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.
141 lines
3.5 KiB
Go
141 lines
3.5 KiB
Go
// Package rollinghash/buzhash implements buzhash as described by
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// https://en.wikipedia.org/wiki/Rolling_hash#Cyclic_polynomial
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package buzhash32
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import (
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"math/rand"
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rollinghash "github.com/chmduquesne/rollinghash"
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)
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var defaultHashes [256]uint32
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func init() {
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defaultHashes = GenerateHashes(1)
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}
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// The size of the checksum.
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const Size = 4
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// Buzhash32 is a digest which satisfies the rollinghash.Hash32 interface.
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// It implements the cyclic polynomial algorithm
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// https://en.wikipedia.org/wiki/Rolling_hash#Cyclic_polynomial
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type Buzhash32 struct {
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sum uint32
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nRotate uint
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nRotateComplement uint // redundant, but pre-computed to spare an operation
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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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bytehash [256]uint32
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}
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// Reset resets the Hash to its initial state.
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func (d *Buzhash32) Reset() {
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d.window = d.window[:0]
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d.oldest = 0
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d.sum = 0
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}
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// GenerateHashes generates a list of hashes to use with buzhash
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func GenerateHashes(seed int64) (res [256]uint32) {
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random := rand.New(rand.NewSource(seed))
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used := make(map[uint32]bool)
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for i, _ := range res {
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x := uint32(random.Int63())
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for used[x] {
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x = uint32(random.Int63())
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}
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used[x] = true
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res[i] = x
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}
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return res
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}
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// New returns a buzhash based on a list of hashes provided by a call to
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// GenerateHashes, seeded with the default value 1.
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func New() *Buzhash32 {
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return NewFromUint32Array(defaultHashes)
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}
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// NewFromUint32Array returns a buzhash based on the provided table uint32 values.
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func NewFromUint32Array(b [256]uint32) *Buzhash32 {
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return &Buzhash32{
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sum: 0,
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window: make([]byte, 0, rollinghash.DefaultWindowCap),
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oldest: 0,
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bytehash: b,
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}
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}
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// Size is 4 bytes
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func (d *Buzhash32) Size() int { return Size }
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// BlockSize is 1 byte
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func (d *Buzhash32) 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 *Buzhash32) 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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d.sum = 0
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for _, c := range d.window {
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d.sum = d.sum<<1 | d.sum>>31
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d.sum ^= d.bytehash[int(c)]
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}
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d.nRotate = uint(len(d.window)) % 32
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d.nRotateComplement = 32 - d.nRotate
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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 *Buzhash32) Sum32() uint32 {
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return d.sum
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}
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// Sum returns the hash as byte slice
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func (d *Buzhash32) 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 *Buzhash32) Roll(c 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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hn := d.bytehash[int(c)]
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h0 := d.bytehash[int(d.window[d.oldest])]
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d.window[d.oldest] = c
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l := len(d.window)
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d.oldest += 1
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if d.oldest >= l {
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d.oldest = 0
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}
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d.sum = (d.sum<<1 | d.sum>>31) ^ (h0<<d.nRotate | h0>>d.nRotateComplement) ^ hn
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}
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