testing: fix rounding error in roundDown10

Fixes #5599.

Thanks to minux.ma for the suggested fix.

As we now have a harness to test testing internal functions I added some coverage for testing.roundUp, as it is the main consumer of roundDown10.

R=minux.ma, kr, r
CC=golang-dev
https://golang.org/cl/9926043

src/pkg/testing/benchmark.go

@@ -138,7 +138,7 @@ func max(x, y int) int {
 func roundDown10(n int) int {
 	var tens = 0
 	// tens = floor(log_10(n))
-	for n > 10 {
+	for n >= 10 {
 		n = n / 10
 		tens++
 	}
@@ -153,14 +153,17 @@ func roundDown10(n int) int {
 // roundUp rounds x up to a number of the form [1eX, 2eX, 5eX].
 func roundUp(n int) int {
 	base := roundDown10(n)
-	if n < (2 * base) {
+	switch {
+	case n <= base:
+		return base
+	case n <= (2 * base):
 		return 2 * base
-	}
+	case n <= (5 * base):
-	if n < (5 * base) {
 		return 5 * base
-	}
+	default:
 		return 10 * base
 	}
+}
 
 // run times the benchmark function in a separate goroutine.
 func (b *B) run() BenchmarkResult {

src/pkg/testing/benchmark_test.go

@@ -13,19 +13,46 @@ var roundDownTests = []struct {
 }{
 	{1, 1},
 	{9, 1},
-	{10, 1},
+	{10, 10},
 	{11, 10},
-	{100, 10},
+	{100, 100},
-	//	{101, 100}, // issue 5599
+	{101, 100},
-	{1000, 100},
+	{999, 100},
-	//	{1001, 1000}, // issue 5599
+	{1000, 1000},
+	{1001, 1000},
 }
 
 func TestRoundDown10(t *testing.T) {
 	for _, tt := range roundDownTests {
 		actual := testing.RoundDown10(tt.v)
 		if tt.expected != actual {
-			t.Errorf("roundDown10: expected %v, actual %v", tt.expected, actual)
+			t.Errorf("roundDown10(%d): expected %d, actual %d", tt.v, tt.expected, actual)
+		}
+	}
+}
+
+var roundUpTests = []struct {
+	v, expected int
+}{
+	{0, 1},
+	{1, 1},
+	{2, 2},
+	{5, 5},
+	{9, 10},
+	{999, 1000},
+	{1000, 1000},
+	{1400, 2000},
+	{1700, 2000},
+	{4999, 5000},
+	{5000, 5000},
+	{5001, 10000},
+}
+
+func TestRoundUp(t *testing.T) {
+	for _, tt := range roundUpTests {
+		actual := testing.RoundUp(tt.v)
+		if tt.expected != actual {
+			t.Errorf("roundUp(%d): expected %d, actual %d", tt.v, tt.expected, actual)
 		}
 	}
 }

src/pkg/testing/export_test.go

@@ -4,4 +4,7 @@
 
 package testing
 
-var RoundDown10 = roundDown10
+var (
+	RoundDown10 = roundDown10
+	RoundUp     = roundUp
+)