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diff --git a/src/hash/maphash/maphash.go b/src/hash/maphash/maphash.go
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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 "unsafe"
+
+// 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, Sum64, or Seed 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   [64]byte  // unflushed byte buffer
+	n     int       // number of unflushed bytes
+}
+
+// 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 {
+		h.setSeed(MakeSeed())
+	}
+}
+
+// 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)
+	for h.n+len(b) > len(h.buf) {
+		k := copy(h.buf[h.n:], b)
+		h.n = len(h.buf)
+		b = b[k:]
+		h.flush()
+	}
+	h.n += copy(h.buf[h.n:], 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) {
+	size := len(s)
+	for h.n+len(s) > len(h.buf) {
+		k := copy(h.buf[h.n:], s)
+		h.n = len(h.buf)
+		s = s[k:]
+		h.flush()
+	}
+	h.n += copy(h.buf[h.n:], 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'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) {
+	h.setSeed(seed)
+	h.n = 0
+}
+
+// setSeed sets seed without discarding accumulated data.
+func (h *Hash) setSeed(seed Seed) {
+	if seed.s == 0 {
+		panic("maphash: use of uninitialized Seed")
+	}
+	h.seed = seed
+	h.state = seed
+}
+
+// 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.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 {
+	h.initSeed()
+	return rthash(h.buf[: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 {
+			break
+		}
+	}
+	return Seed{s: s1<<32 + s2}
+}
+
+//go:linkname runtime_fastrand runtime.fastrand
+func runtime_fastrand() uint32
+
+func rthash(b []byte, seed uint64) uint64 {
+	if len(b) == 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(&b[0]), uintptr(seed), uintptr(len(b))))
+	}
+	lo := runtime_memhash(unsafe.Pointer(&b[0]), uintptr(seed), uintptr(len(b)))
+	hi := runtime_memhash(unsafe.Pointer(&b[0]), uintptr(seed>>32), uintptr(len(b)))
+	return uint64(hi)<<32 | uint64(lo)
+}
+
+//go:linkname runtime_memhash runtime.memhash
+//go:noescape
+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,
+		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) }