Calculating hashCode in Go

Question

Java objects have a hashCode, which are cheaper than a cryptographic hash. How would one implement such a hashCode in Go?

Answer 1

The Go programming language is open source. You can check its standard library for a fast and efficient Go hash implementation.

Here it is:

• runtime/hash64.go for 64-bit architecture

• runtime/hash32.go for 32-bit architecture

They are unexported, but if you'd ever need them in your app, you can just copy the code to your project. Also note that if your CPU supports it, the Go runtime will use aeshash to utilize your CPU's capabilities (more info on this here: How does go calculate a hash value for keys in a map?).

Qutoing the shorter 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))
}