math/big: permit passing of an *Int to Float.Int to avoid allocation

Change-Id: I50e83248357928e56c94b88a8764de828f4f5c76
Reviewed-on: https://go-review.googlesource.com/5890
Reviewed-by: Alan Donovan <adonovan@google.com>

src/math/big/float.go

@@ -788,13 +788,14 @@ func (x *Float) Float64() (float64, Accuracy) {
 // Int returns the result of truncating x towards zero; or nil
 // if x is an infinity. The result is Exact if x.IsInt();
 // otherwise it is Below for x > 0, and Above for x < 0.
-func (x *Float) Int() (res *Int, acc Accuracy) {
-	// TODO(gri) accept z argument for result storage (see Float.Rat below)
+// If a non-nil *Int argument z is provided, it is used to store
+// the result; otherwise a new Int is allocated.
+func (x *Float) Int(z *Int) (*Int, Accuracy) {
 	if debugFloat {
 		validate(x)
 	}
 	// accuracy for inexact results
-	acc = Below // truncation
+	acc := Below // truncation
 	if x.neg {
 		acc = Above
 	}
@@ -807,7 +808,11 @@ func (x *Float) Int() (res *Int, acc Accuracy) {
 		if len(x.mant) == 0 {
 			acc = Exact // ±0
 		}
-		return new(Int), acc // ±0.xxx
+		// ±0.xxx
+		if z == nil {
+			return new(Int), acc
+		}
+		return z.SetUint64(0), acc
 	}
 	// x.exp > 0
 	// x.mant[len(x.mant)-1] != 0
@@ -818,17 +823,20 @@ func (x *Float) Int() (res *Int, acc Accuracy) {
 		acc = Exact
 	}
 	// shift mantissa as needed
-	res = &Int{neg: x.neg}
+	if z == nil {
+		z = new(Int)
+	}
+	z.neg = x.neg
 	// TODO(gri) should have a shift that takes positive and negative shift counts
 	switch {
 	case exp > allBits:
-		res.abs = res.abs.shl(x.mant, exp-allBits)
+		z.abs = z.abs.shl(x.mant, exp-allBits)
 	default:
-		res.abs = res.abs.set(x.mant)
+		z.abs = z.abs.set(x.mant)
 	case exp < allBits:
-		res.abs = res.abs.shr(x.mant, allBits-exp)
+		z.abs = z.abs.shr(x.mant, allBits-exp)
 	}
-	return
+	return z, acc
 }
 
 // Rat returns x converted into an exact fraction; or nil if x is an infinity.

src/math/big/float_test.go

@@ -714,7 +714,7 @@ func TestFloatInt(t *testing.T) {
 		{"1e+100", "10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000", