[project @ 2004-03-19 11:00:02 by simonmar]

- fix one performance bug: we weren't updating the bucket count when
  expanding the hash table, so too many expansions were happening.

- slight improvement to hashString: if we use foldl rather than foldr,
  the resulting code uses an accumulating parameter and runs in linear
  stack space.

diff --git a/Data/HashTable.hs b/Data/HashTable.hs
index cbc3f5c..37ddb5e 100644 (file)
--- a/Data/HashTable.hs
+++ b/Data/HashTable.hs
@@ -41,7 +41,7 @@ import Prelude        hiding  ( lookup )
 import Data.Tuple      ( fst )
 import Data.Bits
 import Data.Maybe
-import Data.List       ( maximumBy, filter, length, concat )
+import Data.List       ( maximumBy, filter, length, concat, foldl )
 import Data.Int                ( Int32 )
 
 #if defined(__GLASGOW_HASKELL__)
@@ -153,8 +153,8 @@ hashInt = (`rem` prime) . fromIntegral
 -- which seems to give reasonable results.
 --
 hashString :: String -> Int32
-hashString = fromIntegral . foldr f 0
-  where f c m = ord c + (m * 128) `rem` fromIntegral prime
+hashString = fromIntegral . foldl f 0
+  where f m c = ord c + (m * 128) `rem` fromIntegral prime
 
 -- | A prime larger than the maximum hash table size
 prime :: Int32
@@ -254,6 +254,7 @@ expandHashTable
       table@HT{ dir=dir,
                split=split,
                max_bucket=max,
+               bcount=bcount,
                mask2=mask2 } = do
   let
       oldsegment = split `shiftR` sEGMENT_SHIFT
@@ -269,10 +270,12 @@ expandHashTable
   --
   let table' =
        if (split+1) < max
-           then table{ split = split+1 }
+           then table{ split = split+1,
+                       bcount = bcount+1 }
                -- we've expanded all the buckets in this table, so start from
                -- the beginning again.
            else table{ split = 0,
+                       bcount = bcount+1,
                        max_bucket = max * 2,
                        mask1 = mask2,
                        mask2 = mask2 `shiftL` 1 .|. 1 }