mirror of
https://github.com/octoleo/syncthing.git
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c0a26c918a
Updates the package and fixes a test that depended on the old behavior of Write() being equivalent to Reset()+Write() which is no longer the case. The scanner already did resets after each block write, so this is fine.
118 lines
2.9 KiB
Go
118 lines
2.9 KiB
Go
// Package rollinghash/adler32 implements a rolling version of hash/adler32
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package adler32
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import (
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"hash"
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vanilla "hash/adler32"
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"github.com/chmduquesne/rollinghash"
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)
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const (
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Mod = 65521
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Size = 4
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)
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// Adler32 is a digest which satisfies the rollinghash.Hash32 interface.
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// It implements the adler32 algorithm https://en.wikipedia.org/wiki/Adler-32
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type Adler32 struct {
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a, b uint32
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n uint32
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// window is treated like a circular buffer, where the oldest element
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// is indicated by d.oldest
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window []byte
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oldest int
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vanilla hash.Hash32
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}
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// Reset resets the digest to its initial state.
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func (d *Adler32) Reset() {
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d.window = d.window[:0] // Reset the size but don't reallocate
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d.oldest = 0
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d.a = 1
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d.b = 0
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d.n = 0
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d.vanilla.Reset()
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}
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// New returns a new Adler32 digest
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func New() *Adler32 {
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return &Adler32{
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a: 1,
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b: 0,
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n: 0,
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window: make([]byte, 0, rollinghash.DefaultWindowCap),
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oldest: 0,
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vanilla: vanilla.New(),
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}
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}
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// Size is 4 bytes
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func (d *Adler32) Size() int { return Size }
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// BlockSize is 1 byte
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func (d *Adler32) BlockSize() int { return 1 }
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// Write appends data to the rolling window and updates the digest.
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func (d *Adler32) Write(data []byte) (int, error) {
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l := len(data)
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if l == 0 {
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return 0, nil
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}
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// Re-arrange the window so that the leftmost element is at index 0
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n := len(d.window)
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if d.oldest != 0 {
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tmp := make([]byte, d.oldest)
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copy(tmp, d.window[:d.oldest])
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copy(d.window, d.window[d.oldest:])
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copy(d.window[n-d.oldest:], tmp)
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d.oldest = 0
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}
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d.window = append(d.window, data...)
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// Piggy-back on the core implementation
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d.vanilla.Reset()
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d.vanilla.Write(d.window)
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s := d.vanilla.Sum32()
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d.a, d.b = s&0xffff, s>>16
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d.n = uint32(len(d.window)) % Mod
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return len(data), nil
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}
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// Sum32 returns the hash as a uint32
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func (d *Adler32) Sum32() uint32 {
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return d.b<<16 | d.a
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}
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// Sum returns the hash as a byte slice
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func (d *Adler32) Sum(b []byte) []byte {
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v := d.Sum32()
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return append(b, byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
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}
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// Roll updates the checksum of the window from the entering byte. You
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// MUST initialize a window with Write() before calling this method.
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func (d *Adler32) Roll(b byte) {
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// This check costs 10-15% performance. If we disable it, we crash
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// when the window is empty. If we enable it, we are always correct
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// (an empty window never changes no matter how much you roll it).
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//if len(d.window) == 0 {
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// return
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//}
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// extract the entering/leaving bytes and update the circular buffer.
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enter := uint32(b)
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leave := uint32(d.window[d.oldest])
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d.window[d.oldest] = b
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d.oldest += 1
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if d.oldest >= len(d.window) {
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d.oldest = 0
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}
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// See http://stackoverflow.com/questions/40985080/why-does-my-rolling-adler32-checksum-not-work-in-go-modulo-arithmetic
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d.a = (d.a + Mod + enter - leave) % Mod
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d.b = (d.b + (d.n*leave/Mod+1)*Mod + d.a - (d.n * leave) - 1) % Mod
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}
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