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// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// 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.)
package maphash
import (
// 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.
type Seed struct {
s uint64
// 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.
type Hash struct {
_ [0]func() // not comparable
seed Seed // initial seed used for this hash
state Seed // current hash of all flushed bytes
buf [bufSize]byte // unflushed byte buffer
n int // number of unflushed bytes
// bufSize is the size of the Hash write buffer.
// The buffer ensures that writes depend only on the sequence of bytes,
// not the sequence of WriteByte/Write/WriteString calls,
// by always calling rthash with a full buffer (except for the tail).
const bufSize = 128
// initSeed seeds the hash if necessary.
// initSeed is called lazily before any operation that actually uses h.seed/h.state.
// Note that this does not include Write/WriteByte/WriteString in the case
// where they only add to h.buf. (If they write too much, they call h.flush,
// which does call h.initSeed.)
func (h *Hash) initSeed() {
if h.seed.s == 0 {
seed := MakeSeed()
h.seed = seed
h.state = seed
// WriteByte adds b to the sequence of bytes hashed by h.
// It never fails; the error result is for implementing io.ByteWriter.
func (h *Hash) WriteByte(b byte) error {
if h.n == len(h.buf) {
h.buf[h.n] = b
return nil
// 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 (h *Hash) Write(b []byte) (int, error) {
size := len(b)
// Deal with bytes left over in h.buf.
// h.n <= bufSize is always true.
// Checking it is ~free and it lets the compiler eliminate a bounds check.
if h.n > 0 && h.n <= bufSize {
k := copy(h.buf[h.n:], b)
h.n += k
if h.n < bufSize {
// Copied the entirety of b to h.buf.
return size, nil
b = b[k:]
// No need to set h.n = 0 here; it happens just before exit.
// Process as many full buffers as possible, without copying, and calling initSeed only once.
if len(b) > bufSize {
for len(b) > bufSize {
h.state.s = rthash(&b[0], bufSize, h.state.s)
b = b[bufSize:]
// Copy the tail.
copy(h.buf[:], b)
h.n = len(b)
return size, nil
// 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.
func (h *Hash) WriteString(s string) (int, error) {
// WriteString mirrors Write. See Write for comments.
size := len(s)
if h.n > 0 && h.n <= bufSize {
k := copy(h.buf[h.n:], s)
h.n += k
if h.n < bufSize {
return size, nil
s = s[k:]
if len(s) > bufSize {
for len(s) > bufSize {
ptr := (*byte)((*unsafeheader.String)(unsafe.Pointer(&s)).Data)
h.state.s = rthash(ptr, bufSize, h.state.s)
s = s[bufSize:]
copy(h.buf[:], s)
h.n = len(s)
return size, nil
// Seed returns h's seed value.
func (h *Hash) Seed() Seed {
return h.seed
// SetSeed sets h to use seed, which must have been returned by MakeSeed
// or by another Hash's 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 (h *Hash) SetSeed(seed Seed) {
if seed.s == 0 {
panic("maphash: use of uninitialized Seed")
h.seed = seed
h.state = seed
h.n = 0
// Reset discards all bytes added to h.
// (The seed remains the same.)
func (h *Hash) Reset() {
h.state = h.seed
h.n = 0
// precondition: buffer is full.
func (h *Hash) flush() {
if h.n != len(h.buf) {
panic("maphash: flush of partially full buffer")
h.state.s = rthash(&h.buf[0], h.n, h.state.s)
h.n = 0
// 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 Reset or 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 (h *Hash) Sum64() uint64 {
return rthash(&h.buf[0], h.n, h.state.s)
// MakeSeed returns a new random seed.
func MakeSeed() Seed {
var s1, s2 uint64
for {
s1 = uint64(runtime_fastrand())
s2 = uint64(runtime_fastrand())
// We use seed 0 to indicate an uninitialized seed/hash,
// so keep trying until we get a non-zero seed.
if s1|s2 != 0 {
return Seed{s: s1<<32 + s2}
//go:linkname runtime_fastrand runtime.fastrand
func runtime_fastrand() uint32
func rthash(ptr *byte, len int, seed uint64) uint64 {
if len == 0 {
return seed
// The runtime hasher only works on uintptr. For 64-bit
// architectures, we use the hasher directly. Otherwise,
// we use two parallel hashers on the lower and upper 32 bits.
if unsafe.Sizeof(uintptr(0)) == 8 {
return uint64(runtime_memhash(unsafe.Pointer(ptr), uintptr(seed), uintptr(len)))
lo := runtime_memhash(unsafe.Pointer(ptr), uintptr(seed), uintptr(len))
hi := runtime_memhash(unsafe.Pointer(ptr), uintptr(seed>>32), uintptr(len))
return uint64(hi)<<32 | uint64(lo)
//go:linkname runtime_memhash runtime.memhash
func runtime_memhash(p unsafe.Pointer, seed, s uintptr) uintptr
// 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 Sum64.
func (h *Hash) Sum(b []byte) []byte {
x := h.Sum64()
return append(b,
// Size returns h's hash value size, 8 bytes.
func (h *Hash) Size() int { return 8 }
// BlockSize returns h's block size.
func (h *Hash) BlockSize() int { return len(h.buf) }