src/runtime/hash_test.go - go - Git at Google

 // Copyright 2013 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 runtime_test

 import (
 	"fmt"
 	"math"
 	"math/rand"
 	. "runtime"
 	"strings"
 	"testing"
 )

 // Smhasher is a torture test for hash functions.
 // https://code.google.com/p/smhasher/
 // This code is a port of some of the Smhasher tests to Go.
 //
 // The current AES hash function passes Smhasher.  Our fallback
 // hash functions don't, so we only enable the difficult tests when
 // we know the AES implementation is available.

 // Sanity checks.
 // hash should not depend on values outside key.
 // hash should not depend on alignment.
 func TestSmhasherSanity(t *testing.T) {
 	r := rand.New(rand.NewSource(1234))
 	const REP = 10
 	const KEYMAX = 128
 	const PAD = 16
 	const OFFMAX = 16
 	for k := 0; k < REP; k++ {
 		for n := 0; n < KEYMAX; n++ {
 			for i := 0; i < OFFMAX; i++ {
 				var b [KEYMAX + OFFMAX + 2*PAD]byte
 				var c [KEYMAX + OFFMAX + 2*PAD]byte
 				randBytes(r, b[:])
 				randBytes(r, c[:])
 				copy(c[PAD+i:PAD+i+n], b[PAD:PAD+n])
 				if BytesHash(b[PAD:PAD+n], 0) != BytesHash(c[PAD+i:PAD+i+n], 0) {
 					t.Errorf("hash depends on bytes outside key")
 				}
 			}
 		}
 	}
 }

 type HashSet struct {
 	m map[uintptr]struct{} // set of hashes added
 	n int                  // number of hashes added
 }

 func newHashSet() *HashSet {
 	return &HashSet{make(map[uintptr]struct{}), 0}
 }
 func (s *HashSet) add(h uintptr) {
 	s.m[h] = struct{}{}
 	s.n++
 }
 func (s *HashSet) addS(x string) {
 	s.add(StringHash(x, 0))
 }
 func (s *HashSet) addB(x []byte) {
 	s.add(BytesHash(x, 0))
 }
 func (s *HashSet) addS_seed(x string, seed uintptr) {
 	s.add(StringHash(x, seed))
 }
 func (s *HashSet) check(t *testing.T) {
 	const SLOP = 10.0
 	collisions := s.n - len(s.m)
 	//fmt.Printf("%d/%d\n", len(s.m), s.n)
 	pairs := int64(s.n) * int64(s.n-1) / 2
 	expected := float64(pairs) / math.Pow(2.0, float64(hashSize))
 	stddev := math.Sqrt(expected)
 	if float64(collisions) > expected+SLOP*(3*stddev+1) {
 		t.Errorf("unexpected number of collisions: got=%d mean=%f stddev=%f", collisions, expected, stddev)
 	}
 }

 // a string plus adding zeros must make distinct hashes
 func TestSmhasherAppendedZeros(t *testing.T) {
 	s := "hello" + strings.Repeat("\x00", 256)
 	h := newHashSet()
 	for i := 0; i <= len(s); i++ {
 		h.addS(s[:i])
 	}
 	h.check(t)
 }

 // All 0-3 byte strings have distinct hashes.
 func TestSmhasherSmallKeys(t *testing.T) {
 	h := newHashSet()
 	var b [3]byte
 	for i := 0; i < 256; i++ {
 		b[0] = byte(i)
 		h.addB(b[:1])
 		for j := 0; j < 256; j++ {
 			b[1] = byte(j)
 			h.addB(b[:2])
 			if !testing.Short() {
 				for k := 0; k < 256; k++ {
 					b[2] = byte(k)
 					h.addB(b[:3])
 				}
 			}
 		}
 	}
 	h.check(t)
 }

 // Different length strings of all zeros have distinct hashes.
 func TestSmhasherZeros(t *testing.T) {
 	N := 256 * 1024
 	if testing.Short() {
 		N = 1024
 	}
 	h := newHashSet()
 	b := make([]byte, N)
 	for i := 0; i <= N; i++ {
 		h.addB(b[:i])
 	}
 	h.check(t)
 }

 // Strings with up to two nonzero bytes all have distinct hashes.
 func TestSmhasherTwoNonzero(t *testing.T) {
 	if testing.Short() {
 		t.Skip("Skipping in short mode")
 	}
 	h := newHashSet()
 	for n := 2; n <= 16; n++ {
 		twoNonZero(h, n)
 	}
 	h.check(t)
 }
 func twoNonZero(h *HashSet, n int) {
 	b := make([]byte, n)

 	// all zero
 	h.addB(b[:])

 	// one non-zero byte
 	for i := 0; i < n; i++ {
 		for x := 1; x < 256; x++ {
 			b[i] = byte(x)
 			h.addB(b[:])
 			b[i] = 0
 		}
 	}

 	// two non-zero bytes
 	for i := 0; i < n; i++ {
 		for x := 1; x < 256; x++ {
 			b[i] = byte(x)
 			for j := i + 1; j < n; j++ {
 				for y := 1; y < 256; y++ {
 					b[j] = byte(y)
 					h.addB(b[:])
 					b[j] = 0
 				}
 			}
 			b[i] = 0
 		}
 	}
 }

