package chunk

import (
	"io"
	"math/bits"

	pool "gitlab.dms3.io/p2p/go-buffer-pool"
)

const (
	buzMin  = 128 << 10
	buzMax  = 512 << 10
	buzMask = 1<<17 - 1
)

type Buzhash struct {
	r   io.Reader
	buf []byte
	n   int

	err error
}

func NewBuzhash(r io.Reader) *Buzhash {
	return &Buzhash{
		r:   r,
		buf: pool.Get(buzMax),
	}
}

func (b *Buzhash) Reader() io.Reader {
	return b.r
}

func (b *Buzhash) NextBytes() ([]byte, error) {
	if b.err != nil {
		return nil, b.err
	}

	n, err := io.ReadFull(b.r, b.buf[b.n:])
	if err != nil {
		if err == io.ErrUnexpectedEOF || err == io.EOF {
			buffered := b.n + n
			if buffered < buzMin {
				b.err = io.EOF
				// Read nothing? Don't return an empty block.
				if buffered == 0 {
					pool.Put(b.buf)
					b.buf = nil
					return nil, b.err
				}
				res := make([]byte, buffered)
				copy(res, b.buf)

				pool.Put(b.buf)
				b.buf = nil
				return res, nil
			}
		} else {
			b.err = err
			pool.Put(b.buf)
			b.buf = nil
			return nil, err
		}
	}

	i := buzMin - 32

	var state uint32 = 0

	if buzMin > len(b.buf) {
		panic("this is impossible")
	}

	for ; i < buzMin; i++ {
		state = bits.RotateLeft32(state, 1)
		state = state ^ bytehash[b.buf[i]]
	}

	{
		max := b.n + n - 32 - 1

		buf := b.buf
		bufshf := b.buf[32:]
		i = buzMin - 32
		_ = buf[max]
		_ = bufshf[max]

		for ; i <= max; i++ {
			if state&buzMask == 0 {
				break
			}
			state = bits.RotateLeft32(state, 1) ^
				bytehash[buf[i]] ^
				bytehash[bufshf[i]]
		}
		i += 32
	}

	res := make([]byte, i)
	copy(res, b.buf)

	b.n = copy(b.buf, b.buf[i:b.n+n])

