math/big: replace Float.NewInf with Float.SetInf for more consistent API

Change-Id: I2a60ea4a196eef1af5d2aae6cc239c64bddb6fb2
Reviewed-on: https://go-review.googlesource.com/6301Reviewed-by: Alan Donovan <adonovan@google.com>

src/math/big/float.go

@@ -82,12 +82,6 @@ const (
 	MaxPrec = math.MaxUint32 // largest (theoretically) supported precision; likely memory-limited
 )
 
-// NewInf returns a new infinite Float value with value +Inf (sign >= 0),
-// or -Inf (sign < 0).
-func NewInf(sign int) *Float {
-	return &Float{neg: sign < 0, exp: infExp}
-}
-
 // Accuracy describes the rounding error produced by the most recent
 // operation that generated a Float value, relative to the exact value:
 //
@@ -633,6 +627,17 @@ func (z *Float) SetRat(x *Rat) *Float {
 	return z.Quo(&a, &b)
 }
 
+// SetInf sets z to the infinite Float +Inf for sign >= 0,
+// or -Inf for sign < 0, and returns z. The precision of
+// z is unchanged and the result is always Exact.
+func (z *Float) SetInf(sign int) *Float {
+	z.acc = Exact
+	z.neg = sign < 0
+	z.mant = z.mant[:0]
+	z.exp = infExp
+	return z
+}
+
 // Set sets z to the (possibly rounded) value of x and returns z.
 // If z's precision is 0, it is changed to the precision of x
 // before setting z (and rounding will have no effect).

src/math/big/float_test.go

@@ -90,13 +90,13 @@ func TestFloatZeroValue(t *testing.T) {
 }
 
 func makeFloat(s string) *Float {
+	var x Float
 	if s == "Inf" || s == "+Inf" {
-		return NewInf(+1)
+		return x.SetInf(+1)
 	}
 	if s == "-Inf" {
-		return NewInf(-1)
+		return x.SetInf(-1)
 	}
-	var x Float
 	x.SetPrec(1000)
 	if _, ok := x.SetString(s); !ok {
 		panic(fmt.Sprintf("%q is not a valid float", s))
@@ -694,6 +694,27 @@ func TestFloatSetRat(t *testing.T) {
 	}
 }
 
+func TestFloatSetInf(t *testing.T) {
+	var f Float
+	for _, test := range []struct {
+		sign int
+		prec uint
+		want string
+	}{
+		{0, 0, "+Inf"},
+		{100, 0, "+Inf"},
+		{-1, 0, "-Inf"},
+		{0, 10, "+Inf"},
+		{100, 20, "+Inf"},
+		{-1, 30, "-Inf"},
+	} {
+		x := f.SetPrec(test.prec).SetInf(test.sign)
+		if got := x.String(); got != test.want || x.Prec() != test.prec {
+			t.Errorf("SetInf(%d) = %s (prec = %d); want %s (prec = %d)", test.sign, got, x.Prec(), test.want, test.prec)
+		}
+	}
+}
+
 func TestFloatUint64(t *testing.T) {
 	for _, test := range []struct {
 		x   string