 // Test strings with repeats, like "abcdabcdabcdabcd..."
 func TestSmhasherCyclic(t *testing.T) {
 	if testing.Short() {
 		t.Skip("Skipping in short mode")
 	}
 	r := rand.New(rand.NewSource(1234))
 	const REPEAT = 8
 	const N = 1000000
 	for n := 4; n <= 12; n++ {
 		h := newHashSet()
 		b := make([]byte, REPEAT*n)
 		for i := 0; i < N; i++ {
 			b[0] = byte(i * 79 % 97)
 			b[1] = byte(i * 43 % 137)
 			b[2] = byte(i * 151 % 197)
 			b[3] = byte(i * 199 % 251)
 			randBytes(r, b[4:n])
 			for j := n; j < n*REPEAT; j++ {
 				b[j] = b[j-n]
 			}
 			h.addB(b)
 		}
 		h.check(t)
 	}
 }

 // Test strings with only a few bits set
 func TestSmhasherSparse(t *testing.T) {
 	if testing.Short() {
 		t.Skip("Skipping in short mode")
 	}
 	sparse(t, 32, 6)
 	sparse(t, 40, 6)
 	sparse(t, 48, 5)
 	sparse(t, 56, 5)
 	sparse(t, 64, 5)
 	sparse(t, 96, 4)
 	sparse(t, 256, 3)
 	sparse(t, 2048, 2)
 }
 func sparse(t *testing.T, n int, k int) {
 	b := make([]byte, n/8)
 	h := newHashSet()
 	setbits(h, b, 0, k)
 	h.check(t)
 }

 // set up to k bits at index i and greater
 func setbits(h *HashSet, b []byte, i int, k int) {
 	h.addB(b)
 	if k == 0 {
 		return
 	}
 	for j := i; j < len(b)*8; j++ {
 		b[j/8] |= byte(1 << uint(j&7))
 		setbits(h, b, j+1, k-1)
 		b[j/8] &= byte(^(1 << uint(j&7)))
 	}
 }

 // Test all possible combinations of n blocks from the set s.
 // "permutation" is a bad name here, but it is what Smhasher uses.
 func TestSmhasherPermutation(t *testing.T) {
 	if testing.Short() {
 		t.Skip("Skipping in short mode")
 	}
 	permutation(t, []uint32{0, 1, 2, 3, 4, 5, 6, 7}, 8)
 	permutation(t, []uint32{0, 1 << 29, 2 << 29, 3 << 29, 4 << 29, 5 << 29, 6 << 29, 7 << 29}, 8)
 	permutation(t, []uint32{0, 1}, 20)
 	permutation(t, []uint32{0, 1 << 31}, 20)
 	permutation(t, []uint32{0, 1, 2, 3, 4, 5, 6, 7, 1 << 29, 2 << 29, 3 << 29, 4 << 29, 5 << 29, 6 << 29, 7 << 29}, 6)
 }
 func permutation(t *testing.T, s []uint32, n int) {
 	b := make([]byte, n*4)
 	h := newHashSet()
 	genPerm(h, b, s, 0)
 	h.check(t)
 }
 func genPerm(h *HashSet, b []byte, s []uint32, n int) {
 	h.addB(b[:n])
 	if n == len(b) {
 		return
 	}
 	for _, v := range s {
 		b[n] = byte(v)
 		b[n+1] = byte(v >> 8)
 		b[n+2] = byte(v >> 16)
 		b[n+3] = byte(v >> 24)
 		genPerm(h, b, s, n+4)
 	}
 }

 type Key interface {
 	clear()              // set bits all to 0
 	random(r *rand.Rand) // set key to something random
 	bits() int           // how many bits key has
 	flipBit(i int)       // flip bit i of the key
 	hash() uintptr       // hash the key
 	name() string        // for error reporting
 }

 type BytesKey struct {
 	b []byte
 }

 func (k *BytesKey) clear() {
 	for i := range k.b {
 		k.b[i] = 0
 	}
 }
 func (k *BytesKey) random(r *rand.Rand) {
 	randBytes(r, k.b)
 }
 func (k *BytesKey) bits() int {
 	return len(k.b) * 8
 }
 func (k *BytesKey) flipBit(i int) {
 	k.b[i>>3] ^= byte(1 << uint(i&7))
 }
 func (k *BytesKey) hash() uintptr {
 	return BytesHash(k.b, 0)
 }
 func (k *BytesKey) name() string {
 	return fmt.Sprintf("bytes%d", len(k.b))
 }

 type Int32Key struct {
 	i uint32
 }

 func (k *Int32Key) clear() {
 	k.i = 0
 }
 func (k *Int32Key) random(r *rand.Rand) {
 	k.i = r.Uint32()
 }
 func (k *Int32Key) bits() int {
 	return 32
 }
 func (k *Int32Key) flipBit(i int) {
 	k.i ^= 1 << uint(i)
 }
 func (k *Int32Key) hash() uintptr {
 	return Int32Hash(k.i, 0)
 }
 func (k *Int32Key) name() string {
 	return "int32"
 }

 type Int64Key struct {
 	i uint64
 }

 func (k *Int64Key) clear() {
 	k.i = 0
 }
 func (k *Int64Key) random(r *rand.Rand) {
 	k.i = uint64(r.Uint32()) + uint64(r.Uint32())<<32
 }
 func (k *Int64Key) bits() int {
 	return 64
 }
 func (k *Int64Key) flipBit(i int) {
 	k.i ^= 1 << uint(i)
 }
 func (k *Int64Key) hash() uintptr {
 	return Int64Hash(k.i, 0)
 }
 func (k *Int64Key) name() string {
 	return "int64"
 }

 type EfaceKey struct {
 	i interface{}
 }

 func (k *EfaceKey) clear() {
 	k.i = nil
 }
 func (k *EfaceKey) random(r *rand.Rand) {
 	k.i = uint64(r.Int63())
 }
 func (k *EfaceKey) bits() int {
 	// use 64 bits.  This tests inlined interfaces
 	// on 64-bit targets and indirect interfaces on
 	// 32-bit targets.
 	return 64
 }
 func (k *EfaceKey) flipBit(i int) {
 	k.i = k.i.(uint64) ^ uint64(1)<<uint(i)
 }
 func (k *EfaceKey) hash() uintptr {
 	return EfaceHash(k.i, 0)
 }
 func (k *EfaceKey) name() string {
 	return "Eface"
 }

 type IfaceKey struct {
 	i interface {
 		F()
 	}
 }
 type fInter uint64

 func (x fInter) F() {
 }

 func (k *IfaceKey) clear() {
 	k.i = nil
 }
 func (k *IfaceKey) random(r *rand.Rand) {
 	k.i = fInter(r.Int63())
 }
 func (k *IfaceKey) bits() int {
 	// use 64 bits.  This tests inlined interfaces
 	// on 64-bit targets and indirect interfaces on
 	// 32-bit targets.
 	return 64
 }
 func (k *IfaceKey) flipBit(i int) {
 	k.i = k.i.(fInter) ^ fInter(1)<<uint(i)
 }
 func (k *IfaceKey) hash() uintptr {
 	return IfaceHash(k.i, 0)
 }
 func (k *IfaceKey) name() string {
 	return "Iface"
 }

 // Flipping a single bit of a key should flip each output bit with 50% probability.
 func TestSmhasherAvalanche(t *testing.T) {
 	if testing.Short() {
 		t.Skip("Skipping in short mode")
 	}
 	avalancheTest1(t, &BytesKey{make([]byte, 2)})
 	avalancheTest1(t, &BytesKey{make([]byte, 4)})
 	avalancheTest1(t, &BytesKey{make([]byte, 8)})
 	avalancheTest1(t, &BytesKey{make([]byte, 16)})
 	avalancheTest1(t, &BytesKey{make([]byte, 32)})
 	avalancheTest1(t, &BytesKey{make([]byte, 200)})
 	avalancheTest1(t, &Int32Key{})
 	avalancheTest1(t, &Int64Key{})
 	avalancheTest1(t, &EfaceKey{})
 	avalancheTest1(t, &IfaceKey{})
 }
 func avalancheTest1(t *testing.T, k Key) {
 	const REP = 100000
 	r := rand.New(rand.NewSource(1234))
 	n := k.bits()

 	// grid[i][j] is a count of whether flipping
 	// input bit i affects output bit j.
 	grid := make([][hashSize]int, n)

 	for z := 0; z < REP; z++ {
 		// pick a random key, hash it
 		k.random(r)
 		h := k.hash()

 		// flip each bit, hash & compare the results
 		for i := 0; i < n; i++ {
 			k.flipBit(i)
 			d := h ^ k.hash()
 			k.flipBit(i)

 			// record the effects of that bit flip
 			g := &grid[i]
 			for j := 0; j < hashSize; j++ {
 				g[j] += int(d & 1)
 				d >>= 1
 			}
 		}
 	}

 	// Each entry in the grid should be about REP/2.
 	// More precisely, we did N = k.bits() * hashSize experiments where
 	// each is the sum of REP coin flips.  We want to find bounds on the
 	// sum of coin flips such that a truly random experiment would have
 	// all sums inside those bounds with 99% probability.
 	N := n * hashSize
 	var c float64
 	// find c such that Prob(mean-c*stddev < x < mean+c*stddev)^N > .9999
 	for c = 0.0; math.Pow(math.Erf(c/math.Sqrt(2)), float64(N)) < .9999; c += .1 {
 	}
 	c *= 4.0 // allowed slack - we don't need to be perfectly random
 	mean := .5 * REP
 	stddev := .5 * math.Sqrt(REP)
 	low := int(mean - c*stddev)
 	high := int(mean + c*stddev)
 	for i := 0; i < n; i++ {
 		for j := 0; j < hashSize; j++ {
 			x := grid[i][j]
 			if x < low || x > high {
 				t.Errorf("bad bias for %s bit %d -> bit %d: %d/%d\n", k.name(), i, j, x, REP)
 			}
 		}
 	}
 }

 // All bit rotations of a set of distinct keys
 func TestSmhasherWindowed(t *testing.T) {
 	windowed(t, &Int32Key{})
 	windowed(t, &Int64Key{})
 	windowed(t, &BytesKey{make([]byte, 128)})
 }
 func windowed(t *testing.T, k Key) {
 	if testing.Short() {
 		t.Skip("Skipping in short mode")
 	}
 	const BITS = 16

 	for r := 0; r < k.bits(); r++ {
 		h := newHashSet()
 		for i := 0; i < 1<<BITS; i++ {
 			k.clear()
 			for j := 0; j < BITS; j++ {
 				if i>>uint(j)&1 != 0 {
 					k.flipBit((j + r) % k.bits())
 				}
 			}
 			h.add(k.hash())
 		}
 		h.check(t)
 	}
 }

 // All keys of the form prefix + [A-Za-z0-9]*N + suffix.
 func TestSmhasherText(t *testing.T) {
 	if testing.Short() {
 		t.Skip("Skipping in short mode")
 	}
 	text(t, "Foo", "Bar")
 	text(t, "FooBar", "")
 	text(t, "", "FooBar")
 }
 func text(t *testing.T, prefix, suffix string) {
 	const N = 4
 	const S = "ABCDEFGHIJKLMNOPQRSTabcdefghijklmnopqrst0123456789"
 	const L = len(S)
 	b := make([]byte, len(prefix)+N+len(suffix))
 	copy(b, prefix)
 	copy(b[len(prefix)+N:], suffix)
 	h := newHashSet()
 	c := b[len(prefix):]
 	for i := 0; i < L; i++ {
 		c[0] = S[i]
 		for j := 0; j < L; j++ {
 			c[1] = S[j]
 			for k := 0; k < L; k++ {
 				c[2] = S[k]
 				for x := 0; x < L; x++ {
 					c[3] = S[x]
 					h.addB(b)
 				}
 			}
 		}
 	}
 	h.check(t)
 }

 // Make sure different seed values generate different hashes.
 func TestSmhasherSeed(t *testing.T) {
 	h := newHashSet()
 	const N = 100000
 	s := "hello"
 	for i := 0; i < N; i++ {
 		h.addS_seed(s, uintptr(i))
 	}
 	h.check(t)
 }

 // size of the hash output (32 or 64 bits)
 const hashSize = 32 + int(^uintptr(0)>>63<<5)

 func randBytes(r *rand.Rand, b []byte) {
 	for i := range b {
 		b[i] = byte(r.Uint32())
 	}
 }

 func benchmarkHash(b *testing.B, n int) {
 	s := strings.Repeat("A", n)

 	for i := 0; i < b.N; i++ {
 		StringHash(s, 0)
 	}
 	b.SetBytes(int64(n))
 }

 func BenchmarkHash5(b *testing.B)     { benchmarkHash(b, 5) }
 func BenchmarkHash16(b *testing.B)    { benchmarkHash(b, 16) }
 func BenchmarkHash64(b *testing.B)    { benchmarkHash(b, 64) }
 func BenchmarkHash1024(b *testing.B)  { benchmarkHash(b, 1024) }
 func BenchmarkHash65536(b *testing.B) { benchmarkHash(b, 65536) }

 func TestArrayHash(t *testing.T) {
 	// Make sure that "" in arrays hash correctly.  The hash
 	// should at least scramble the input seed so that, e.g.,
 	// {"","foo"} and {"foo",""} have different hashes.

 	// If the hash is bad, then all (8 choose 4) = 70 keys
 	// have the same hash. If so, we allocate 70/8 = 8
 	// overflow buckets.  If the hash is good we don't
 	// normally allocate any overflow buckets, and the
 	// probability of even one or two overflows goes down rapidly.
 	// (There is always 1 allocation of the bucket array.  The map
 	// header is allocated on the stack.)
 	f := func() {
 		// Make the key type at most 128 bytes.  Otherwise,
 		// we get an allocation per key.
 		type key [8]string
 		m := make(map[key]bool, 70)

 		// fill m with keys that have 4 "foo"s and 4 ""s.
 		for i := 0; i < 256; i++ {
 			var k key
 			cnt := 0
 			for j := uint(0); j < 8; j++ {
 				if i>>j&1 != 0 {
 					k[j] = "foo"
 					cnt++
 				}
 			}
 			if cnt == 4 {
 				m[k] = true
 			}
 		}
 		if len(m) != 70 {
 			t.Errorf("bad test: (8 choose 4) should be 70, not %d", len(m))
 		}
 	}
 	if n := testing.AllocsPerRun(10, f); n > 6 {
 		t.Errorf("too many allocs %f - hash not balanced", n)
 	}
 }
 func TestStructHash(t *testing.T) {
 	// See the comment in TestArrayHash.
 	f := func() {
 		type key struct {
 			a, b, c, d, e, f, g, h string
 		}
 		m := make(map[key]bool, 70)

