+name: ptrdiff
+version: 0
+synopsis: Type-checked pointer diffs
+homepage: https://github.com/ekmett/codex/tree/master/ptrdiff#readme
+license: BSD-2-Clause OR Apache-2.0
+license-file: LICENSE.md
+author: Edward Kmett
+maintainer: Edward Kmett
+copyright: Copyright (c) 2019 Edward Kmett
+stability: experimental
+category: FFI
+build-type: Simple
+description: Type-checked pointer diffs.
+extra-doc-files:
+ README.md,
+ CHANGELOG.md
+
+source-repository head
+ type: git
+ location: https://github.com/ekmett/codex
+ subdir: ptrdiff
+
+library
+ hs-source-dirs: src
+ default-language: Haskell2010
+ ghc-options:
+ -Wall -Wincomplete-uni-patterns -Wincomplete-record-updates
+ -Wredundant-constraints -Widentities -Wmissing-export-lists
+ build-depends: base >= 4.11 && < 5
+ exposed-modules:
+ Foreign.Ptr.Diff
diff --git a/src/Foreign/Ptr/Diff.hs b/src/Foreign/Ptr/Diff.hs
new file mode 100644
--- /dev/null
+++ b/src/Foreign/Ptr/Diff.hs
@@ -0,0 +1,105 @@
+{-# language ScopedTypeVariables #-}
+{-# language TypeApplications #-}
+{-# language DeriveGeneric #-}
+{-# language DeriveDataTypeable #-}
+-- |
+-- Copyright : (c) 2019-2021 Edward Kmett
+-- License : BSD-2-Clause OR Apache-2.0
+-- Maintainer: Edward Kmett
+-- Stability : experimental
+-- Portability: non-portable
+--
+module Foreign.Ptr.Diff
+( Diff(..)
+, inv
+, next, prev
+, (.*)
+, advance
+, DiffTorsor(..)
+, peekDiffOff
+, pokeDiffOff
+) where
+
+import Control.Category
+import Control.Monad.IO.Class
+import Data.Coerce
+import Data.Data (Data)
+import Foreign.ForeignPtr
+import Foreign.ForeignPtr.Unsafe
+import Foreign.Ptr
+import Foreign.Storable
+import GHC.Generics
+import Prelude hiding (id,(.))
+
+-- | So long as 'minusPtr' returns an 'Int', we're stuck with size of 'Int' = size of 'ptrdiff_t'
+newtype Diff a b = Diff { getDiff :: Int }
+ deriving (Eq,Ord,Show,Read,Data,Generic)
+
+instance Category Diff where
+ id = Diff 0
+ {-# inline id #-}
+ Diff a . Diff b = Diff (a + b)
+ {-# inline (.) #-}
+
+-- | 'Diff' is a groupoid, so @Diff a a@ is a group, and as such it forms a module over the integers
+(.*) :: Int -> Diff a a -> Diff a a
+n .* Diff a = Diff (n*a)
+{-# inline (.*) #-}
+
+infixr 6 .*
+
+-- | Invert a pointer 'Diff', e.g. convert a pointer to a field member to one that computes the location
+-- of the parent, or one that computes the offset of the next item into one that computes the offset of
+-- the previous element.
+inv :: Diff a b -> Diff b a
+inv (Diff a) = Diff (negate a)
+{-# inline inv #-}
+
+-- | Calculate the position of the next elment of a given type when they are packed in densely.
+next :: forall a. Storable a => Diff a a
+next = Diff (sizeOf @a undefined)
+{-# inline next #-}
+
+-- | Calculate the position of the previous element of a given type when they are packed in densely.
+prev :: Storable a => Diff a a
+prev = inv next
+{-# inline prev #-}
+
+-- | Advance @n@ (possibly negative) items in a dense packing.
+advance :: Storable a => Int -> Diff a a
+advance n = n .* next
+{-# inline advance #-}
+
+class DiffTorsor t where
+ act :: Diff a b -> t a -> t b
+ diff :: t b -> t a -> Diff a b
+
+-- 'Ptr' is a torsor w/ structure category 'Diff'
+instance DiffTorsor Ptr where
+ act = coerce (flip plusPtr)
+ diff = coerce minusPtr
+ {-# inline act #-}
+ {-# inline diff #-}
+
+instance DiffTorsor FunPtr where
+ act d p = castPtrToFunPtr $ act d (castFunPtrToPtr p)
+ diff p q = diff (castFunPtrToPtr p) (castFunPtrToPtr q)
+ {-# inline act #-}
+ {-# inline diff #-}
+
+-- | due to finalizers this doesn't _quite_ satisfy ForeignPtr a * Diff a b <-> ForeignPtr a * ForeignPtr b
+instance DiffTorsor ForeignPtr where
+ act = coerce (flip plusForeignPtr)
+ diff p q = diff (unsafeForeignPtrToPtr p) (unsafeForeignPtrToPtr q)
+ {-# inline act #-}
+ {-# inline diff #-}
+
+-- | 'peek' using a 'Diff' between pointers to calculate the offset.
+peekDiffOff :: (MonadIO m, Storable b) => Ptr a -> Diff a b -> m b
+peekDiffOff p (Diff d) = liftIO $ peekByteOff p d
+{-# inline peekDiffOff #-}
+
+-- | 'poke' using a 'Diff' between pointers to calculate the offset.
+pokeDiffOff :: (MonadIO m, Storable b) => Ptr a -> Diff a b -> b -> m ()
+pokeDiffOff p (Diff d) a = liftIO $ pokeByteOff p d a
+{-# inline pokeDiffOff #-}