2 files changed, 173 insertions, 0 deletions
diff --git a/internal/statsengine/histogram.go b/internal/statsengine/histogram.go
new file mode 100644
index 0000000..550efe0
--- /dev/null
+++ b/internal/statsengine/histogram.go
@@ -0,0 +1,81 @@
+package statsengine
+
+const histogramBucketCount = 8
+
+type histogram struct {
+	counts [histogramBucketCount]uint64
+	total  uint64
+}
+
+var histogramBoundariesNs = [histogramBucketCount - 1]uint64{
+	1_000,
+	10_000,
+	100_000,
+	1_000_000,
+	10_000_000,
+	100_000_000,
+	1_000_000_000,
+}
+
+var histogramLabels = [histogramBucketCount]string{
+	"[0,1us)",
+	"[1us,10us)",
+	"[10us,100us)",
+	"[100us,1ms)",
+	"[1ms,10ms)",
+	"[10ms,100ms)",
+	"[100ms,1s)",
+	"[1s,+inf)",
+}
+
+func newHistogram() *histogram {
+	return &histogram{}
+}
+
+func (h *histogram) Increment(durationNs uint64) {
+	if h == nil {
+		return
+	}
+
+	idx := histogramBucketIndex(durationNs)
+	h.counts[idx]++
+	h.total++
+}
+
+func (h *histogram) Snapshot() HistogramSnapshot {
+	if h == nil {
+		return NewHistogramSnapshot(0, nil)
+	}
+
+	buckets := make([]HistogramBucketSnapshot, 0, histogramBucketCount)
+	for i := 0; i < histogramBucketCount; i++ {
+		lower, upper := histogramBucketRange(i)
+		buckets = append(buckets, HistogramBucketSnapshot{
+			Label:   histogramLabels[i],
+			LowerNs: lower,
+			UpperNs: upper,
+			Count:   h.counts[i],
+		})
+	}
+
+	return NewHistogramSnapshot(h.total, buckets)
+}
+
+func histogramBucketIndex(durationNs uint64) int {
+	for i, upper := range histogramBoundariesNs {
+		if durationNs < upper {
+			return i
+		}
+	}
+	return histogramBucketCount - 1
+}
+
+func histogramBucketRange(i int) (lower uint64, upper uint64) {
+	if i == 0 {
+		return 0, histogramBoundariesNs[0]
+	}
+	if i == histogramBucketCount-1 {
+		return histogramBoundariesNs[i-1], 0
+	}
+	return histogramBoundariesNs[i-1], histogramBoundariesNs[i]
+}
diff --git a/internal/statsengine/histogram_test.go b/internal/statsengine/histogram_test.go
new file mode 100644
index 0000000..96ad95a
--- /dev/null
+++ b/internal/statsengine/histogram_test.go
@@ -0,0 +1,92 @@
+package statsengine
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestHistogramBucketIndexBoundaries(t *testing.T) {
+	tests := []struct {
+		name string
+		dur  uint64
+		idx  int
+	}{
+		{name: "zero", dur: 0, idx: 0},
+		{name: "just below 1us", dur: 999, idx: 0},
+		{name: "at 1us", dur: 1_000, idx: 1},
+		{name: "at 10us", dur: 10_000, idx: 2},
+		{name: "at 100us", dur: 100_000, idx: 3},
+		{name: "at 1ms", dur: 1_000_000, idx: 4},
+		{name: "at 10ms", dur: 10_000_000, idx: 5},
+		{name: "at 100ms", dur: 100_000_000, idx: 6},
+		{name: "at 1s", dur: 1_000_000_000, idx: 7},
+		{name: "above 1s", dur: 5_000_000_000, idx: 7},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := histogramBucketIndex(tc.dur); got != tc.idx {
+				t.Fatalf("wrong bucket index for %d: got %d want %d", tc.dur, got, tc.idx)
+			}
+		})
+	}
+}
+
+func TestHistogramSnapshotCountsAndRanges(t *testing.T) {
+	h := newHistogram()
+
+	h.Increment(0)
+	h.Increment(1_000)
+	h.Increment(1_000)
+	h.Increment(1_500_000_000)
+
+	snap := h.Snapshot()
+	if snap.Total != 4 {
+		t.Fatalf("wrong total: got %d want 4", snap.Total)
+	}
+
+	buckets := snap.Buckets()
+	if len(buckets) != histogramBucketCount {
+		t.Fatalf("wrong bucket count: got %d want %d", len(buckets), histogramBucketCount)
+	}
+
+	if buckets[0].Count != 1 || buckets[1].Count != 2 || buckets[7].Count != 1 {
+		t.Fatalf("unexpected bucket counts: %+v", buckets)
+	}
+
+	if buckets[0].LowerNs != 0 || buckets[0].UpperNs != 1_000 {
+		t.Fatalf("unexpected first range: %+v", buckets[0])
+	}
+	if buckets[7].LowerNs != 1_000_000_000 || buckets[7].UpperNs != 0 {
+		t.Fatalf("unexpected last range: %+v", buckets[7])
+	}
+}
+
+func TestHistogramSnapshotIsDefensive(t *testing.T) {
+	h := newHistogram()
+	h.Increment(1)
+
+	s1 := h.Snapshot()
+	b1 := s1.Buckets()
+	b1[0].Count = 99
+
+	s2 := h.Snapshot()
+	if got := s2.Buckets()[0].Count; got != 1 {
+		t.Fatalf("snapshot leaked mutable buckets: got %d", got)
+	}
+
+	if reflect.DeepEqual(b1, s2.Buckets()) {
+		t.Fatalf("expected defensive copies of buckets")
+	}
+}
+
+func TestNilHistogramSnapshot(t *testing.T) {
+	var h *histogram
+	s := h.Snapshot()
+	if s.Total != 0 {
+		t.Fatalf("expected zero total, got %d", s.Total)
+	}
+	if got := s.Buckets(); got != nil {
+		t.Fatalf("expected nil buckets, got %#v", got)
+	}
+}