 		// fill m with keys that have 4 "foo"s and 4 ""s.
 		for i := 0; i < 256; i++ {
 			var k key
 			cnt := 0
 			if i&1 != 0 {
 				k.a = "foo"
 				cnt++
 			}
 			if i&2 != 0 {
 				k.b = "foo"
 				cnt++
 			}
 			if i&4 != 0 {
 				k.c = "foo"
 				cnt++
 			}
 			if i&8 != 0 {
 				k.d = "foo"
 				cnt++
 			}
 			if i&16 != 0 {
 				k.e = "foo"
 				cnt++
 			}
 			if i&32 != 0 {
 				k.f = "foo"
 				cnt++
 			}
 			if i&64 != 0 {
 				k.g = "foo"
 				cnt++
 			}
 			if i&128 != 0 {
 				k.h = "foo"
 				cnt++
 			}
 			if cnt == 4 {
 				m[k] = true
 			}
 		}
 		if len(m) != 70 {
 			t.Errorf("bad test: (8 choose 4) should be 70, not %d", len(m))
 		}
 	}
 	if n := testing.AllocsPerRun(10, f); n > 6 {
 		t.Errorf("too many allocs %f - hash not balanced", n)
 	}
 }
	// Copyright 2013 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 runtime_test

	import (
	"fmt"
	"math"
	"math/rand"
	. "runtime"
	"strings"
	"testing"
	)

	// Smhasher is a torture test for hash functions.
	// https://code.google.com/p/smhasher/
	// This code is a port of some of the Smhasher tests to Go.
	//
	// The current AES hash function passes Smhasher. Our fallback
	// hash functions don't, so we only enable the difficult tests when
	// we know the AES implementation is available.

	// Sanity checks.
	// hash should not depend on values outside key.
	// hash should not depend on alignment.
	func TestSmhasherSanity(t *testing.T) {
	r := rand.New(rand.NewSource(1234))
	const REP = 10
	const KEYMAX = 128
	const PAD = 16
	const OFFMAX = 16
	for k := 0; k < REP; k++ {
	for n := 0; n < KEYMAX; n++ {
	for i := 0; i < OFFMAX; i++ {
	var b [KEYMAX + OFFMAX + 2*PAD]byte
	var c [KEYMAX + OFFMAX + 2*PAD]byte
	randBytes(r, b[:])
	randBytes(r, c[:])
	copy(c[PAD+i:PAD+i+n], b[PAD:PAD+n])
	if BytesHash(b[PAD:PAD+n], 0) != BytesHash(c[PAD+i:PAD+i+n], 0) {
	t.Errorf("hash depends on bytes outside key")
	}
	}
	}
	}
	}

	type HashSet struct {
	m map[uintptr]struct{} // set of hashes added
	n int // number of hashes added
	}

	func newHashSet() *HashSet {
	return &HashSet{make(map[uintptr]struct{}), 0}
	}
	func (s *HashSet) add(h uintptr) {
	s.m[h] = struct{}{}
	s.n++
	}
	func (s *HashSet) addS(x string) {
	s.add(StringHash(x, 0))
	}
	func (s *HashSet) addB(x []byte) {
	s.add(BytesHash(x, 0))
	}
	func (s *HashSet) addS_seed(x string, seed uintptr) {
	s.add(StringHash(x, seed))
	}
	func (s HashSet) check(t testing.T) {
	const SLOP = 10.0
	collisions := s.n - len(s.m)
	//fmt.Printf("%d/%d\n", len(s.m), s.n)
	pairs := int64(s.n) * int64(s.n-1) / 2
	expected := float64(pairs) / math.Pow(2.0, float64(hashSize))
	stddev := math.Sqrt(expected)
	if float64(collisions) > expected+SLOP(3stddev+1) {
	t.Errorf("unexpected number of collisions: got=%d mean=%f stddev=%f", collisions, expected, stddev)
	}
	}

	// a string plus adding zeros must make distinct hashes
	func TestSmhasherAppendedZeros(t *testing.T) {
	s := "hello" + strings.Repeat("\x00", 256)
	h := newHashSet()
	for i := 0; i <= len(s); i++ {
	h.addS(s[:i])
	}
	h.check(t)
	}

	// All 0-3 byte strings have distinct hashes.
	func TestSmhasherSmallKeys(t *testing.T) {
	h := newHashSet()
	var b [3]byte
	for i := 0; i < 256; i++ {
	b[0] = byte(i)
	h.addB(b[:1])
	for j := 0; j < 256; j++ {
	b[1] = byte(j)
	h.addB(b[:2])
	if !testing.Short() {
	for k := 0; k < 256; k++ {
	b[2] = byte(k)
	h.addB(b[:3])
	}
	}
	}
	}
	h.check(t)
	}

	// Different length strings of all zeros have distinct hashes.
	func TestSmhasherZeros(t *testing.T) {
	N := 256 * 1024
	if testing.Short() {
	N = 1024
	}
	h := newHashSet()
	b := make([]byte, N)
	for i := 0; i <= N; i++ {
	h.addB(b[:i])
	}
	h.check(t)
	}

	// Strings with up to two nonzero bytes all have distinct hashes.
	func TestSmhasherTwoNonzero(t *testing.T) {
	if testing.Short() {
	t.Skip("Skipping in short mode")
	}
	h := newHashSet()
	for n := 2; n <= 16; n++ {
	twoNonZero(h, n)
	}
	h.check(t)
	}
	func twoNonZero(h *HashSet, n int) {
	b := make([]byte, n)

	// all zero
	h.addB(b[:])

	// one non-zero byte
	for i := 0; i < n; i++ {
	for x := 1; x < 256; x++ {
	b[i] = byte(x)
	h.addB(b[:])
	b[i] = 0
	}
	}

