restic/chunker/chunker.go

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