xmath += CeilLog2, FloorLog2

CeilLog2(x)	= min i: 2^i >= x
FloorLog2(x)	= max i: 2^i <= x

xmath/math19.go

@@ -34,5 +34,30 @@ func CeilPow2(x uint64) uint64 {
 	}
 }
 
+// CeilLog2 returns minimal i: 2^i >= x
+func CeilLog2(x uint64) int {
+	switch bits.OnesCount64(x) {
+	case 0:
+		return 0
+	case 1:
+		return bits.Len64(x) - 1
+	default:
+		return bits.Len64(x)
+	}
+}
+
+// FloorLog2 returns maximal i: 2^i <= x
+//
+// x=0 gives -> -1.
+func FloorLog2(x uint64) int {
+	switch bits.OnesCount64(x) {
+	case 0:
+		return -1
+	default:
+		return bits.Len64(x) - 1
+	}
+}
+
+
 // XXX if needed: NextPow2 (y > x, such that y = 2^i) is
 //	1 << bits.Len64(x)

xmath/math_test.go

@@ -21,38 +21,64 @@ import (
 	"testing"
 )
 
-func TestCeilPow2(t *testing.T) {
-	testv := []struct {in, out uint64} {
-		{0, 0},
-		{1, 1},
-		{2, 2},
-		{3, 4},
-		{4, 4},
-		{5, 8},
-		{5, 8},
-		{6, 8},
-		{7, 8},
-		{8, 8},
-		{9, 16},
-		{10, 16},
-		{11, 16},
-		{12, 16},
-		{13, 16},
-		{14, 16},
-		{15, 16},
-		{16, 16},
-		{1<<62 - 1, 1<<62},
-		{1<<62, 1<<62},
-		{1<<62+1, 1<<63},
-		{1<<63 - 1, 1<<63},
-		{1<<63, 1<<63},
+func TestPow2(t *testing.T) {
+	testv := []struct {x, xcpow2 uint64; xclog2 int} {
+		{0, 0, 0},
+		{1, 1, 0},
+		{2, 2, 1},
+		{3, 4, 2},
+		{4, 4, 2},
+		{5, 8, 3},
+		{5, 8, 3},
+		{6, 8, 3},
+		{7, 8, 3},
+		{8, 8, 3},
+		{9, 16, 4},
+		{10, 16, 4},
+		{11, 16, 4},
+		{12, 16, 4},
+		{13, 16, 4},
+		{14, 16, 4},
+		{15, 16, 4},
+		{16, 16, 4},
+		{1<<62 - 1, 1<<62, 62},
+		{1<<62, 1<<62, 62},
+		{1<<62+1, 1<<63, 63},
+		{1<<63 - 1, 1<<63, 63},
+		{1<<63, 1<<63, 63},
 	}
 
 	for _, tt := range testv {
-		out := CeilPow2(tt.in)
-		if out != tt.out {
-			t.Errorf("CeilPow2(%v) -> %v  ; want %v", tt.in, out, tt.out)
+		xcpow2 := CeilPow2(tt.x)
+		if xcpow2 != tt.xcpow2 {
+			t.Errorf("CeilPow2(%v) -> %v  ; want %v", tt.x, xcpow2, tt.xcpow2)
+		}
+
+		xclog2 := CeilLog2(tt.xcpow2)
+		if xclog2 != tt.xclog2 {
+			t.Errorf("CeilLog2(%v) -> %v  ; want %v", tt.xcpow2, xclog2, tt.xclog2)
 		}
-	}
 
+		xclog2 = CeilLog2(tt.x)
+		if xclog2 != tt.xclog2 {
+			t.Errorf("CeilLog2(%v) -> %v  ; want %v", tt.x, xclog2, tt.xclog2)
+		}
+
+		xflog2 := FloorLog2(tt.xcpow2)
+		xflog2Ok := tt.xclog2
+		if tt.x == 0 {
+			xflog2Ok = -1
+		}
+		if xflog2 != xflog2Ok {
+			t.Errorf("FloorLog2(%v) -> %v  ; want %v", tt.xcpow2, xflog2, xflog2Ok)
+		}
+
+		if tt.x != tt.xcpow2 {
+			xflog2Ok--
+		}
+		xflog2 = FloorLog2(tt.x)
+		if xflog2 != xflog2Ok {
+			t.Errorf("FloorLog2(%v) -> %v  ; want %v", tt.x, xflog2, xflog2Ok)
+		}
+	}
 }