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1 files changed, 43 insertions, 0 deletions

diff --git a/src/ImmutGrowVector.hs b/src/ImmutGrowVector.hs
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+module ImmutGrowVector where
+
+import Foreign.Storable
+import Data.Vector.Storable qualified as VS
+
+
+-- | Acts like an immutable storable vector, except that it's split in a long
+-- prefix and a short suffix so that modifications at the end are cheap. If the
+-- suffix gets longer (by appending elements), the suffix elements are promoted
+-- to prefix elements once in a while, resulting in a big copy at those times.
+data ImmutGrowVector a = ImmutGrowVector (VS.Vector a) (VS.Vector a)
+  deriving (Show)
+
+empty :: Storable a => ImmutGrowVector a
+empty = ImmutGrowVector VS.empty VS.empty
+
+fromListN :: Storable a => Int -> [a] -> ImmutGrowVector a
+fromListN n l
+  | n > 2 =
+      let (l1, l2) = splitAt (n - 2) l
+      in ImmutGrowVector (VS.fromListN (n - 2) l1) (VS.fromListN 2 l2)
+  | otherwise =
+      ImmutGrowVector VS.empty (VS.fromListN n l)
+
+(!) :: Storable a => ImmutGrowVector a -> Int -> a
+ImmutGrowVector prefix suffix ! i
+  | i < VS.length prefix = prefix VS.! i
+  | otherwise = suffix VS.! (i - VS.length prefix)
+
+length :: Storable a => ImmutGrowVector a -> Int
+length (ImmutGrowVector prefix suffix) = VS.length prefix + VS.length suffix
+
+set :: Storable a => ImmutGrowVector a -> Int -> a -> ImmutGrowVector a
+set (ImmutGrowVector prefix suffix) idx value
+  | idx < VS.length prefix = error "ImmutGrowVector: mutation in slow part"
+  | otherwise = ImmutGrowVector prefix (suffix VS.// [(idx - VS.length prefix, value)])
+
+append :: Storable a => ImmutGrowVector a -> a -> ImmutGrowVector a
+append (ImmutGrowVector prefix suffix) value
+  | VS.length suffix < 8 = ImmutGrowVector prefix (suffix `VS.snoc` value)
+  | otherwise =
+      let n = VS.length suffix
+      in ImmutGrowVector (prefix <> VS.take (n - 1) suffix) (VS.drop (n - 1) suffix `VS.snoc` value)