util: add FibHash function

This adds a FibHash function that implements a "fibonacci hash." This is a
simple multiplicative hash that we use occasionally for testing Pebble and would
like to use for similar purposes in CockroachDB as well.

We also add Max and Min functions that take an iter.Seq.

Epic: none
Release note: None

Author: stevendanna

pkg/util/BUILD.bazel

@@ -46,6 +46,7 @@ go_test(
     srcs = [
         "every_n_test.go",
         "fast_int_map_test.go",
+        "hash_test.go",
         "slices_test.go",
         "smalltrace_test.go",
         "strings_test.go",

pkg/util/hash.go

@@ -56,3 +56,12 @@ func (f *FNV64) Add(c uint64) {
 func (f *FNV64) Sum() uint64 {
 	return f.sum
 }
+
+// Fibonacci Hash is a multiplicative hash that maps the given `v` into 2^bits
+// values.
+//
+// Reference:
+//   - https://probablydance.com/2018/06/16/fibonacci-hashing-the-optimization-that-the-world-forgot-or-a-better-alternative-to-integer-modulo/
+func FibHash(v uint64, bits int) uint64 {
+	return (v * 11400714819323198485) >> (64 - bits)
+}

pkg/util/hash_test.go

@@ -0,0 +1,40 @@
+// Copyright 2016 The Cockroach Authors.
+//
+// Use of this software is governed by the CockroachDB Software License
+// included in the /LICENSE file.
+
+package util
+
+import (
+	"testing"
+	"unsafe"
+
+	"github.com/cockroachdb/cockroach/pkg/util/randutil"
+	"github.com/stretchr/testify/require"
+)
+
+func TestFibHash(t *testing.T) {
+	rng, _ := randutil.NewTestRand()
+	counts := make(map[uint64]int)
+	for range 1000 {
+		h := FibHash(rng.Uint64(), 3)
+		counts[h]++
+	}
+
+	// Let's test using a ptr value. We use this slice to keep the pointers alive
+	// so that we don't just get the same pointer each time.
+	vs := make([]*int, 0, 1000)
+	for range 1000 {
+		v := new(int)
+		vs = append(vs, v)
+		h := FibHash(uint64(uintptr(unsafe.Pointer(v))), 3)
+		counts[h]++
+	}
+	require.Equal(t, 1000, len(vs))
+	// We can only make some weak assertions about the distribution here.
+	for k, v := range counts {
+		require.True(t, k < 8)
+		perfectDistributionCount := 2000 / 8
+		require.Less(t, v, perfectDistributionCount*2)
+	}
+}