rollinghash

README

Rolling Hashes

Philosophy

This package contains several various rolling hashes for you to play with crazy ideas. The API design philosophy is to stick as closely as possible to the interface provided by the builtin hash package (the hashes implemented here are effectively drop-in replacements for their builtin counterparts), while providing simultaneously the highest speed and simplicity.

Usage

A rollinghash.Hash is just a hash.Hash which implements the Roller interface. Here is how it is typically used:

data := []byte("here is some data to roll on")
h := buzhash64.New()
n := 16

// Initialize the rolling window
h.Write(data[:n])

for _, c := range(data[n:]) {

    // Slide the window and update the hash
    h.Roll(c)

    // Get the updated hash value
    fmt.Println(h.Sum64())
}

Gotchas

The rolling window MUST be initialized by calling Write first (which saves a copy). The byte leaving the rolling window is inferred from the internal copy of the rolling window, which is updated with every call to Roll.

If you want your code to run at the highest speed, do NOT cast the result of a New() as a rollinghash.Hash. Instead, use the native type returned by New(). This is because the go compiler cannot inline calls from an interface. When later you call Roll(), the native type call will be inlined by the compiler, but not the casted type call.

var h1 rollinghash.Hash
h1 = buzhash32.New()
h2 := buzhash32.New()

[...]

h1.Roll(b) // Not inlined (slow)
h2.Roll(b) // inlined (fast)

What's new in v4

In v4:

• Write has become fully consistent with hash.Hash. As opposed to previous versions, where writing data would reinitialize the window, it now appends this data to the existing window. In order to reset the window, one should instead use the Reset method.

• Calling Roll on an empty window is considered a bug, and now triggers a panic.

Brief reminder of the behaviors in previous versions:

• From v0.x.x to v2.x.x: Roll returns an error for an empty window. Write reinitializes the rolling window.

• v3.x.x : Roll does not return anything. Write still reinitializes the rolling window. The rolling window always has a minimum size of 1, which yields wrong results when using roll before having initialized the window.

Go versions

The RabinKarp64 rollinghash does not yield consistent results before go1.7. This is because it uses Rand.Read() from the builtin math/rand. This function was fixed in go 1.7 to produce a consistent stream of bytes that is independant of the size of the input buffer. If you depend on this hash, it is strongly recommended to stick to versions of go superior to 1.7.

License

This code is delivered to you under the terms of the MIT public license, except the rabinkarp64 subpackage, which has been adapted from restic (BSD 2-clause "Simplified").

Notable users

• syncthing, a decentralized synchronisation solution
• muscato, a genome analysis tool

If you are using this in production or for research, let me know and I will happily put a link here!

Documentation

Overview

Package rollinghash implements rolling versions of some hashes

Example

package main

import (
	"hash"
	"log"

	_adler32 "github.com/chmduquesne/rollinghash/adler32"
)

func main() {
	s := []byte("The quick brown fox jumps over the lazy dog")

	// This example works with adler32, but the api is identical for all
	// other rolling checksums. Consult the documentation of the checksum
	// of interest.
	classic := hash.Hash32(_adler32.New())
	rolling := _adler32.New()

	// Window len
	n := 16

	// You MUST load an initial window into the rolling hash before being
	// able to roll bytes
	rolling.Write(s[:n])

	// Roll it and compare the result with full re-calculus every time
	for i := n; i < len(s); i++ {

		// Reset and write the window in classic
		classic.Reset()
		classic.Write(s[i-n+1 : i+1])

		// Roll the incoming byte in rolling
		rolling.Roll(s[i])

		// Compare the hashes
		if classic.Sum32() != rolling.Sum32() {
			log.Fatalf("%v: expected %x, got %x",
				s[i-n+1:i+1], classic.Sum32(), rolling.Sum32())
		}
	}

}

Index

• Constants
• type Hash
• type Hash32
• type Hash64
• type Roller

Examples

• Package

Constants

const DefaultWindowCap = 64

DefaultWindowCap is the default capacity of the internal window of a new Hash.

Types

type Hash

type Hash interface {
	hash.Hash
	Roller
}

rollinghash.Hash extends hash.Hash by adding the method Roll. A rollinghash.Hash can be updated byte by byte, by specifying which byte enters the window.

type Hash32

type Hash32 interface {
	hash.Hash32
	Roller
}

rollinghash.Hash32 extends hash.Hash by adding the method Roll. A rollinghash.Hash32 can be updated byte by byte, by specifying which byte enters the window.

type Hash64

type Hash64 interface {
	hash.Hash64
	Roller
}

rollinghash.Hash64 extends hash.Hash by adding the method Roll. A rollinghash.Hash64 can be updated byte by byte, by specifying which byte enters the window.

type Roller

type Roller interface {
	Roll(b byte)
}

A Roller is a type that has the method Roll. Roll updates the hash of a rolling window from just the entering byte. You MUST call Write() BEFORE using this method and provide it with an initial window of size at least 1 byte. You can then call this method for every new byte entering the window. The byte leaving the window is automatically computed from a copy of the window internally kept in the checksum. This window is updated along with the internal state of the checksum every time Roll() is called.