metrics: add linear probabilistic counter

go.mod

@@ -13,6 +13,7 @@ require (
 	github.com/aws/aws-sdk-go-v2 v1.22.2
 	github.com/aws/aws-sdk-go-v2/config v1.18.42
 	github.com/aws/aws-sdk-go-v2/service/s3 v1.42.1
+	github.com/bits-and-blooms/bitset v1.11.0
 	github.com/caddyserver/certmagic v0.19.2
 	github.com/cenkalti/backoff/v4 v4.2.1
 	github.com/cespare/xxhash/v2 v2.2.0

go.sum

@@ -137,6 +137,8 @@ github.com/beorn7/perks v0.0.0-20180321164747-3a771d992973/go.mod h1:Dwedo/Wpr24
 github.com/beorn7/perks v1.0.0/go.mod h1:KWe93zE9D1o94FZ5RNwFwVgaQK1VOXiVxmqh+CedLV8=
 github.com/beorn7/perks v1.0.1 h1:VlbKKnNfV8bJzeqoa4cOKqO6bYr3WgKZxO8Z16+hsOM=
 github.com/beorn7/perks v1.0.1/go.mod h1:G2ZrVWU2WbWT9wwq4/hrbKbnv/1ERSJQ0ibhJ6rlkpw=
+github.com/bits-and-blooms/bitset v1.11.0 h1:RMyy2mBBShArUAhfVRZJ2xyBO58KCBCtZFShw3umo6k=
+github.com/bits-and-blooms/bitset v1.11.0/go.mod h1:7hO7Gc7Pp1vODcmWvKMRA9BNmbv6a/7QIWpPxHddWR8=
 github.com/bmatcuk/doublestar v1.1.1/go.mod h1:UD6OnuiIn0yFxxA2le/rnRU1G4RaI4UvFv1sNto9p6w=
 github.com/bufbuild/buf v1.26.1 h1:+GdU4z2paCmDclnjLv7MqnVi3AGviImlIKhG0MHH9FA=
 github.com/bufbuild/buf v1.26.1/go.mod h1:UMPncXMWgrmIM+0QpwTEwjNr2SA0z2YIVZZsmNflvB4=

pkg/counter/counter.go

@@ -0,0 +1,64 @@
+// Package counter implements linear counter estimator
+package counter
+
+import (
+	"hash/crc32"
+	"math"
+
+	"github.com/bits-and-blooms/bitset"
+)
+
+const (
+	// DefaultCap max capacity for the counter
+	DefaultCap = 1 << 19
+	loadFactor = 4
+)
+
+// Counter implements a simple probabilistic counter estimator with 1% estimation accuracy
+// as described in https://www.waitingforcode.com/big-data-algorithms/cardinality-estimation-linear-probabilistic-counting/read
+type Counter struct {
+	Bits *bitset.BitSet `json:"bits"`
+}
+
+// New creates a counter for the maximum amount unique elements provided
+func New(cap uint) *Counter {
+	return &Counter{
+		// from paper: a load factor (number of unique values/hash table size) much larger
+		// than 1.0 (e.g., 12) can be used for accurate estimation (e.g., 1% of error)
+		Bits: bitset.New(cap / loadFactor),
+	}
+}
+
+// FromBinary unmarshals counter state
+func FromBinary(data []byte) (*Counter, error) {
+	pc := &Counter{
+		Bits: &bitset.BitSet{},
+	}
+	if err := pc.Bits.UnmarshalBinary(data); err != nil {
+		return nil, err
+	}
+	return pc, nil
+}
+
+// ToBinary marshals counter state
+func (c *Counter) ToBinary() ([]byte, error) {
+	return c.Bits.MarshalBinary()
+}
+
+// Reset the counter
+func (c *Counter) Reset() {
+	c.Bits.ClearAll()
+}
+
+// Mark marks key as present in the set
+func (c *Counter) Mark(key string) {
+	hash := crc32.ChecksumIEEE([]byte(key))
+	c.Bits.Set(uint(hash) % c.Bits.Len())
+}
+
+// Count returns an estimate of distinct elements in the set
+func (c *Counter) Count() uint {
+	size := float64(c.Bits.Len())
+	zeros := size - float64(c.Bits.Count())
+	return uint(-1 * size * math.Log(zeros/size))
+}

pkg/counter/counter_test.go

@@ -0,0 +1,77 @@
+package counter_test
+
+import (
+	"fmt"
+	"math"
+	"math/rand"
+	"testing"
+
+	"github.com/google/uuid"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/pomerium/pomerium/pkg/counter"
+)
+
+func stableRandomUUIDs(n int) []string {
+	r := rand.New(rand.NewSource(1234567890))
+	out := make([]string, 0, n)
+	for i := 0; i < n; i++ {
+		u, _ := uuid.NewRandomFromReader(r)
+		out = append(out, u.String())
+	}
+	return out
+}
+
+func TestStableRandomUUIDs(t *testing.T) {
+	t.Parallel()
+
+	assert.Equal(t, stableRandomUUIDs(20), stableRandomUUIDs(20))
+}
+
+func TestCounter(t *testing.T) {
+	t.Parallel()
+
+	limit := 1000
+	n := (limit * 8) / 10
+	for j := 0; j < 20; j++ {
+		t.Run(fmt.Sprint(j), func(t *testing.T) {
+			c := counter.New(uint(limit))
+			for _, id := range stableRandomUUIDs(n) {
+				c.Mark(id)
+			}
+			est := c.Count()
+			assert.LessOrEqual(t, math.Abs(float64(n)-float64(est)), math.Ceil(float64(n)*0.01))
+		})
+	}
+}
+
+func TestSerialize(t *testing.T) {
+	t.Parallel()
+
+	c := counter.New(counter.DefaultCap)
+	for _, id := range stableRandomUUIDs(20) {
+		c.Mark(id)
+	}
+	assert.EqualValues(t, 20, c.Count())
+
+	data, err := c.ToBinary()
+	require.NoError(t, err)
+
+	c2, err := counter.FromBinary(data)
+	require.NoError(t, err)
+
+	assert.EqualValues(t, 20, c2.Count())
+}
+
+func TestReset(t *testing.T) {
+	t.Parallel()
+
+	c := counter.New(counter.DefaultCap)
+	for _, id := range stableRandomUUIDs(20) {
+		c.Mark(id)
+	}
+	assert.EqualValues(t, 20, c.Count())
+	c.Reset()
+	assert.EqualValues(t, 0, c.Count())
+}