big: fix panic and round correctly in Rat.FloatString

R=gri, rsc
CC=golang-dev
https://golang.org/cl/2212044

src/pkg/big/rat.go

@@ -294,25 +294,34 @@ func (z *Rat) FloatString(prec int) string {
 
 	q, r := nat{}.div(nat{}, z.a.abs, z.b)
 
-	s := q.string(10)
-	if z.a.neg {
-		s = "-" + s
+	p := natOne
+	if prec > 0 {
+		p = nat{}.expNN(natTen, nat{}.setUint64(uint64(prec)), nil)
 	}
 
-	p := nat{}.expNN(natTen, nat{Word(prec)}, nil)
 	r = r.mul(r, p)
 	r, r2 := r.div(nat{}, r, z.b)
 
 	// see if we need to round up
-	r2 = r2.mul(r2, natTwo)
+	r2 = r2.add(r2, r2)
 	if z.b.cmp(r2) <= 0 {
 		r = r.add(r, natOne)
+		if r.cmp(p) >= 0 {
+			q = nat{}.add(q, natOne)
+			r = nat{}.sub(r, p)
+		}
+	}
+
+	s := q.string(10)
+	if z.a.neg {
+		s = "-" + s
 	}
 
+	if prec > 0 {
 		rs := r.string(10)
 		leadingZeros := prec - len(rs)
 		s += "." + strings.Repeat("0", leadingZeros) + rs
-	s = strings.TrimRight(s, "0")
+	}
 
 	return s
 }

src/pkg/big/rat_test.go

@@ -71,6 +71,11 @@ var floatStringTests = []floatStringTest{
 	floatStringTest{".25", 1, "0.3"},
 	floatStringTest{"-1/3", 3, "-0.333"},
 	floatStringTest{"-2/3", 4, "-0.6667"},
+	floatStringTest{"0.96", 1, "1.0"},
+	floatStringTest{"0.999", 2, "1.00"},
+	floatStringTest{"0.9", 0, "1"},
+	floatStringTest{".25", -1, "0"},
+	floatStringTest{".55", -1, "1"},
 }
 
 func TestFloatString(t *testing.T) {