source: code/trunk/vendor/github.com/cespare/xxhash/v2/xxhash_other.go@ 823

// +build !amd64 appengine !gc purego

package xxhash

// Sum64 computes the 64-bit xxHash digest of b.
func Sum64(b []byte) uint64 {
        // A simpler version would be
        //   d := New()
        //   d.Write(b)
        //   return d.Sum64()
        // but this is faster, particularly for small inputs.

        n := len(b)
        var h uint64

        if n >= 32 {
                v1 := prime1v + prime2
                v2 := prime2
                v3 := uint64(0)
                v4 := -prime1v
                for len(b) >= 32 {
                        v1 = round(v1, u64(b[0:8:len(b)]))
                        v2 = round(v2, u64(b[8:16:len(b)]))
                        v3 = round(v3, u64(b[16:24:len(b)]))
                        v4 = round(v4, u64(b[24:32:len(b)]))
                        b = b[32:len(b):len(b)]
                }
                h = rol1(v1) + rol7(v2) + rol12(v3) + rol18(v4)
                h = mergeRound(h, v1)
                h = mergeRound(h, v2)
                h = mergeRound(h, v3)
                h = mergeRound(h, v4)
        } else {
                h = prime5
        }

        h += uint64(n)

        i, end := 0, len(b)
        for ; i+8 <= end; i += 8 {
                k1 := round(0, u64(b[i:i+8:len(b)]))
                h ^= k1
                h = rol27(h)*prime1 + prime4
        }
        if i+4 <= end {
                h ^= uint64(u32(b[i:i+4:len(b)])) * prime1
                h = rol23(h)*prime2 + prime3
                i += 4
        }
        for ; i < end; i++ {
                h ^= uint64(b[i]) * prime5
                h = rol11(h) * prime1
        }

        h ^= h >> 33
        h *= prime2
        h ^= h >> 29
        h *= prime3
        h ^= h >> 32

        return h
}

func writeBlocks(d *Digest, b []byte) int {
        v1, v2, v3, v4 := d.v1, d.v2, d.v3, d.v4
        n := len(b)
        for len(b) >= 32 {
                v1 = round(v1, u64(b[0:8:len(b)]))
                v2 = round(v2, u64(b[8:16:len(b)]))
                v3 = round(v3, u64(b[16:24:len(b)]))
                v4 = round(v4, u64(b[24:32:len(b)]))
                b = b[32:len(b):len(b)]
        }
        d.v1, d.v2, d.v3, d.v4 = v1, v2, v3, v4
        return n - len(b)
}