src/runtime/hash32.go

// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Hashing algorithm inspired by
//   xxhash: https://code.google.com/p/xxhash/
// cityhash: https://code.google.com/p/cityhash/

// +build 386 arm

package runtime

import "unsafe"

const (
	// Constants for multiplication: four random odd 32-bit numbers.
	m1 = 3168982561
	m2 = 3339683297
	m3 = 832293441
	m4 = 2336365089
)

func memhash(p unsafe.Pointer, seed, s uintptr) uintptr {
	if GOARCH == "386" && GOOS != "nacl" && useAeshash {
		return aeshash(p, seed, s)
	}
	h := uint32(seed + s*hashkey[0])
tail:
	switch {
	case s == 0:
	case s < 4:
		h ^= uint32(*(*byte)(p))
		h ^= uint32(*(*byte)(add(p, s>>1))) << 8
		h ^= uint32(*(*byte)(add(p, s-1))) << 16
		h = rotl_15(h*m1) * m2
	case s == 4:
		h ^= readUnaligned32(p)
		h = rotl_15(h*m1) * m2
	case s <= 8:
		h ^= readUnaligned32(p)
		h = rotl_15(h*m1) * m2
		h ^= readUnaligned32(add(p, s-4))
		h = rotl_15(h*m1) * m2
	case s <= 16:
		h ^= readUnaligned32(p)
		h = rotl_15(h*m1) * m2
		h ^= readUnaligned32(add(p, 4))
		h = rotl_15(h*m1) * m2
		h ^= readUnaligned32(add(p, s-8))
		h = rotl_15(h*m1) * m2
		h ^= readUnaligned32(add(p, s-4))
		h = rotl_15(h*m1) * m2
	default:
		v1 := h
		v2 := uint32(seed * hashkey[1])
		v3 := uint32(seed * hashkey[2])
		v4 := uint32(seed * hashkey[3])
		for s >= 16 {
			v1 ^= readUnaligned32(p)
			v1 = rotl_15(v1*m1) * m2
			p = add(p, 4)
			v2 ^= readUnaligned32(p)
			v2 = rotl_15(v2*m2) * m3
			p = add(p, 4)
			v3 ^= readUnaligned32(p)
			v3 = rotl_15(v3*m3) * m4
			p = add(p, 4)
			v4 ^= readUnaligned32(p)
			v4 = rotl_15(v4*m4) * m1
			p = add(p, 4)
			s -= 16
		}
		h = v1 ^ v2 ^ v3 ^ v4
		goto tail
	}
	h ^= h >> 17
	h *= m3
	h ^= h >> 13
	h *= m4
	h ^= h >> 16
	return uintptr(h)
}

// Note: in order to get the compiler to issue rotl instructions, we
// need to constant fold the shift amount by hand.
// TODO: convince the compiler to issue rotl instructions after inlining.
func rotl_15(x uint32) uint32 {
	return (x << 15) | (x >> (32 - 15))
}