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Skip likely-zero initial probe, speed up map.rs.

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This commit is contained in:
Graydon Hoare 2011-04-25 03:58:53 +00:00
parent 0cffc58225
commit 5fe21b5af0
1 changed files with 5 additions and 5 deletions
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10
src/lib/map.rs
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@ -36,26 +36,26 @@ fn mk_hashmap[K, V](&hashfn[K] hasher, &eqfn[K] eqer) -> hashmap[K, V] {
// half and lower half of the uint bits. Our bucket probing
// sequence is then defined by
//
// hash(key, i) := hashl(key) + i * hashr(key) for i = 0, 1, 2, ...
// hash(key, i) := hashl(key) * i + hashr(key) for i = 0, 1, 2, ...
//
// Tearing the hash function apart this way is kosher in practice
// as, assuming 32-bit uints, the table would have to be at 2^32
// buckets before the resulting pair of hash functions no longer
// probes all buckets for a fixed key. Note that hashr is made to
// probes all buckets for a fixed key. Note that hashl is made to
// output odd numbers (hence coprime to the number of nbkts, which
// is always a power of 2), so that all buckets are probed for a
// fixed key.
fn hashl(uint n, uint nbkts) -> uint {
ret (n >>> 16u);
ret ((n >>> 16u) * 2u + 1u);
}
fn hashr(uint n, uint nbkts) -> uint {
ret ((((~ 0u) >>> 16u) & n) * 2u + 1u);
ret (0x0000_ffff_u & n);
}
fn hash(uint h, uint nbkts, uint i) -> uint {
ret (hashl(h, nbkts) + i * hashr(h, nbkts)) % nbkts;
ret (hashl(h, nbkts) * i + hashr(h, nbkts)) % nbkts;
}
/**