mirror of
https://github.com/octoleo/syncthing.git
synced 2024-11-06 05:17:49 +00:00
104 lines
2.4 KiB
Go
104 lines
2.4 KiB
Go
// Package rollinghash/bozo32 is a wrong implementation of the rabinkarp
|
|
// checksum. In practice, it works very well and exhibits all the
|
|
// properties wanted from a rolling checksum, so after realising that this
|
|
// code did not implement the rabinkarp checksum as described in the
|
|
// original paper, it was renamed from rabinkarp32 to bozo32 and kept
|
|
// in this package.
|
|
|
|
package bozo32
|
|
|
|
import rollinghash "github.com/chmduquesne/rollinghash"
|
|
|
|
// The size of the checksum.
|
|
const Size = 4
|
|
|
|
// Bozo32 is a digest which satisfies the rollinghash.Hash32 interface.
|
|
type Bozo32 struct {
|
|
a uint32
|
|
h uint32
|
|
aPowerN uint32
|
|
|
|
// window is treated like a circular buffer, where the oldest element
|
|
// is indicated by d.oldest
|
|
window []byte
|
|
oldest int
|
|
}
|
|
|
|
// Reset resets the Hash to its initial state.
|
|
func (d *Bozo32) Reset() {
|
|
d.h = 0
|
|
d.aPowerN = 1
|
|
d.window = nil
|
|
d.oldest = 0
|
|
}
|
|
|
|
func NewFromInt(a uint32) *Bozo32 {
|
|
return &Bozo32{
|
|
a: a,
|
|
h: 0,
|
|
aPowerN: 1,
|
|
window: make([]byte, 1, rollinghash.DefaultWindowCap),
|
|
oldest: 0,
|
|
}
|
|
}
|
|
|
|
func New() *Bozo32 {
|
|
return NewFromInt(65521) // largest prime fitting in 16 bits
|
|
}
|
|
|
|
// Size is 4 bytes
|
|
func (d *Bozo32) Size() int { return Size }
|
|
|
|
// BlockSize is 1 byte
|
|
func (d *Bozo32) BlockSize() int { return 1 }
|
|
|
|
// Write (re)initializes the rolling window with the input byte slice and
|
|
// adds its data to the digest. It never returns an error.
|
|
func (d *Bozo32) Write(data []byte) (int, error) {
|
|
// Copy the window
|
|
l := len(data)
|
|
if l == 0 {
|
|
l = 1
|
|
}
|
|
if len(d.window) >= l {
|
|
d.window = d.window[:l]
|
|
} else {
|
|
d.window = make([]byte, l)
|
|
}
|
|
copy(d.window, data)
|
|
|
|
for _, c := range d.window {
|
|
d.h *= d.a
|
|
d.h += uint32(c)
|
|
d.aPowerN *= d.a
|
|
}
|
|
return len(d.window), nil
|
|
}
|
|
|
|
// Sum32 returns the hash as a uint32
|
|
func (d *Bozo32) Sum32() uint32 {
|
|
return d.h
|
|
}
|
|
|
|
// Sum returns the hash as byte slice
|
|
func (d *Bozo32) Sum(b []byte) []byte {
|
|
v := d.Sum32()
|
|
return append(b, byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
|
|
}
|
|
|
|
// Roll updates the checksum of the window from the entering byte. You
|
|
// MUST initialize a window with Write() before calling this method.
|
|
func (d *Bozo32) Roll(c byte) {
|
|
// extract the entering/leaving bytes and update the circular buffer.
|
|
enter := uint32(c)
|
|
leave := uint32(d.window[d.oldest])
|
|
d.window[d.oldest] = c
|
|
l := len(d.window)
|
|
d.oldest += 1
|
|
if d.oldest >= l {
|
|
d.oldest = 0
|
|
}
|
|
|
|
d.h = d.h*d.a + enter - leave*d.aPowerN
|
|
}
|