From 28cca28e623c1ac5a2a3e7dcdd31b3a90552c9eb Mon Sep 17 00:00:00 2001 From: Andrea Canciani Date: Sat, 3 Jan 2015 10:44:54 +0100 Subject: [PATCH] Improve `make_hash` function The `make_hash` function is used to prevent hashes of non-empty buckets to collide with `EMPTY_HASH = 0u64`. Ideally this function also preserve the uniform distribution of hashes and is cheap to compute. The new implementation reduces the input hash size by one bit, simply by setting the most significant bit. This obviously prevent output hashes to collide with `EMPTY_HASH` and guarantees that the uniform distribution is preserved. Moreover, the new function is simpler (no comparisons, just an OR) and (under the same assumptions as the old function, i.e. only the least significant bit will contribute to the bucket index) no additional collisions are caused. --- src/libstd/collections/hash/table.rs | 14 ++++++-------- 1 file changed, 6 insertions(+), 8 deletions(-) diff --git a/src/libstd/collections/hash/table.rs b/src/libstd/collections/hash/table.rs index a687ba3da8d7f..8529866502183 100644 --- a/src/libstd/collections/hash/table.rs +++ b/src/libstd/collections/hash/table.rs @@ -1,4 +1,4 @@ -// Copyright 2014 The Rust Project Developers. See the COPYRIGHT +// Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // @@ -139,13 +139,11 @@ impl SafeHash { /// This function wraps up `hash_keyed` to be the only way outside this /// module to generate a SafeHash. pub fn make_hash, S, H: Hasher>(hasher: &H, t: &T) -> SafeHash { - match hasher.hash(t) { - // This constant is exceedingly likely to hash to the same - // bucket, but it won't be counted as empty! Just so we can maintain - // our precious uniform distribution of initial indexes. - EMPTY_BUCKET => SafeHash { hash: 0x8000_0000_0000_0000 }, - h => SafeHash { hash: h }, - } + // We need to avoid 0u64 in order to prevent collisions with + // EMPTY_HASH. We can maintain our precious uniform distribution + // of initial indexes by unconditionally setting the MSB, + // effectively reducing 64-bits hashes to 63 bits. + SafeHash { hash: 0x8000_0000_0000_0000 | hasher.hash(t) } } // `replace` casts a `*u64` to a `*SafeHash`. Since we statically