Adding upstream version 1.21.8.upstream/1.21.8

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 786 insertions, 0 deletions

diff --git a/src/runtime/hash_test.go b/src/runtime/hash_test.go
new file mode 100644
index 0000000..6562829
--- /dev/null
+++ b/src/runtime/hash_test.go
@@ -0,0 +1,786 @@
+// 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"
+	"internal/race"
+	"math"
+	"math/rand"
+	. "runtime"
+	"strings"
+	"testing"
+	"unsafe"
+)
+
+func TestMemHash32Equality(t *testing.T) {
+	if *UseAeshash {
+		t.Skip("skipping since AES hash implementation is used")
+	}
+	var b [4]byte
+	r := rand.New(rand.NewSource(1234))
+	seed := uintptr(r.Uint64())
+	for i := 0; i < 100; i++ {
+		randBytes(r, b[:])
+		got := MemHash32(unsafe.Pointer(&b), seed)
+		want := MemHash(unsafe.Pointer(&b), seed, 4)
+		if got != want {
+			t.Errorf("MemHash32(%x, %v) = %v; want %v", b, seed, got, want)
+		}
+	}
+}
+
+func TestMemHash64Equality(t *testing.T) {
+	if *UseAeshash {
+		t.Skip("skipping since AES hash implementation is used")
+	}
+	var b [8]byte
+	r := rand.New(rand.NewSource(1234))
+	seed := uintptr(r.Uint64())
+	for i := 0; i < 100; i++ {
+		randBytes(r, b[:])
+		got := MemHash64(unsafe.Pointer(&b), seed)
+		want := MemHash(unsafe.Pointer(&b), seed, 8)
+		if got != want {
+			t.Errorf("MemHash64(%x, %v) = %v; want %v", b, seed, got, want)
+		}
+	}
+}
+
+// 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 = 50.0
+	collisions := s.n - len(s.m)
+	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 threshold=%f", collisions, expected, stddev, expected+SLOP*(3*stddev+1))
+	}
+}
+
+// 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) {
+	if race.Enabled {
+		t.Skip("Too long for race mode")
+	}
+	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 GOARCH == "wasm" {
+		t.Skip("Too slow on wasm")
+	}
+	if testing.Short() {
+		t.Skip("Skipping in short mode")
+	}
+	if race.Enabled {
+		t.Skip("Too long for race 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")
+	}
+	if race.Enabled {
+		t.Skip("Too long for race 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 GOARCH == "wasm" {
+		t.Skip("Too slow on wasm")
+	}
+	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 GOARCH == "wasm" {
+		t.Skip("Too slow on wasm")
+	}
+	if testing.Short() {
+		t.Skip("Skipping in short mode")
+	}
+	if race.Enabled {
+		t.Skip("Too long for race 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 any
+}
+
+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 GOARCH == "wasm" {
+		t.Skip("Too slow on wasm")
+	}
+	if testing.Short() {
+		t.Skip("Skipping in short mode")
+	}
+	if race.Enabled {
+		t.Skip("Too long for race 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 *= 8.0 // 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) {
+	if race.Enabled {
+		t.Skip("Too long for race mode")
+	}
+	t.Logf("32 bit keys")
+	windowed(t, &Int32Key{})
+	t.Logf("64 bit keys")
+	windowed(t, &Int64Key{})
+	t.Logf("string keys")
+	windowed(t, &BytesKey{make([]byte, 128)})
+}
+func windowed(t *testing.T, k Key) {
+	if GOARCH == "wasm" {
+		t.Skip("Too slow on wasm")
+	}
+	if PtrSize == 4 {
+		// This test tends to be flaky on 32-bit systems.
+		// There's not enough bits in the hash output, so we
+		// expect a nontrivial number of collisions, and it is
+		// often quite a bit higher than expected. See issue 43130.
+		t.Skip("Flaky on 32-bit systems")
+	}
+	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)
+	}
+}
+
+var sink uint64
+
+func BenchmarkAlignedLoad(b *testing.B) {
+	var buf [16]byte
+	p := unsafe.Pointer(&buf[0])
+	var s uint64
+	for i := 0; i < b.N; i++ {
+		s += ReadUnaligned64(p)
+	}
+	sink = s
+}
+
+func BenchmarkUnalignedLoad(b *testing.B) {
+	var buf [16]byte
+	p := unsafe.Pointer(&buf[1])
+	var s uint64
+	for i := 0; i < b.N; i++ {
+		s += ReadUnaligned64(p)
+	}
+	sink = s
+}
+
+func TestCollisions(t *testing.T) {
+	if testing.Short() {
+		t.Skip("Skipping in short mode")
+	}
+	for i := 0; i < 16; i++ {
+		for j := 0; j < 16; j++ {
+			if j == i {
+				continue
+			}
+			var a [16]byte
+			m := make(map[uint16]struct{}, 1<<16)
+			for n := 0; n < 1<<16; n++ {
+				a[i] = byte(n)
+				a[j] = byte(n >> 8)
+				m[uint16(BytesHash(a[:], 0))] = struct{}{}
+			}
+			// N balls in N bins, for N=65536
+			avg := 41427
+			stdDev := 123
+			if len(m) < avg-40*stdDev || len(m) > avg+40*stdDev {
+				t.Errorf("bad number of collisions i=%d j=%d outputs=%d out of 65536\n", i, j, len(m))
+			}
+		}
+	}
+}