Implement measure_off in pure Haskell

src/Data/Text.hs

@@ -232,6 +232,7 @@ import Data.Monoid (Monoid(..))
 import Data.Semigroup (Semigroup(..))
 import Data.String (IsString(..))
 import Data.Text.Internal.Reverse (reverse)
+import Data.Text.Internal.Measure (measure_off)
 import Data.Text.Internal.Encoding.Utf8 (utf8Length, utf8LengthByLeader, chr2, chr3, chr4, ord2, ord3, ord4)
 import qualified Data.Text.Internal.Fusion as S
 import Data.Text.Internal.Fusion.CaseMapping (foldMapping, lowerMapping, upperMapping)
@@ -1387,12 +1388,7 @@ take n t@(Text arr off len)
 measureOff :: Int -> Text -> Int
 measureOff !n (Text (A.ByteArray arr) off len) = if len == 0 then 0 else
   cSsizeToInt $
-    c_measure_off arr (intToCSize off) (intToCSize len) (intToCSize n)
-
--- | The input buffer (arr :: ByteArray#, off :: CSize, len :: CSize)
--- must specify a valid UTF-8 sequence, this condition is not checked.
-foreign import ccall unsafe "_hs_text_measure_off" c_measure_off
-    :: ByteArray# -> CSize -> CSize -> CSize -> CSsize
+    measure_off arr (intToCSize off) (intToCSize len) (intToCSize n)
 
 -- | /O(n)/ 'takeEnd' @n@ @t@ returns the suffix remaining after
 -- taking @n@ characters from the end of @t@.

src/Data/Text/Internal/Measure.hs

@@ -0,0 +1,53 @@
+{-# LANGUAGE CPP #-}
+{-# LANGUAGE MagicHash #-}
+
+#if defined(PURE_HASKELL)
+{-# LANGUAGE BangPatterns #-}
+{-# LANGUAGE MultiWayIf #-}
+#endif
+
+#if !defined(PURE_HASKELL)
+{-# LANGUAGE UnliftedFFITypes #-}
+#endif
+
+{-# OPTIONS_HADDOCK not-home #-}
+
+-- | Implements 'measure_off', using efficient C routines by default.
+module Data.Text.Internal.Measure
+  ( measure_off
+  )
+where
+
+import GHC.Exts
+
+#if defined(PURE_HASKELL)
+import GHC.Word
+import Data.Text.Internal.Encoding.Utf8 (utf8LengthByLeader)
+#endif
+
+import Foreign.C.Types (CSize(..))
+import System.Posix.Types (CSsize(..))
+
+#if defined(PURE_HASKELL)
+
+measure_off :: ByteArray# -> CSize -> CSize -> CSize -> CSsize
+measure_off ba off len cnt = go 0 0
+  where
+    go !cc !i
+      -- return the number of bytes for the first cnt codepoints,
+      | cc == cnt = fromIntegral i
+      -- return negated number of codepoints if there are fewer than cnt
+      | i >= len  = negate (fromIntegral cc)
+      | otherwise =
+          let !(I# o) = fromIntegral (off+i)
+              !b = indexWord8Array# ba o
+          in go (cc+1) (i + fromIntegral (utf8LengthByLeader (W8# b)))
+
+#else
+
+-- | The input buffer (arr :: ByteArray#, off :: CSize, len :: CSize)
+-- must specify a valid UTF-8 sequence, this condition is not checked.
+foreign import ccall unsafe "_hs_text_measure_off" measure_off
+    :: ByteArray# -> CSize -> CSize -> CSize -> CSsize
+
+#endif

text.cabal

@@ -97,7 +97,6 @@ library
   if arch(javascript) || flag(pure-haskell)
     cpp-options: -DPURE_HASKELL
     c-sources:  cbits/is_ascii.c
-                cbits/measure_off.c
                 cbits/utils.c
   else
     c-sources:  cbits/is_ascii.c
@@ -207,6 +206,7 @@ library
   other-modules:
     Data.Text.Show
     Data.Text.Internal.Reverse
+    Data.Text.Internal.Measure
 
   build-depends:
     array            >= 0.3 && < 0.6,