	return res, nil
}

var bytehash = [256]uint32{
	0x6236e7d5, 0x10279b0b, 0x72818182, 0xdc526514, 0x2fd41e3d, 0x777ef8c8,
	0x83ee5285, 0x2c8f3637, 0x2f049c1a, 0x57df9791, 0x9207151f, 0x9b544818,
	0x74eef658, 0x2028ca60, 0x0271d91a, 0x27ae587e, 0xecf9fa5f, 0x236e71cd,
	0xf43a8a2e, 0xbb13380, 0x9e57912c, 0x89a26cdb, 0x9fcf3d71, 0xa86da6f1,
	0x9c49f376, 0x346aecc7, 0xf094a9ee, 0xea99e9cb, 0xb01713c6, 0x88acffb,
	0x2960a0fb, 0x344a626c, 0x7ff22a46, 0x6d7a1aa5, 0x6a714916, 0x41d454ca,
	0x8325b830, 0xb65f563, 0x447fecca, 0xf9d0ea5e, 0xc1d9d3d4, 0xcb5ec574,
	0x55aae902, 0x86edc0e7, 0xd3a9e33, 0xe70dc1e1, 0xe3c5f639, 0x9b43140a,
	0xc6490ac5, 0x5e4030fb, 0x8e976dd5, 0xa87468ea, 0xf830ef6f, 0xcc1ed5a5,
	0x611f4e78, 0xddd11905, 0xf2613904, 0x566c67b9, 0x905a5ccc, 0x7b37b3a4,
	0x4b53898a, 0x6b8fd29d, 0xaad81575, 0x511be414, 0x3cfac1e7, 0x8029a179,
	0xd40efeda, 0x7380e02, 0xdc9beffd, 0x2d049082, 0x99bc7831, 0xff5002a8,
	0x21ce7646, 0x1cd049b, 0xf43994f, 0xc3c6c5a5, 0xbbda5f50, 0xec15ec7,
	0x9adb19b6, 0xc1e80b9, 0xb9b52968, 0xae162419, 0x2542b405, 0x91a42e9d,
	0x6be0f668, 0x6ed7a6b9, 0xbc2777b4, 0xe162ce56, 0x4266aad5, 0x60fdb704,
	0x66f832a5, 0x9595f6ca, 0xfee83ced, 0x55228d99, 0x12bf0e28, 0x66896459,
	0x789afda, 0x282baa8, 0x2367a343, 0x591491b0, 0x2ff1a4b1, 0x410739b6,
	0x9b7055a0, 0x2e0eb229, 0x24fc8252, 0x3327d3df, 0xb0782669, 0x1c62e069,
	0x7f503101, 0xf50593ae, 0xd9eb275d, 0xe00eb678, 0x5917ccde, 0x97b9660a,
	0xdd06202d, 0xed229e22, 0xa9c735bf, 0xd6316fe6, 0x6fc72e4c, 0x206dfa2,
	0xd6b15c5a, 0x69d87b49, 0x9c97745, 0x13445d61, 0x35a975aa, 0x859aa9b9,
	0x65380013, 0xd1fb6391, 0xc29255fd, 0x784a3b91, 0xb9e74c26, 0x63ce4d40,
	0xc07cbe9e, 0xe6e4529e, 0xfb3632f, 0x9438d9c9, 0x682f94a8, 0xf8fd4611,
	0x257ec1ed, 0x475ce3d6, 0x60ee2db1, 0x2afab002, 0x2b9e4878, 0x86b340de,
	0x1482fdca, 0xfe41b3bf, 0xd4a412b0, 0xe09db98c, 0xc1af5d53, 0x7e55e25f,
	0xd3346b38, 0xb7a12cbd, 0x9c6827ba, 0x71f78bee, 0x8c3a0f52, 0x150491b0,
	0xf26de912, 0x233e3a4e, 0xd309ebba, 0xa0a9e0ff, 0xca2b5921, 0xeeb9893c,
	0x33829e88, 0x9870cc2a, 0x23c4b9d0, 0xeba32ea3, 0xbdac4d22, 0x3bc8c44c,
	0x1e8d0397, 0xf9327735, 0x783b009f, 0xeb83742, 0x2621dc71, 0xed017d03,
	0x5c760aa1, 0x5a69814b, 0x96e3047f, 0xa93c9cde, 0x615c86f5, 0xb4322aa5,
	0x4225534d, 0xd2e2de3, 0xccfccc4b, 0xbac2a57, 0xf0a06d04, 0xbc78d737,
	0xf2d1f766, 0xf5a7953c, 0xbcdfda85, 0x5213b7d5, 0xbce8a328, 0xd38f5f18,
	0xdb094244, 0xfe571253, 0x317fa7ee, 0x4a324f43, 0x3ffc39d9, 0x51b3fa8e,
	0x7a4bee9f, 0x78bbc682, 0x9f5c0350, 0x2fe286c, 0x245ab686, 0xed6bf7d7,
	0xac4988a, 0x3fe010fa, 0xc65fe369, 0xa45749cb, 0x2b84e537, 0xde9ff363,
	0x20540f9a, 0xaa8c9b34, 0x5bc476b3, 0x1d574bd7, 0x929100ad, 0x4721de4d,
	0x27df1b05, 0x58b18546, 0xb7e76764, 0xdf904e58, 0x97af57a1, 0xbd4dc433,
	0xa6256dfd, 0xf63998f3, 0xf1e05833, 0xe20acf26, 0xf57fd9d6, 0x90300b4d,
	0x89df4290, 0x68d01cbc, 0xcf893ee3, 0xcc42a046, 0x778e181b, 0x67265c76,
	0xe981a4c4, 0x82991da1, 0x708f7294, 0xe6e2ae62, 0xfc441870, 0x95e1b0b6,
	0x445f825, 0x5a93b47f, 0x5e9cf4be, 0x84da71e7, 0x9d9582b0, 0x9bf835ef,
	0x591f61e2, 0x43325985, 0x5d2de32e, 0x8d8fbf0f, 0x95b30f38, 0x7ad5b6e,
	0x4e934edf, 0x3cd4990e, 0x9053e259, 0x5c41857d}