maphash

Documentation

Overview

Package maphash provides hash functions on byte sequences. These hash functions are intended to be used to implement hash tables or other data structures that need to map arbitrary strings or byte sequences to a uniform distribution on unsigned 64-bit integers. Each different instance of a hash table or data structure should use its own Seed.

The hash functions are not cryptographically secure. (See crypto/sha256 and crypto/sha512 for cryptographic use.)

Example

package main

import (
	"fmt"
	"hash/maphash"
)

func main() {
	// The zero Hash value is valid and ready to use; setting an
	// initial seed is not necessary.
	var h maphash.Hash

	// Add a string to the hash, and print the current hash value.
	h.WriteString("hello, ")
	fmt.Printf("%#x\n", h.Sum64())

	// Append additional data (in the form of a byte array).
	h.Write([]byte{'w', 'o', 'r', 'l', 'd'})
	fmt.Printf("%#x\n", h.Sum64())

	// Reset discards all data previously added to the Hash, without
	// changing its seed.
	h.Reset()

	// Use SetSeed to create a new Hash h2 which will behave
	// identically to h.
	var h2 maphash.Hash
	h2.SetSeed(h.Seed())

	h.WriteString("same")
	h2.WriteString("same")
	fmt.Printf("%#x == %#x\n", h.Sum64(), h2.Sum64())
}

Index

• func Bytes(seed Seed, b []byte) uint64
• func String(seed Seed, s string) uint64
• type Hash
  • func (h *Hash) BlockSize() int
  • func (h *Hash) Reset()
  • func (h *Hash) Seed() Seed
  • func (h *Hash) SetSeed(seed Seed)
  • func (h *Hash) Size() int
  • func (h *Hash) Sum(b []byte) []byte
  • func (h *Hash) Sum64() uint64
  • func (h *Hash) Write(b []byte) (int, error)
  • func (h *Hash) WriteByte(b byte) error
  • func (h *Hash) WriteString(s string) (int, error)
• type Seed
  • func MakeSeed() Seed

Examples

• Package

Functions

func Bytes

func Bytes(seed Seed, b []byte) uint64

Bytes returns the hash of b with the given seed.

Bytes is equivalent to, but more convenient and efficient than:

var h Hash
h.SetSeed(seed)
h.Write(b)
return h.Sum64()

func String

func String(seed Seed, s string) uint64

String returns the hash of s with the given seed.

String is equivalent to, but more convenient and efficient than:

var h Hash
h.SetSeed(seed)
h.WriteString(s)
return h.Sum64()

Types

type Hash

type Hash struct {
	// contains filtered or unexported fields
}

A Hash computes a seeded hash of a byte sequence.

The zero Hash is a valid Hash ready to use. A zero Hash chooses a random seed for itself during the first call to a Reset, Write, Seed, or Sum64 method. For control over the seed, use SetSeed.

The computed hash values depend only on the initial seed and the sequence of bytes provided to the Hash object, not on the way in which the bytes are provided. For example, the three sequences

h.Write([]byte{'f','o','o'})
h.WriteByte('f'); h.WriteByte('o'); h.WriteByte('o')
h.WriteString("foo")

all have the same effect.

Hashes are intended to be collision-resistant, even for situations where an adversary controls the byte sequences being hashed.

A Hash is not safe for concurrent use by multiple goroutines, but a Seed is. If multiple goroutines must compute the same seeded hash, each can declare its own Hash and call SetSeed with a common Seed.

func (*Hash) BlockSize

func (h *Hash) BlockSize() int

BlockSize returns h's block size.

func (*Hash) Reset

func (h *Hash) Reset()

Reset discards all bytes added to h. (The seed remains the same.)

func (*Hash) Seed

func (h *Hash) Seed() Seed

Seed returns h's seed value.

func (*Hash) SetSeed

func (h *Hash) SetSeed(seed Seed)

SetSeed sets h to use seed, which must have been returned by MakeSeed or by another Hash.Seed method. Two Hash objects with the same seed behave identically. Two Hash objects with different seeds will very likely behave differently. Any bytes added to h before this call will be discarded.

func (*Hash) Size

func (h *Hash) Size() int

Size returns h's hash value size, 8 bytes.

func (*Hash) Sum

func (h *Hash) Sum(b []byte) []byte

Sum appends the hash's current 64-bit value to b. It exists for implementing hash.Hash. For direct calls, it is more efficient to use Hash.Sum64.

func (*Hash) Sum64

func (h *Hash) Sum64() uint64

Sum64 returns h's current 64-bit value, which depends on h's seed and the sequence of bytes added to h since the last call to Hash.Reset or Hash.SetSeed.

All bits of the Sum64 result are close to uniformly and independently distributed, so it can be safely reduced by using bit masking, shifting, or modular arithmetic.

func (*Hash) Write

func (h *Hash) Write(b []byte) (int, error)

Write adds b to the sequence of bytes hashed by h. It always writes all of b and never fails; the count and error result are for implementing io.Writer.

func (*Hash) WriteByte

func (h *Hash) WriteByte(b byte) error

WriteByte adds b to the sequence of bytes hashed by h. It never fails; the error result is for implementing io.ByteWriter.

func (*Hash) WriteString

func (h *Hash) WriteString(s string) (int, error)

WriteString adds the bytes of s to the sequence of bytes hashed by h. It always writes all of s and never fails; the count and error result are for implementing io.StringWriter.

type Seed

type Seed struct {
	// contains filtered or unexported fields
}

A Seed is a random value that selects the specific hash function computed by a Hash. If two Hashes use the same Seeds, they will compute the same hash values for any given input. If two Hashes use different Seeds, they are very likely to compute distinct hash values for any given input.

A Seed must be initialized by calling MakeSeed. The zero seed is uninitialized and not valid for use with Hash's SetSeed method.

Each Seed value is local to a single process and cannot be serialized or otherwise recreated in a different process.

func MakeSeed

func MakeSeed() Seed

MakeSeed returns a new random seed.