From: Ian Lynagh <igloo@earth.li>
Date: Tue, 21 Aug 2007 14:07:06 +0000 (+0000)
Subject: Fix hashInt
X-Git-Url: http://git.megacz.com/?a=commitdiff_plain;h=0c685143cbdfcfc3964cf945f8df1b2f3cb0cef8;p=ghc-base.git

Fix hashInt
As pointed out in
http://www.haskell.org/pipermail/glasgow-haskell-bugs/2007-August/009545.html
the old behaviour was
Prelude Data.HashTable> map hashInt [0..10]
[0,-1,-1,-2,-2,-2,-3,-3,-4,-4,-4]

Fixed according to the "Fibonacci Hashing" algorithm described in
http://www.brpreiss.com/books/opus4/html/page213.html
http://www.brpreiss.com/books/opus4/html/page214.html
---

diff --git a/Data/HashTable.hs b/Data/HashTable.hs
index 0cee737..391876f 100644
--- a/Data/HashTable.hs
+++ b/Data/HashTable.hs
@@ -189,13 +189,13 @@ golden :: Int32
 golden = -1640531527
 
 -- | A sample (and useful) hash function for Int and Int32,
--- implemented by extracting the uppermost 32 bits of the 64-bit
+-- implemented by extracting the lowermost 32 bits of the
 -- result of multiplying by a 32-bit constant.  The constant is from
 -- Knuth, derived from the golden ratio:
 --
 -- > golden = round ((sqrt 5 - 1) * 2^31) :: Int
 hashInt :: Int -> Int32
-hashInt x = mulHi (fromIntegral x) golden
+hashInt x = fromIntegral x * golden
 
 -- hi 32 bits of a x-bit * 32 bit -> 64-bit multiply
 mulHi :: Int32 -> Int32 -> Int32
@@ -208,7 +208,7 @@ mulHi a b = fromIntegral (r `shiftR` 32)
 --
 -- > hashString = foldl' f 0
 -- >   where f m c = fromIntegral (fromEnum c + 1) * golden + mulHi m golden
--- 
+--
 -- Note that this has not been extensively tested for reasonability,
 -- but Knuth argues that repeated multiplication by the golden ratio
 -- will minimize gaps in the hash space.