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]
my_hash(key: u32, salt: u32, n: usize) -> usize16 fn my_hash(key: u32, salt: u32, n: usize) -> usize {
17 let y = key.wrapping_add(salt).wrapping_mul(2654435769);
18 let y = 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]
mph_lookup<KV, V, FK, FV>( x: u32, salt: &[u16], kv: &[KV], fk: FK, fv: FV, default: V, ) -> V where KV: Copy, FK: Fn(KV) -> u32, FV: Fn(KV) -> V,30 pub(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
38 where
39 KV: Copy,
40 FK: Fn(KV) -> u32,
41 FV: Fn(KV) -> V,
42 {
43 let s = salt[my_hash(x, 0, salt.len())] as u32;
44 let key_val = kv[my_hash(x, s, salt.len())];
45 if x == fk(key_val) {
46 fv(key_val)
47 } else {
48 default
49 }
50 }
51