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equal hash code: avoid dropping useful bits

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Closes PR 14059

Symbols in the PR were mapped to coliding hashes in
groups of 4 because the low 2 bits of the `eq?` hash
code were begin dropped to generate an `equal?` hash
code. Those two bits got lost due to a refectoring
a while back that moved the dropping of two useless
bits to a more centralized place, but the 2-bit shift
did not get removed from the `equal` hash code comparision.

The PR's example program will still generate groups of 2
when hashing around 10k symbols (which used to be groups of 8).
That's because there's a bit in the hash-code counter that
turns out to be forced to 1.
This commit is contained in:
Matthew Flatt 2013-10-12 08:15:22 -06:00
parent 8ff0193ebe
commit 8b7b96215b
1 changed files with 3 additions and 3 deletions
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6
racket/src/racket/src/hash.c
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@ -1371,7 +1371,7 @@ static uintptr_t equal_hash_key(Scheme_Object *o, uintptr_t k, Hash_Info *hi)
return k;
} else
return k + (PTR_TO_LONG(o) >> 2);
return k + PTR_TO_LONG(o);
}
}
case scheme_box_type:
@ -1496,7 +1496,7 @@ static uintptr_t equal_hash_key(Scheme_Object *o, uintptr_t k, Hash_Info *hi)
return k + (MZ_OPT_HASH_KEY(&s->iso) & 0xFFFC);
} else
return k + (PTR_TO_LONG(o) >> 2);
return k + PTR_TO_LONG(o);
}
# endif
case scheme_resolved_module_path_type:
@ -1532,7 +1532,7 @@ static uintptr_t equal_hash_key(Scheme_Object *o, uintptr_t k, Hash_Info *hi)
if (h1)
return h1(o, k, hi);
else
return k + (PTR_TO_LONG(o) >> 2);
return k + PTR_TO_LONG(o);
}
}