1// Copyright 2019 The Rust Project Developers. See the COPYRIGHT
2// file at the top-level directory of this distribution and at
3// http://rust-lang.org/COPYRIGHT.
4//
5// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8// option. This file may not be copied, modified, or distributed
9// except according to those terms.
10
11//! Support for lookups based on minimal perfect hashing.
12
13// This function is based on multiplication being fast and is "good enough". Also
14// it can share some work between the unsalted and salted versions.
15#[inline]
16fn my_hash(key: u32, salt: u32, n: usize) -> usize {
17 let y: u32 = key.wrapping_add(salt).wrapping_mul(2654435769);
18 let y: u32 = y ^ key.wrapping_mul(0x31415926);
19 (((y as u64) * (n as u64)) >> 32) as usize
20}
21
22/// Do a lookup using minimal perfect hashing.
23///
24/// The table is stored as a sequence of "salt" values, then a sequence of
25/// values that contain packed key/value pairs. The strategy is to hash twice.
26/// The first hash retrieves a salt value that makes the second hash unique.
27/// The hash function doesn't have to be very good, just good enough that the
28/// resulting map is unique.
29#[inline]
30pub(crate) fn mph_lookup<KV, V, FK, FV>(
31 x: u32,
32 salt: &[u16],
33 kv: &[KV],
34 fk: FK,
35 fv: FV,
36 default: V,
37) -> V
38where
39 KV: Copy,
40 FK: Fn(KV) -> u32,
41 FV: Fn(KV) -> V,
42{
43 let s: u32 = salt[my_hash(key:x, salt:0, n:salt.len())] as u32;
44 let key_val: KV = kv[my_hash(key:x, salt:s, n:salt.len())];
45 if x == fk(key_val) {
46 fv(key_val)
47 } else {
48 default
49 }
50}
51