Preview: maphash.go
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/proc/thread-self/root/opt/golang/1.22.0/src/hash/maphash/maphash.go
// 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
// 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
}
// 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 Bytes(seed Seed, b []byte) uint64 {
state := seed.s
if state == 0 {
panic("maphash: use of uninitialized Seed")
}
if len(b) > bufSize {
b = b[:len(b):len(b)] // merge len and cap calculations when reslicing
for len(b) > bufSize {
state = rthash(b[:bufSize], state)
b = b[bufSize:]
}
}
return rthash(b, state)
}
// 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()
func String(seed Seed, s string) uint64 {
state := seed.s
if state == 0 {
panic("maphash: use of uninitialized Seed")
}
for len(s) > bufSize {
state = rthashString(s[:bufSize], state)
s = s[bufSize:]
}
return rthashString(s, state)
}
// 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.flush()
}
h.buf[h.n] = b
h.n++
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:]
h.flush()
// 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 {
h.initSeed()
for len(b) > bufSize {
h.state.s = rthash(b[: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:]
h.flush()
}
if len(s) > bufSize {
h.initSeed()
for len(s) > bufSize {
h.state.s = rthashString(s[: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 {
h.initSeed()
return h.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 (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.initSeed()
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.initSeed()
h.state.s = rthash(h.buf[: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 [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 (h *Hash) Sum64() uint64 {
h.initSeed()
return rthash(h.buf[:h.n], h.state.s)
}
// MakeSeed returns a new random seed.
func MakeSeed() Seed {
var s uint64
for {
s = randUint64()
// We use seed 0 to indicate an uninitialized seed/hash,
// so keep trying until we get a non-zero seed.
if s != 0 {
break
}
}
return Seed{s: s}
}
// 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 (h *Hash) Sum(b []byte) []byte {
x := h.Sum64()
return append(b,
byte(x>>0),
byte(x>>8),
byte(x>>16),
byte(x>>24),
byte(x>>32),
byte(x>>40),
byte(x>>48),
byte(x>>56))
}
// 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) }
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