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110 lines
2.5 KiB
Go
110 lines
2.5 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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// 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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n uint32
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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[:1] // Reset the size but don't reallocate
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d.window[0] = 0
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d.a = 1
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d.b = 0
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d.oldest = 0
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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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window: make([]byte, 1, 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 (re)initializes the rolling window with the input byte slice and
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// adds its data to the digest.
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func (d *Adler32) Write(p []byte) (int, error) {
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// Copy the window, avoiding allocations where possible
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l := len(p)
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if l == 0 {
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l = 1
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}
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if len(d.window) != l {
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if cap(d.window) >= l {
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d.window = d.window[:l]
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} else {
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d.window = make([]byte, len(p))
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}
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}
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copy(d.window, p)
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// Piggy-back on the core implementation
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d.vanilla.Reset()
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d.vanilla.Write(p)
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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(p)) % Mod
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return len(d.window), 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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// 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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