nip45: hyperloglog implementation.

nip45/helpers.go

@@ -0,0 +1,56 @@
+package nip45
+
+import "math"
+
+const two32 = 1 << 32
+
+// Extract bits from uint64 using LSB 0 numbering, including lo.
+func eb(bits uint64, hi uint8, lo uint8) uint64 {
+	m := uint64(((1 << (hi - lo)) - 1) << lo)
+	return (bits & m) >> lo
+}
+
+func linearCounting(m uint32, v uint32) float64 {
+	fm := float64(m)
+	return fm * math.Log(fm/float64(v))
+}
+
+func clz64(x uint64) uint8 {
+	var c uint8
+	for m := uint64(1 << 63); m&x == 0 && m != 0; m >>= 1 {
+		c++
+	}
+	return c
+}
+
+func countZeros(s []uint8) uint32 {
+	var c uint32
+	for _, v := range s {
+		if v == 0 {
+			c++
+		}
+	}
+	return c
+}
+
+func calculateEstimate(s []uint8) float64 {
+	sum := 0.0
+	for _, val := range s {
+		sum += 1.0 / float64(uint64(1)<<val)
+	}
+
+	m := uint32(len(s))
+	fm := float64(m)
+	return alpha(m) * fm * fm / sum
+}
+
+func alpha(m uint32) float64 {
+	if m == 16 {
+		return 0.673
+	} else if m == 32 {
+		return 0.697
+	} else if m == 64 {
+		return 0.709
+	}
+	return 0.7213 / (1 + 1.079/float64(m))
+}