	// two non-zero bytes
	for i := 0; i < n; i++ {
	for x := 1; x < 256; x++ {
	b[i] = byte(x)
	for j := i + 1; j < n; j++ {
	for y := 1; y < 256; y++ {
	b[j] = byte(y)
	h.addB(b[:])
	b[j] = 0
	}
	}
	b[i] = 0
	}
	}
	}

	// Test strings with repeats, like "abcdabcdabcdabcd..."
	func TestSmhasherCyclic(t *testing.T) {
	if testing.Short() {
	t.Skip("Skipping in short mode")
	}
	r := rand.New(rand.NewSource(1234))
	const REPEAT = 8
	const N = 1000000
	for n := 4; n <= 12; n++ {
	h := newHashSet()
	b := make([]byte, REPEAT*n)
	for i := 0; i < N; i++ {
	b[0] = byte(i * 79 % 97)
	b[1] = byte(i * 43 % 137)
	b[2] = byte(i * 151 % 197)
	b[3] = byte(i * 199 % 251)
	randBytes(r, b[4:n])
	for j := n; j < n*REPEAT; j++ {
	b[j] = b[j-n]
	}
	h.addB(b)
	}
	h.check(t)
	}
	}

	// Test strings with only a few bits set
	func TestSmhasherSparse(t *testing.T) {
	if testing.Short() {
	t.Skip("Skipping in short mode")
	}
	sparse(t, 32, 6)
	sparse(t, 40, 6)
	sparse(t, 48, 5)
	sparse(t, 56, 5)
	sparse(t, 64, 5)
	sparse(t, 96, 4)
	sparse(t, 256, 3)
	sparse(t, 2048, 2)
	}
	func sparse(t *testing.T, n int, k int) {
	b := make([]byte, n/8)
	h := newHashSet()
	setbits(h, b, 0, k)
	h.check(t)
	}

	// set up to k bits at index i and greater
	func setbits(h *HashSet, b []byte, i int, k int) {
	h.addB(b)
	if k == 0 {
	return
	}
	for j := i; j < len(b)*8; j++ {
	b[j/8] \|= byte(1 << uint(j&7))
	setbits(h, b, j+1, k-1)
	b[j/8] &= byte(^(1 << uint(j&7)))
	}
	}

	// Test all possible combinations of n blocks from the set s.
	// "permutation" is a bad name here, but it is what Smhasher uses.
	func TestSmhasherPermutation(t *testing.T) {
	if testing.Short() {
	t.Skip("Skipping in short mode")
	}
	permutation(t, []uint32{0, 1, 2, 3, 4, 5, 6, 7}, 8)
	permutation(t, []uint32{0, 1 << 29, 2 << 29, 3 << 29, 4 << 29, 5 << 29, 6 << 29, 7 << 29}, 8)
	permutation(t, []uint32{0, 1}, 20)
	permutation(t, []uint32{0, 1 << 31}, 20)
	permutation(t, []uint32{0, 1, 2, 3, 4, 5, 6, 7, 1 << 29, 2 << 29, 3 << 29, 4 << 29, 5 << 29, 6 << 29, 7 << 29}, 6)
	}
	func permutation(t *testing.T, s []uint32, n int) {
	b := make([]byte, n*4)
	h := newHashSet()
	genPerm(h, b, s, 0)
	h.check(t)
	}
	func genPerm(h *HashSet, b []byte, s []uint32, n int) {
	h.addB(b[:n])
	if n == len(b) {
	return
	}
	for _, v := range s {
	b[n] = byte(v)
	b[n+1] = byte(v >> 8)
	b[n+2] = byte(v >> 16)
	b[n+3] = byte(v >> 24)
	genPerm(h, b, s, n+4)
	}
	}

	type Key interface {
	clear() // set bits all to 0
	random(r *rand.Rand) // set key to something random
	bits() int // how many bits key has
	flipBit(i int) // flip bit i of the key
	hash() uintptr // hash the key
	name() string // for error reporting
	}

	type BytesKey struct {
	b []byte
	}

	func (k *BytesKey) clear() {
	for i := range k.b {
	k.b[i] = 0
	}
	}
	func (k BytesKey) random(r rand.Rand) {
	randBytes(r, k.b)
	}
	func (k *BytesKey) bits() int {
	return len(k.b) * 8
	}
	func (k *BytesKey) flipBit(i int) {
	k.b[i>>3] ^= byte(1 << uint(i&7))
	}
	func (k *BytesKey) hash() uintptr {
	return BytesHash(k.b, 0)
	}
	func (k *BytesKey) name() string {
	return fmt.Sprintf("bytes%d", len(k.b))
	}

	type Int32Key struct {
	i uint32
	}

	func (k *Int32Key) clear() {
	k.i = 0
	}
	func (k Int32Key) random(r rand.Rand) {
	k.i = r.Uint32()
	}
	func (k *Int32Key) bits() int {
	return 32
	}
	func (k *Int32Key) flipBit(i int) {
	k.i ^= 1 << uint(i)
	}
	func (k *Int32Key) hash() uintptr {
	return Int32Hash(k.i, 0)
	}
	func (k *Int32Key) name() string {
	return "int32"
	}

