[Delegations prereq] Add hash bins helpers

Splitting up #175

internal/targets/hash_bins.go

@@ -0,0 +1,82 @@
+package targets
+
+import (
+	"strconv"
+	"strings"
+)
+
+// hexEncode formats x as a hex string, left padded with zeros to padWidth.
+func hexEncode(x uint64, padWidth int) string {
+	// Benchmarked to be more than 10x faster than padding with Sprintf.
+	s := strconv.FormatUint(x, 16)
+	if len(s) >= padWidth {
+		return s
+	}
+	return strings.Repeat("0", padWidth-len(s)) + s
+}
+
+// HashBin represents a hex prefix range. First should be less than Last.
+type HashBin struct {
+	First uint64
+	Last  uint64
+}
+
+// Name returns the of the role that signs for the HashBin.
+func (b HashBin) Name(prefix string, padWidth int) string {
+	if b.First == b.Last {
+		return prefix + hexEncode(b.First, padWidth)
+	}
+
+	return prefix + hexEncode(b.First, padWidth) + "-" + hexEncode(b.Last, padWidth)
+}
+
+// Enumerate returns a slice of hash prefixes in the range from First to Last.
+func (b HashBin) Enumerate(padWidth int) []string {
+	n := int(b.Last - b.First + 1)
+	ret := make([]string, int(n))
+
+	x := b.First
+	for i := 0; i < n; i++ {
+		ret[i] = hexEncode(x, padWidth)
+		x++
+	}
+
+	return ret
+}
+
+// HashPrefixLength returns the width of hash prefixes if there are
+// 2^(log2NumBins) hash bins.
+func HashPrefixLength(log2NumBins uint8) int {
+	if log2NumBins == 0 {
+		// Hash prefix of "" is represented equivalently as "0-f".
+		return 1
+	}
+
+	// ceil(log2NumBins / 4.0)
+	return int((log2NumBins-1)/4) + 1
+}
+
+// GenerateHashBins returns a slice of length 2^(log2NumBins) that partitions
+// the space of path hashes into HashBin ranges.
+func GenerateHashBins(log2NumBins uint8) []HashBin {
+	numBins := uint64(1) << log2NumBins
+
+	// numPrefixes = 16^(HashPrefixLength(log2NumBins))
+	numPrefixes := uint64(1) << (4 * HashPrefixLength(log2NumBins))
+
+	p := make([]HashBin, numBins)
+
+	first := uint64(0)
+	interval := numPrefixes / numBins
+	last := first + interval - 1
+	for i := uint64(0); i < numBins; i++ {
+		p[i] = HashBin{
+			First: first,
+			Last:  last,
+		}
+		first += interval
+		last += interval
+	}
+
+	return p
+}

internal/targets/hash_bins_test.go

@@ -0,0 +1,95 @@
+package targets
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func BenchmarkHexEncode1(b *testing.B) {
+	for n := 0; n <= b.N; n++ {
+		for x := uint64(0); x <= 0xf; x += 1 {
+			hexEncode(x, 1)
+		}
+	}
+}
+
+func BenchmarkHexEncode4(b *testing.B) {
+	for n := 0; n <= b.N; n++ {
+		for x := uint64(0); x <= 0xffff; x += 1 {
+			hexEncode(x, 4)
+		}
+	}
+}
+
+func TestHashBin(t *testing.T) {
+	h := HashBin{
+		First: 0x0,
+		Last:  0xf,
+	}
+	assert.Equal(t, "abc_0-f", h.Name("abc_", 1))
+	assert.Equal(t, "abc_0000-000f", h.Name("abc_", 4))
+	assert.Equal(t, []string{
+		"00", "01", "02", "03", "04", "05", "06", "07",
+		"08", "09", "0a", "0b", "0c", "0d", "0e", "0f",
+	}, h.Enumerate(2))
+
+	h = HashBin{
+		First: 0xcd,
+		Last:  0xce,
+	}
+	assert.Equal(t, "abc_00cd-00ce", h.Name("abc_", 4))
+	assert.Equal(t, []string{"00cd", "00ce"}, h.Enumerate(4))
+
+	h = HashBin{
+		First: 0x0abc,
+		Last:  0xbcde,
+	}
+	assert.Equal(t, "test_0abc-bcde", h.Name("test_", 4))
+}
+
+func TestHashPrefixLength(t *testing.T) {
+	assert.Equal(t, 1, HashPrefixLength(0))
+	assert.Equal(t, 1, HashPrefixLength(1))
+	assert.Equal(t, 1, HashPrefixLength(2))
+	assert.Equal(t, 1, HashPrefixLength(3))
+	assert.Equal(t, 1, HashPrefixLength(4))
+	assert.Equal(t, 2, HashPrefixLength(5))
+	assert.Equal(t, 2, HashPrefixLength(6))
+	assert.Equal(t, 2, HashPrefixLength(7))
+	assert.Equal(t, 2, HashPrefixLength(8))
+	assert.Equal(t, 3, HashPrefixLength(9))
+	assert.Equal(t, 3, HashPrefixLength(10))
+	assert.Equal(t, 3, HashPrefixLength(11))
+	assert.Equal(t, 3, HashPrefixLength(12))
+}
+
+func TestGenerateHashBins(t *testing.T) {
+	tcs := []struct {
+		Log2NumBins uint8
+		BinNames    []string
+	}{
+		{0, []string{"0-f"}},
+		{1, []string{"0-7", "8-f"}},
+		{2, []string{"0-3", "4-7", "8-b", "c-f"}},
+		{3, []string{"0-1", "2-3", "4-5", "6-7", "8-9", "a-b", "c-d", "e-f"}},
+		{4, []string{
+			"0", "1", "2", "3", "4", "5", "6", "7",
+			"8", "9", "a", "b", "c", "d", "e", "f",
+		}},
+		{5, []string{
+			"00-07", "08-0f", "10-17", "18-1f", "20-27", "28-2f", "30-37", "38-3f",
+			"40-47", "48-4f", "50-57", "58-5f", "60-67", "68-6f", "70-77", "78-7f",
+			"80-87", "88-8f", "90-97", "98-9f", "a0-a7", "a8-af", "b0-b7", "b8-bf",
+			"c0-c7", "c8-cf", "d0-d7", "d8-df", "e0-e7", "e8-ef", "f0-f7", "f8-ff",
+		}},
+	}
+	for _, tc := range tcs {
+		bn := []string{}
+		bins := GenerateHashBins(tc.Log2NumBins)
+		for _, b := range bins {
+			bn = append(bn, b.Name("", HashPrefixLength(tc.Log2NumBins)))
+		}
+		assert.Equal(t, tc.BinNames, bn, "GenerateHashBins(%v)", tc.Log2NumBins)
+	}
+}