mirror of
https://github.com/restic/restic.git
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273 lines
5.8 KiB
Go
273 lines
5.8 KiB
Go
package chunker
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import (
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"io"
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"sync"
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)
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const (
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KiB = 1024
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MiB = 1024 * KiB
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// randomly generated irreducible polynomial of degree 53 in Z_2[X]
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Polynomial = 0x3DA3358B4DC173
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// use a sliding window of 64 byte.
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WindowSize = 64
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// aim to create chunks of 20 bits or about 1MiB on average.
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AverageBits = 20
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// Chunks should be in the range of 512KiB to 8MiB.
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MinSize = 512 * KiB
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MaxSize = 8 * MiB
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splitmask = (1 << AverageBits) - 1
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)
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var (
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pol_shift = deg(Polynomial) - 8
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once sync.Once
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mod_table [256]uint64
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out_table [256]uint64
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)
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// A chunk is one content-dependent chunk of bytes whose end was cut when the
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// Rabin Fingerprint had the value stored in Cut.
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type Chunk struct {
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Start int
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Length int
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Cut uint64
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Data []byte
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}
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// A chunker takes a stream of bytes and emits average size chunks.
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type Chunker interface {
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// Next returns the next chunk of data. If an error occurs while reading,
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// the error is returned with a nil chunk. The state of the current chunk
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// is undefined. When the last chunk has been returned, all subsequent
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// calls yield a nil chunk and an io.EOF error.
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Next() (*Chunk, error)
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}
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// A chunker internally holds everything needed to split content.
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type chunker struct {
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rd io.Reader
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closed bool
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window []byte
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wpos int
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buf []byte
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bpos int
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bmax int
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data []byte
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start int
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count int
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pos int
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digest uint64
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}
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// New returns a new Chunker that reads from data from rd.
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func New(rd io.Reader) Chunker {
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c := &chunker{
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rd: rd,
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window: make([]byte, WindowSize),
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buf: make([]byte, MaxSize),
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data: make([]byte, 0, MaxSize),
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}
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once.Do(c.fill_tables)
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c.reset()
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return c
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}
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func (c *chunker) reset() {
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for i := 0; i < WindowSize; i++ {
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c.window[i] = 0
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}
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c.digest = 0
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c.wpos = 0
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c.pos = 0
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c.count = 0
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c.slide(1)
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c.data = make([]byte, 0, MaxSize)
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}
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// Calculate out_table and mod_table for optimization. Must be called only once.
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func (c *chunker) fill_tables() {
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// calculate table for sliding out bytes. The byte to slide out is used as
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// the index for the table, the value contains the following:
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// out_table[b] = Hash(b || 0 || ... || 0)
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// \ windowsize-1 zero bytes /
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// To slide out byte b_0 for window size w with known hash
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// H := H(b_0 || ... || b_w), it is sufficient to add out_table[b_0]:
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// H(b_0 || ... || b_w) + H(b_0 || 0 || ... || 0)
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// = H(b_0 + b_0 || b_1 + 0 || ... || b_w + 0)
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// = H( 0 || b_1 || ... || b_w)
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//
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// Afterwards a new byte can be shifted in.
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for b := 0; b < 256; b++ {
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var hash uint64
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hash = append_byte(hash, byte(b), Polynomial)
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for i := 0; i < WindowSize-1; i++ {
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hash = append_byte(hash, 0, Polynomial)
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}
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out_table[b] = hash
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}
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// calculate table for reduction mod Polynomial
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k := deg(Polynomial)
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for b := 0; b < 256; b++ {
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// mod_table[b] = A | B, where A = (b(x) * x^k mod pol) and B = b(x) * x^k
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//
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// The 8 bits above deg(Polynomial) determine what happens next and so
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// these bits are used as a lookup to this table. The value is split in
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// two parts: Part A contains the result of the modulus operation, part
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// B is used to cancel out the 8 top bits so that one XOR operation is
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// enough to reduce modulo Polynomial
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mod_table[b] = mod(uint64(b)<<uint(k), Polynomial) | (uint64(b) << uint(k))
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}
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}
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func (c *chunker) Next() (*Chunk, error) {
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for {
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if c.bpos >= c.bmax {
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n, err := io.ReadFull(c.rd, c.buf)
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if err == io.ErrUnexpectedEOF {
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err = nil
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}
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// io.ReadFull only returns io.EOF when no bytes could be read. If
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// this is the case and we're in this branch, there are no more
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// bytes to buffer, so this was the last chunk. If a different
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// error has occurred, return that error and abandon the current
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// chunk.
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if err == io.EOF && !c.closed {
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c.closed = true
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// return current chunk
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return &Chunk{
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Start: c.start,
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Length: c.count,
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Cut: c.digest,
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Data: c.data,
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}, nil
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}
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if err != nil {
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return nil, err
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}
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c.bpos = 0
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c.bmax = n
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}
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for i, b := range c.buf[c.bpos:c.bmax] {
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// inline c.slide(b) and append(b) to increase performance
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out := c.window[c.wpos]
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c.window[c.wpos] = b
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c.digest ^= out_table[out]
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c.wpos = (c.wpos + 1) % WindowSize
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// c.append(b)
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index := c.digest >> uint(pol_shift)
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c.digest <<= 8
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c.digest |= uint64(b)
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c.digest ^= mod_table[index]
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if (c.count+i+1 >= MinSize && (c.digest&splitmask) == 0) || c.count+i+1 >= MaxSize {
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c.data = append(c.data, c.buf[c.bpos:c.bpos+i]...)
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c.count += i + 1
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c.pos += i + 1
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c.bpos += i + 1
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chunk := &Chunk{
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Start: c.start,
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Length: c.count,
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Cut: c.digest,
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Data: c.data,
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}
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// keep position
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pos := c.pos
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c.reset()
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c.pos = pos
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c.start = pos
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return chunk, nil
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}
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}
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steps := c.bmax - c.bpos
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if steps > 0 {
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c.data = append(c.data, c.buf[c.bpos:c.bpos+steps]...)
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}
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c.count += steps
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c.pos += steps
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c.bpos = c.bmax
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}
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return nil, nil
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}
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func (c *chunker) append(b byte) {
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index := c.digest >> uint(pol_shift)
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c.digest <<= 8
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c.digest |= uint64(b)
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c.digest ^= mod_table[index]
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}
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func (c *chunker) slide(b byte) {
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out := c.window[c.wpos]
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c.window[c.wpos] = b
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c.digest ^= out_table[out]
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c.wpos = (c.wpos + 1) % WindowSize
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c.append(b)
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}
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func append_byte(hash uint64, b byte, pol uint64) uint64 {
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hash <<= 8
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hash |= uint64(b)
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return mod(hash, pol)
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}
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// Mod calculates the remainder of x divided by p.
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func mod(x, p uint64) uint64 {
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for deg(x) >= deg(p) {
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shift := uint(deg(x) - deg(p))
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x = x ^ (p << shift)
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}
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return x
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}
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// Deg returns the degree of the polynomial p, this is equivalent to the number
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// of the highest bit set in p.
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func deg(p uint64) int {
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var mask uint64 = 0x8000000000000000
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for i := 0; i < 64; i++ {
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if mask&p > 0 {
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return 63 - i
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}
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mask >>= 1
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}
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return -1
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}
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