	type Int64Key struct {
	i uint64
	}

	func (k *Int64Key) clear() {
	k.i = 0
	}
	func (k Int64Key) random(r rand.Rand) {
	k.i = uint64(r.Uint32()) + uint64(r.Uint32())<<32
	}
	func (k *Int64Key) bits() int {
	return 64
	}
	func (k *Int64Key) flipBit(i int) {
	k.i ^= 1 << uint(i)
	}
	func (k *Int64Key) hash() uintptr {
	return Int64Hash(k.i, 0)
	}
	func (k *Int64Key) name() string {
	return "int64"
	}

	type EfaceKey struct {
	i interface{}
	}

	func (k *EfaceKey) clear() {
	k.i = nil
	}
	func (k EfaceKey) random(r rand.Rand) {
	k.i = uint64(r.Int63())
	}
	func (k *EfaceKey) bits() int {
	// use 64 bits. This tests inlined interfaces
	// on 64-bit targets and indirect interfaces on
	// 32-bit targets.
	return 64
	}
	func (k *EfaceKey) flipBit(i int) {
	k.i = k.i.(uint64) ^ uint64(1)<<uint(i)
	}
	func (k *EfaceKey) hash() uintptr {
	return EfaceHash(k.i, 0)
	}
	func (k *EfaceKey) name() string {
	return "Eface"
	}

	type IfaceKey struct {
	i interface {
	F()
	}
	}
	type fInter uint64

	func (x fInter) F() {
	}

	func (k *IfaceKey) clear() {
	k.i = nil
	}
	func (k IfaceKey) random(r rand.Rand) {
	k.i = fInter(r.Int63())
	}
	func (k *IfaceKey) bits() int {
	// use 64 bits. This tests inlined interfaces
	// on 64-bit targets and indirect interfaces on
	// 32-bit targets.
	return 64
	}
	func (k *IfaceKey) flipBit(i int) {
	k.i = k.i.(fInter) ^ fInter(1)<<uint(i)
	}
	func (k *IfaceKey) hash() uintptr {
	return IfaceHash(k.i, 0)
	}
	func (k *IfaceKey) name() string {
	return "Iface"
	}

	// Flipping a single bit of a key should flip each output bit with 50% probability.
	func TestSmhasherAvalanche(t *testing.T) {
	if testing.Short() {
	t.Skip("Skipping in short mode")
	}
	avalancheTest1(t, &BytesKey{make([]byte, 2)})
	avalancheTest1(t, &BytesKey{make([]byte, 4)})
	avalancheTest1(t, &BytesKey{make([]byte, 8)})
	avalancheTest1(t, &BytesKey{make([]byte, 16)})
	avalancheTest1(t, &BytesKey{make([]byte, 32)})
	avalancheTest1(t, &BytesKey{make([]byte, 200)})
	avalancheTest1(t, &Int32Key{})
	avalancheTest1(t, &Int64Key{})
	avalancheTest1(t, &EfaceKey{})
	avalancheTest1(t, &IfaceKey{})
	}
	func avalancheTest1(t *testing.T, k Key) {
	const REP = 100000
	r := rand.New(rand.NewSource(1234))
	n := k.bits()

	// grid[i][j] is a count of whether flipping
	// input bit i affects output bit j.
	grid := make([][hashSize]int, n)

	for z := 0; z < REP; z++ {
	// pick a random key, hash it
	k.random(r)
	h := k.hash()

	// flip each bit, hash & compare the results
	for i := 0; i < n; i++ {
	k.flipBit(i)
	d := h ^ k.hash()
	k.flipBit(i)

	// record the effects of that bit flip
	g := &grid[i]
	for j := 0; j < hashSize; j++ {
	g[j] += int(d & 1)
	d >>= 1
	}
	}
	}

	// Each entry in the grid should be about REP/2.
	// More precisely, we did N = k.bits() * hashSize experiments where
	// each is the sum of REP coin flips. We want to find bounds on the
	// sum of coin flips such that a truly random experiment would have
	// all sums inside those bounds with 99% probability.
	N := n * hashSize
	var c float64
	// find c such that Prob(mean-cstddev < x < mean+cstddev)^N > .9999
	for c = 0.0; math.Pow(math.Erf(c/math.Sqrt(2)), float64(N)) < .9999; c += .1 {
	}
	c *= 4.0 // allowed slack - we don't need to be perfectly random
	mean := .5 * REP
	stddev := .5 * math.Sqrt(REP)
	low := int(mean - c*stddev)
	high := int(mean + c*stddev)
	for i := 0; i < n; i++ {
	for j := 0; j < hashSize; j++ {
	x := grid[i][j]
	if x < low \|\| x > high {
	t.Errorf("bad bias for %s bit %d -> bit %d: %d/%d\n", k.name(), i, j, x, REP)
	}
	}
	}
	}

	// All bit rotations of a set of distinct keys
	func TestSmhasherWindowed(t *testing.T) {
	windowed(t, &Int32Key{})
	windowed(t, &Int64Key{})
	windowed(t, &BytesKey{make([]byte, 128)})
	}
	func windowed(t *testing.T, k Key) {
	if testing.Short() {
	t.Skip("Skipping in short mode")
	}
	const BITS = 16

	for r := 0; r < k.bits(); r++ {
	h := newHashSet()
	for i := 0; i < 1<<BITS; i++ {
	k.clear()
	for j := 0; j < BITS; j++ {
	if i>>uint(j)&1 != 0 {
	k.flipBit((j + r) % k.bits())
	}
	}
	h.add(k.hash())
	}
	h.check(t)
	}
	}

