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weakhash.go
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weakhash.go
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// Copyright (C) 2016 The Syncthing Authors.
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this file,
// You can obtain one at http://mozilla.org/MPL/2.0/.
package weakhash
import (
"bufio"
"hash"
"io"
"os"
)
const (
Size = 4
)
func NewHash(size int) hash.Hash32 {
return &digest{
buf: make([]byte, size),
size: size,
}
}
// Find finds all the blocks of the given size within io.Reader that matches
// the hashes provided, and returns a hash -> slice of offsets within reader
// map, that produces the same weak hash.
func Find(ir io.Reader, hashesToFind []uint32, size int) (map[uint32][]int64, error) {
if ir == nil {
return nil, nil
}
r := bufio.NewReader(ir)
hf := NewHash(size)
n, err := io.CopyN(hf, r, int64(size))
if err == io.EOF {
return nil, nil
}
if err != nil {
return nil, err
}
if n != int64(size) {
return nil, io.ErrShortBuffer
}
offsets := make(map[uint32][]int64)
for _, hashToFind := range hashesToFind {
offsets[hashToFind] = nil
}
var i int64
var hash uint32
for {
hash = hf.Sum32()
if existing, ok := offsets[hash]; ok {
offsets[hash] = append(existing, i)
}
i++
bt, err := r.ReadByte()
if err == io.EOF {
break
} else if err != nil {
return offsets, err
}
hf.Write([]byte{bt})
}
return offsets, nil
}
// Using this: http://tutorials.jenkov.com/rsync/checksums.html
// Example implementations: https://gist.github.com/csabahenk/1096262/revisions
// Alternative that could be used is adler32 http://blog.liw.fi/posts/rsync-in-python/#comment-fee8d5e07794fdba3fe2d76aa2706a13
type digest struct {
buf []byte
size int
a uint16
b uint16
j int
}
func (d *digest) Write(data []byte) (int, error) {
for _, c := range data {
// TODO: Use this in Go 1.6
// d.a = d.a - uint16(d.buf[d.j]) + uint16(c)
// d.b = d.b - uint16(d.size)*uint16(d.buf[d.j]) + d.a
d.a -= uint16(d.buf[d.j])
d.a += uint16(c)
d.b -= uint16(d.size) * uint16(d.buf[d.j])
d.b += d.a
d.buf[d.j] = c
d.j = (d.j + 1) % d.size
}
return len(data), nil
}
func (d *digest) Reset() {
for i := range d.buf {
d.buf[i] = 0x0
}
d.a = 0
d.b = 0
d.j = 0
}
func (d *digest) Sum(b []byte) []byte {
r := d.Sum32()
return append(b, byte(r>>24), byte(r>>16), byte(r>>8), byte(r))
}
func (d *digest) Sum32() uint32 { return uint32(d.a) | (uint32(d.b) << 16) }
func (digest) Size() int { return Size }
func (digest) BlockSize() int { return 1 }
func NewFinder(path string, size int, hashesToFind []uint32) (*Finder, error) {
file, err := os.Open(path)
if err != nil {
return nil, err
}
offsets, err := Find(file, hashesToFind, size)
if err != nil {
file.Close()
return nil, err
}
return &Finder{
file: file,
size: size,
offsets: offsets,
}, nil
}
type Finder struct {
file *os.File
size int
offsets map[uint32][]int64
}
// Iterate iterates all available blocks that matches the provided hash, reads
// them into buf, and calls the iterator function. The iterator function should
// return wether it wishes to continue interating.
func (h *Finder) Iterate(hash uint32, buf []byte, iterFunc func(int64) bool) (bool, error) {
if h == nil || hash == 0 || len(buf) != h.size {
return false, nil
}
for _, offset := range h.offsets[hash] {
_, err := h.file.ReadAt(buf, offset)
if err != nil {
return false, err
}
if !iterFunc(offset) {
return true, nil
}
}
return false, nil
}
// Close releases any resource associated with the finder
func (h *Finder) Close() {
if h != nil {
h.file.Close()
}
}