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104 lines
2.4 KiB
Go
104 lines
2.4 KiB
Go
// Package rollinghash/bozo32 is a wrong implementation of the rabinkarp
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// checksum. In practice, it works very well and exhibits all the
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// properties wanted from a rolling checksum, so after realising that this
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// code did not implement the rabinkarp checksum as described in the
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// original paper, it was renamed from rabinkarp32 to bozo32 and kept
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// in this package.
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package bozo32
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import rollinghash "github.com/chmduquesne/rollinghash"
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// The size of the checksum.
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const Size = 4
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// Bozo32 is a digest which satisfies the rollinghash.Hash32 interface.
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type Bozo32 struct {
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a uint32
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h uint32
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aPowerN 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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}
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// Reset resets the Hash to its initial state.
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func (d *Bozo32) Reset() {
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d.h = 0
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d.aPowerN = 1
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d.window = nil
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d.oldest = 0
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}
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func NewFromInt(a uint32) *Bozo32 {
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return &Bozo32{
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a: a,
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h: 0,
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aPowerN: 1,
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window: make([]byte, 1, rollinghash.DefaultWindowCap),
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oldest: 0,
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}
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}
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func New() *Bozo32 {
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return NewFromInt(65521) // largest prime fitting in 16 bits
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}
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// Size is 4 bytes
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func (d *Bozo32) Size() int { return Size }
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// BlockSize is 1 byte
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func (d *Bozo32) 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. It never returns an error.
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func (d *Bozo32) Write(data []byte) (int, error) {
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// Copy the window
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l := len(data)
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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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d.window = d.window[:l]
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} else {
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d.window = make([]byte, l)
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}
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copy(d.window, data)
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for _, c := range d.window {
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d.h *= d.a
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d.h += uint32(c)
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d.aPowerN *= d.a
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}
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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 *Bozo32) Sum32() uint32 {
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return d.h
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}
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// Sum returns the hash as byte slice
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func (d *Bozo32) 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 *Bozo32) Roll(c byte) {
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// extract the entering/leaving bytes and update the circular buffer.
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enter := uint32(c)
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leave := uint32(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.h = d.h*d.a + enter - leave*d.aPowerN
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}
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