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Avoid calling length on chunks in lazy splitAt #676

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Bodigrim merged 2 commits into from
Dec 16, 2025
+12 −5
Merged

Avoid calling length on chunks in lazy splitAt #676

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f3a572f
Avoid calling `length` on chunks in lazy `splitAt`
dolio Dec 15, 2025
2091e7e
Check for maxBound in lazy splitAt in its own case
dolio Dec 15, 2025
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17 changes: 12 additions & 5 deletions src/Data/Text/Lazy.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -1306,12 +1306,19 @@ splitAt = loop
loop :: Int64 -> Text -> (Text, Text)
loop !_ Empty = (empty, empty)
loop n t | n <= 0 = (empty, t)
loop n (Chunk t ts)
| n < len = let (t',t'') = T.splitAt (int64ToInt n) t
in (Chunk t' Empty, Chunk t'' ts)
| otherwise = let (ts',ts'') = loop (n - len) ts
loop n (Chunk t@(T.Text arr off len) ts)
| n > mx = let (ts', ts'') = loop (n - intToInt64 (T.length t)) ts
in (Chunk t ts', ts'')
| m > 0, m >= len = (Chunk t Empty, ts)
| m > 0 = let t' = T.Text arr off m
t'' = T.Text arr (off+m) (len-m)
in (Chunk t' Empty, Chunk t'' ts)
| otherwise = let (ts', ts'') = loop (n + intToInt64 m) ts
in (Chunk t ts', ts'')
where len = intToInt64 (T.length t)
where
mx = intToInt64 P.maxBound
m = T.measureOff (int64ToInt n) t


-- | /O(n)/ 'splitAtWord' @n t@ returns a strict pair whose first
-- element is a prefix of @t@ whose chunks contain @n@ 'Word8'
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