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 ) // 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 takes a stream of bytes and emits average size chunks. type Chunker interface { // 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. Next() (*Chunk, error) } // 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 data []byte 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 := &chunker{ rd: rd, window: make([]byte, WindowSize), buf: make([]byte, MaxSize), data: make([]byte, 0, MaxSize), } once.Do(c.fill_tables) c.reset() return c } func (c *chunker) reset() { for i := 0; i < WindowSize; i++ { c.window[i] = 0 } c.digest = 0 c.wpos = 0 c.pos = 0 c.count = 0 c.slide(1) c.data = make([]byte, 0, MaxSize) } // 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)<= 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 // 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: c.data, }, 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 { c.data = append(c.data, 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: c.data, } // keep position pos := c.pos c.reset() c.pos = pos c.start = pos return chunk, nil } } steps := c.bmax - c.bpos if steps > 0 { c.data = append(c.data, 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 }