strconv: fix %.1f, 0.09

Fixes #822.
Credit to https://golang.org/cl/1442041 by danielfleischman

R=rsc
CC=golang-dev
https://golang.org/cl/1738047

src/pkg/strconv/atof_test.go

@@ -74,8 +74,10 @@ var atoftests = []atofTest{
 	atofTest{"1e-322", "1e-322", nil},
 	// smallest denormal
 	atofTest{"5e-324", "5e-324", nil},
+	atofTest{"4e-324", "5e-324", nil},
+	atofTest{"3e-324", "5e-324", nil},
 	// too small
-	atofTest{"4e-324", "0", nil},
+	atofTest{"2e-324", "0", nil},
 	// way too small
 	atofTest{"1e-350", "0", nil},
 	atofTest{"1e-400000", "0", nil},

src/pkg/strconv/decimal.go

@@ -289,11 +289,11 @@ func (a *decimal) Shift(k int) *decimal {
 
 // If we chop a at nd digits, should we round up?
 func shouldRoundUp(a *decimal, nd int) bool {
-	if nd <= 0 || nd >= a.nd {
+	if nd < 0 || nd >= a.nd {
 		return false
 	}
 	if a.d[nd] == '5' && nd+1 == a.nd { // exactly halfway - round to even
-		return (a.d[nd-1]-'0')%2 != 0
+		return nd > 0 && (a.d[nd-1]-'0')%2 != 0
 	}
 	// not halfway - digit tells all
 	return a.d[nd] >= '5'
@@ -301,8 +301,11 @@ func shouldRoundUp(a *decimal, nd int) bool {
 
 // Round a to nd digits (or fewer).
 // Returns receiver for convenience.
+// If nd is zero, it means we're rounding
+// just to the left of the digits, as in
+// 0.09 -> 0.1.
 func (a *decimal) Round(nd int) *decimal {
-	if nd <= 0 || nd >= a.nd {
+	if nd < 0 || nd >= a.nd {
 		return a
 	}
 	if shouldRoundUp(a, nd) {
@@ -314,7 +317,7 @@ func (a *decimal) Round(nd int) *decimal {
 // Round a down to nd digits (or fewer).
 // Returns receiver for convenience.
 func (a *decimal) RoundDown(nd int) *decimal {
-	if nd <= 0 || nd >= a.nd {
+	if nd < 0 || nd >= a.nd {
 		return a
 	}
 	a.nd = nd
@@ -325,7 +328,7 @@ func (a *decimal) RoundDown(nd int) *decimal {
 // Round a up to nd digits (or fewer).
 // Returns receiver for convenience.
 func (a *decimal) RoundUp(nd int) *decimal {
-	if nd <= 0 || nd >= a.nd {
+	if nd < 0 || nd >= a.nd {
 		return a
 	}
 

src/pkg/strconv/ftoa_test.go

@@ -108,6 +108,16 @@ var ftoatests = []ftoaTest{
 	ftoaTest{-math.Inf(0), 'g', -1, "-Inf"},
 
 	ftoaTest{-1, 'b', -1, "-4503599627370496p-52"},
+
+	// fixed bugs
+	ftoaTest{0.9, 'f', 1, "0.9"},
+	ftoaTest{0.09, 'f', 1, "0.1"},
+	ftoaTest{0.0999, 'f', 1, "0.1"},
+	ftoaTest{0.05, 'f', 1, "0.1"},
+	ftoaTest{0.05, 'f', 0, "0"},
+	ftoaTest{0.5, 'f', 1, "0.5"},
+	ftoaTest{0.5, 'f', 0, "0"},
+	ftoaTest{1.5, 'f', 0, "2"},
 }
 
 func TestFtoa(t *testing.T) {