package hasher

// Implementation borrowed from https://github.com/spaolacci/murmur3,
// distributed under BSD-3 license.

import (
	"hash"
	"unsafe"
)

type bmixer interface {
	bmix(p []byte) (tail []byte)
	Size() (n int)
	reset()
}

type digest struct {
	clen int      // Digested input cumulative length.
	tail []byte   // 0 to Size()-1 bytes view of `buf'.
	buf  [16]byte // Expected (but not required) to be Size() large.
	seed uint32   // Seed for initializing the hash.
	bmixer
}

func (d *digest) BlockSize() int { return 1 }

func (d *digest) Write(p []byte) (n int, err error) {
	n = len(p)
	d.clen += n

	if len(d.tail) > 0 {
		// Stick back pending bytes.
		nfree := d.Size() - len(d.tail) // nfree ∈ [1, d.Size()-1].
		if nfree < len(p) {
			// One full block can be formed.
			block := append(d.tail, p[:nfree]...)
			p = p[nfree:]
			_ = d.bmix(block) // No tail.
		} else {
			// Tail's buf is large enough to prevent reallocs.
			p = append(d.tail, p...)
		}
	}

	d.tail = d.bmix(p)

	// Keep own copy of the 0 to Size()-1 pending bytes.
	nn := copy(d.buf[:], d.tail)
	d.tail = d.buf[:nn]

	return n, nil
}

func (d *digest) Reset() {
	d.clen = 0
	d.tail = nil
	d.bmixer.reset()
}

// Make sure interfaces are correctly implemented.
var (
	_ hash.Hash   = new(digest32)
	_ hash.Hash32 = new(digest32)
	_ bmixer      = new(digest32)
)

const (
	c1_32 uint32 = 0xcc9e2d51
	c2_32 uint32 = 0x1b873593
)

// digest32 represents a partial evaluation of a 32 bites hash.
type digest32 struct {
	digest
	h1 uint32 // Unfinalized running hash.
}

func (d *digest32) Size() int { return 4 }

func (d *digest32) reset() { d.h1 = d.seed }

func (d *digest32) Sum(b []byte) []byte {
	h := d.Sum32()
	return append(b, byte(h>>24), byte(h>>16), byte(h>>8), byte(h))
}

// Digest as many blocks as possible.
func (d *digest32) bmix(p []byte) (tail []byte) {
	h1 := d.h1

	nblocks := len(p) / 4
	for i := 0; i < nblocks; i++ {
		k1 := *(*uint32)(unsafe.Pointer(&p[i*4]))

		k1 *= c1_32
		k1 = (k1 << 15) | (k1 >> 17) // rotl32(k1, 15)
		k1 *= c2_32

		h1 ^= k1
		h1 = (h1 << 13) | (h1 >> 19) // rotl32(h1, 13)
		h1 = h1*4 + h1 + 0xe6546b64
	}
	d.h1 = h1
	return p[nblocks*d.Size():]
}

func (d *digest32) Sum32() (h1 uint32) {

	h1 = d.h1

	var k1 uint32
	switch len(d.tail) & 3 {
	case 3:
		k1 ^= uint32(d.tail[2]) << 16
		fallthrough
	case 2:
		k1 ^= uint32(d.tail[1]) << 8
		fallthrough
	case 1:
		k1 ^= uint32(d.tail[0])
		k1 *= c1_32
		k1 = (k1 << 15) | (k1 >> 17) // rotl32(k1, 15)
		k1 *= c2_32
		h1 ^= k1
	}

	h1 ^= uint32(d.clen)

	h1 ^= h1 >> 16
	h1 *= 0x85ebca6b
	h1 ^= h1 >> 13
	h1 *= 0xc2b2ae35
	h1 ^= h1 >> 16

	return h1
}

/*
func rotl32(x uint32, r byte) uint32 {
	return (x << r) | (x >> (32 - r))
}
*/

// sum32WithSeed returns the MurmurHash3 sum of data. It is equivalent to the
// following sequence (without the extra burden and the extra allocation):
//     hasher := New32WithSeed(seed)
//     hasher.Write(data)
//     return hasher.Sum32()
func sum32WithSeed(data []byte, seed uint32) uint32 {

	h1 := seed

	nblocks := len(data) / 4
	for blocks := 0; blocks < nblocks; blocks++ {
		k1 := *(*uint32)(unsafe.Pointer(&data[blocks*4]))

		k1 *= c1_32
		k1 = (k1 << 15) | (k1 >> 17) // rotl32(k1, 15)
		k1 *= c2_32

		h1 ^= k1
		h1 = (h1 << 13) | (h1 >> 19) // rotl32(h1, 13)
		h1 = h1*4 + h1 + 0xe6546b64
	}

	tail := data[nblocks*4:]

	var k1 uint32
	switch len(tail) & 3 {
	case 3:
		k1 ^= uint32(tail[2]) << 16
		fallthrough
	case 2:
		k1 ^= uint32(tail[1]) << 8
		fallthrough
	case 1:
		k1 ^= uint32(tail[0])
		k1 *= c1_32
		k1 = (k1 << 15) | (k1 >> 17) // rotl32(k1, 15)
		k1 *= c2_32
		h1 ^= k1
	}

	h1 ^= uint32(len(data))

	h1 ^= h1 >> 16
	h1 *= 0x85ebca6b
	h1 ^= h1 >> 13
	h1 *= 0xc2b2ae35
	h1 ^= h1 >> 16

	return h1
}