mirror of
https://github.com/octoleo/syncthing.git
synced 2024-11-19 19:45:12 +00:00
107 lines
2.7 KiB
Go
107 lines
2.7 KiB
Go
// Package rollinghash/adler32 implements a rolling version of hash/adler32
|
|
|
|
package adler32
|
|
|
|
import (
|
|
"hash"
|
|
vanilla "hash/adler32"
|
|
|
|
"github.com/chmduquesne/rollinghash"
|
|
)
|
|
|
|
const (
|
|
mod = 65521
|
|
)
|
|
|
|
const Size = 4
|
|
|
|
type digest struct {
|
|
a, b uint32
|
|
|
|
// window is treated like a circular buffer, where the oldest element
|
|
// is indicated by d.oldest
|
|
window []byte
|
|
oldest int
|
|
n uint32
|
|
|
|
vanilla hash.Hash32
|
|
}
|
|
|
|
// Reset resets the Hash to its initial state.
|
|
func (d *digest) Reset() {
|
|
d.a = 1
|
|
d.b = 0
|
|
d.window = d.window[:0]
|
|
d.oldest = 0
|
|
}
|
|
|
|
// New returns a new rollinghash.Hash32 computing the rolling Adler-32
|
|
// checksum. The window is copied from the last Write(). This window is
|
|
// only used to determine which is the oldest element (leaving the
|
|
// window). The calls to Roll() do not recompute the whole checksum.
|
|
func New() rollinghash.Hash32 {
|
|
return &digest{a: 1, b: 0, window: nil, oldest: 0, vanilla: vanilla.New()}
|
|
}
|
|
|
|
// Size returns the number of bytes Sum will return.
|
|
func (d *digest) Size() int { return Size }
|
|
|
|
// BlockSize returns the hash's underlying block size.
|
|
// The Write method must be able to accept any amount
|
|
// of data, but it may operate more efficiently if all
|
|
// writes are a multiple of the block size.
|
|
func (d *digest) BlockSize() int { return 1 }
|
|
|
|
// Write (via the embedded io.Writer interface) adds more data to the
|
|
// running hash. It never returns an error.
|
|
func (d *digest) Write(p []byte) (int, error) {
|
|
// Copy the window, avoiding allocations where possible
|
|
if len(d.window) != len(p) {
|
|
if cap(d.window) >= len(p) {
|
|
d.window = d.window[:len(p)]
|
|
} else {
|
|
d.window = make([]byte, len(p))
|
|
}
|
|
}
|
|
copy(d.window, p)
|
|
|
|
// Piggy-back on the core implementation
|
|
d.vanilla.Reset()
|
|
d.vanilla.Write(p)
|
|
s := d.vanilla.Sum32()
|
|
d.a, d.b = s&0xffff, s>>16
|
|
d.n = uint32(len(p)) % mod
|
|
return len(d.window), nil
|
|
}
|
|
|
|
func (d *digest) Sum32() uint32 {
|
|
return d.b<<16 | d.a
|
|
}
|
|
|
|
func (d *digest) 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 leaving byte and the
|
|
// entering byte. See
|
|
// http://stackoverflow.com/questions/40985080/why-does-my-rolling-adler32-checksum-not-work-in-go-modulo-arithmetic
|
|
func (d *digest) Roll(b byte) {
|
|
if len(d.window) == 0 {
|
|
d.window = make([]byte, 1)
|
|
d.window[0] = b
|
|
}
|
|
// extract the entering/leaving bytes and update the circular buffer.
|
|
enter := uint32(b)
|
|
leave := uint32(d.window[d.oldest])
|
|
d.window[d.oldest] = b
|
|
d.oldest += 1
|
|
if d.oldest >= len(d.window) {
|
|
d.oldest = 0
|
|
}
|
|
|
|
// compute
|
|
d.a = (d.a + mod + enter - leave) % mod
|
|
d.b = (d.b + (d.n*leave/mod+1)*mod + d.a - (d.n * leave) - 1) % mod
|
|
}
|