51 lines
1.6 KiB
Rust
51 lines
1.6 KiB
Rust
// Copyright 2019 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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//! Support for lookups based on minimal perfect hashing.
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// This function is based on multiplication being fast and is "good enough". Also
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// it can share some work between the unsalted and salted versions.
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#[inline]
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fn my_hash(key: u32, salt: u32, n: usize) -> usize {
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let y = key.wrapping_add(salt).wrapping_mul(2654435769);
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let y = y ^ key.wrapping_mul(0x31415926);
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(((y as u64) * (n as u64)) >> 32) as usize
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}
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/// Do a lookup using minimal perfect hashing.
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///
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/// The table is stored as a sequence of "salt" values, then a sequence of
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/// values that contain packed key/value pairs. The strategy is to hash twice.
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/// The first hash retrieves a salt value that makes the second hash unique.
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/// The hash function doesn't have to be very good, just good enough that the
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/// resulting map is unique.
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#[inline]
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pub(crate) fn mph_lookup<KV, V, FK, FV>(
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x: u32,
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salt: &[u16],
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kv: &[KV],
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fk: FK,
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fv: FV,
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default: V,
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) -> V
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where
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KV: Copy,
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FK: Fn(KV) -> u32,
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FV: Fn(KV) -> V,
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{
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let s = salt[my_hash(x, 0, salt.len())] as u32;
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let key_val = kv[my_hash(x, s, salt.len())];
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if x == fk(key_val) {
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fv(key_val)
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} else {
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default
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}
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}
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