	// All keys of the form prefix + [A-Za-z0-9]*N + suffix.
	func TestSmhasherText(t *testing.T) {
	if testing.Short() {
	t.Skip("Skipping in short mode")
	}
	text(t, "Foo", "Bar")
	text(t, "FooBar", "")
	text(t, "", "FooBar")
	}
	func text(t *testing.T, prefix, suffix string) {
	const N = 4
	const S = "ABCDEFGHIJKLMNOPQRSTabcdefghijklmnopqrst0123456789"
	const L = len(S)
	b := make([]byte, len(prefix)+N+len(suffix))
	copy(b, prefix)
	copy(b[len(prefix)+N:], suffix)
	h := newHashSet()
	c := b[len(prefix):]
	for i := 0; i < L; i++ {
	c[0] = S[i]
	for j := 0; j < L; j++ {
	c[1] = S[j]
	for k := 0; k < L; k++ {
	c[2] = S[k]
	for x := 0; x < L; x++ {
	c[3] = S[x]
	h.addB(b)
	}
	}
	}
	}
	h.check(t)
	}

	// Make sure different seed values generate different hashes.
	func TestSmhasherSeed(t *testing.T) {
	h := newHashSet()
	const N = 100000
	s := "hello"
	for i := 0; i < N; i++ {
	h.addS_seed(s, uintptr(i))
	}
	h.check(t)
	}

	// size of the hash output (32 or 64 bits)
	const hashSize = 32 + int(^uintptr(0)>>63<<5)

	func randBytes(r *rand.Rand, b []byte) {
	for i := range b {
	b[i] = byte(r.Uint32())
	}
	}

	func benchmarkHash(b *testing.B, n int) {
	s := strings.Repeat("A", n)

	for i := 0; i < b.N; i++ {
	StringHash(s, 0)
	}
	b.SetBytes(int64(n))
	}

	func BenchmarkHash5(b *testing.B) { benchmarkHash(b, 5) }
	func BenchmarkHash16(b *testing.B) { benchmarkHash(b, 16) }
	func BenchmarkHash64(b *testing.B) { benchmarkHash(b, 64) }
	func BenchmarkHash1024(b *testing.B) { benchmarkHash(b, 1024) }
	func BenchmarkHash65536(b *testing.B) { benchmarkHash(b, 65536) }

	func TestArrayHash(t *testing.T) {
	// Make sure that "" in arrays hash correctly. The hash
	// should at least scramble the input seed so that, e.g.,
	// {"","foo"} and {"foo",""} have different hashes.

	// If the hash is bad, then all (8 choose 4) = 70 keys
	// have the same hash. If so, we allocate 70/8 = 8
	// overflow buckets. If the hash is good we don't
	// normally allocate any overflow buckets, and the
	// probability of even one or two overflows goes down rapidly.
	// (There is always 1 allocation of the bucket array. The map
	// header is allocated on the stack.)
	f := func() {
	// Make the key type at most 128 bytes. Otherwise,
	// we get an allocation per key.
	type key [8]string
	m := make(map[key]bool, 70)

	// fill m with keys that have 4 "foo"s and 4 ""s.
	for i := 0; i < 256; i++ {
	var k key
	cnt := 0
	for j := uint(0); j < 8; j++ {
	if i>>j&1 != 0 {
	k[j] = "foo"
	cnt++
	}
	}
	if cnt == 4 {
	m[k] = true
	}
	}
	if len(m) != 70 {
	t.Errorf("bad test: (8 choose 4) should be 70, not %d", len(m))
	}
	}
	if n := testing.AllocsPerRun(10, f); n > 6 {
	t.Errorf("too many allocs %f - hash not balanced", n)
	}
	}
	func TestStructHash(t *testing.T) {
	// See the comment in TestArrayHash.
	f := func() {
	type key struct {
	a, b, c, d, e, f, g, h string
	}
	m := make(map[key]bool, 70)

	// fill m with keys that have 4 "foo"s and 4 ""s.
	for i := 0; i < 256; i++ {
	var k key
	cnt := 0
	if i&1 != 0 {
	k.a = "foo"
	cnt++
	}
	if i&2 != 0 {
	k.b = "foo"
	cnt++
	}
	if i&4 != 0 {
	k.c = "foo"
	cnt++
	}
	if i&8 != 0 {
	k.d = "foo"
	cnt++
	}
	if i&16 != 0 {
	k.e = "foo"
	cnt++
	}
	if i&32 != 0 {
	k.f = "foo"
	cnt++
	}
	if i&64 != 0 {
	k.g = "foo"
	cnt++
	}
	if i&128 != 0 {
	k.h = "foo"
	cnt++
	}
	if cnt == 4 {
	m[k] = true
	}
	}
	if len(m) != 70 {
	t.Errorf("bad test: (8 choose 4) should be 70, not %d", len(m))
	}
	}
	if n := testing.AllocsPerRun(10, f); n > 6 {
	t.Errorf("too many allocs %f - hash not balanced", n)
	}
	}