adding percentile in histogram

z/histogram.go

@@ -149,3 +149,25 @@ func (histogram *HistogramData) String() string {
 	b.WriteString(" --\n")
 	return b.String()
 }
+
+// Percentile returns the percentile value for the histogram.
+// value of p should be between [0.0-1.0]
+func (histogram *HistogramData) Percentile(p float64) float64 {
+	if histogram.Count == 0 {
+		// if no data return the minimum range
+		return histogram.Bounds[0]
+	}
+
+	pval := int64(float64(histogram.Count) * p)
+	for i, v := range histogram.CountPerBucket {
+		pval = pval - v
+		if pval <= 0 {
+			if i == len(histogram.Bounds) {
+				break
+			}
+			return histogram.Bounds[i]
+		}
+	}
+	// default return should be the max range
+	return histogram.Bounds[len(histogram.Bounds) - 1]
+}

z/histogram_test.go

@@ -0,0 +1,80 @@
+package z
+
+import (
+	"github.com/stretchr/testify/require"
+	"math"
+	"testing"
+)
+
+func TestPercentile00(t *testing.T) {
+	size := int(math.Ceil((float64(514) - float64(32)) / float64(4)))
+	bounds := make([]float64, size + 1)
+	for i := range bounds {
+		if i == 0 {
+			bounds[0] = 32
+			continue
+		}
+		if i == size {
+			bounds[i] = 514
+			break
+		}
+		bounds[i] = bounds[i-1] + 4
+	}
+
+	h := NewHistogramData(bounds)
+	for v := 16; v <= 1024; v= v+4 {
+		for i:=0; i < 1000; i++ {
+			h.Update(int64(v))
+		}
+	}
+
+	require.Equal(t, h.Percentile(0.0), 32.0)
+}
+
+func TestPercentile99(t *testing.T) {
+	size := int(math.Ceil((float64(514) - float64(32)) / float64(4)))
+	bounds := make([]float64, size + 1)
+	for i := range bounds {
+		if i == 0 {
+			bounds[0] = 32
+			continue
+		}
+		if i == size {
+			bounds[i] = 514
+			break
+		}
+		bounds[i] = bounds[i-1] + 4
+	}
+	h := NewHistogramData(bounds)
+	for v := 16; v <= 1024; v= v+4 {
+		for i:=0; i < 1000; i++ {
+			h.Update(int64(v))
+		}
+	}
+
+	require.Equal(t, h.Percentile(0.99), 514.0)
+}
+
+func TestPercentile100(t *testing.T) {
+	size := int(math.Ceil((float64(514) - float64(32)) / float64(4)))
+	bounds := make([]float64, size + 1)
+	for i := range bounds {
+		if i == 0 {
+			bounds[0] = 32
+			continue
+		}
+		if i == size {
+			bounds[i] = 514
+			break
+		}
+		bounds[i] = bounds[i-1] + 4
+	}
+	h := NewHistogramData(bounds)
+	for v := 16; v <= 1024; v= v+4 {
+		for i:=0; i < 1000; i++ {
+			h.Update(int64(v))
+		}
+	}
+	require.Equal(t, h.Percentile(1.0), 514.0)
+}
+