go - 在 Go 中计算 hashCode

Question

Java 对象有一个hashCode，它比加密散列更便宜。如何在 Go 中实现这样的 hashCode？

Answer 1

Go programming language是open source .您可以查看其标准库以获取快速高效的 Go 哈希实现。

这里是:

• runtime/hash64.go对于 64 位架构

• runtime/hash32.go对于 32 位架构

它们未导出，但如果您的应用中需要它们，您只需将代码复制到您的项目中即可。另请注意，如果您的 CPU 支持它，Go 运行时将使用 aeshash 来利用您的 CPU 的功能(更多信息在这里:How does go calculate a hash value for keys in a map?)。

引用较短的 32-bit version :

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))
}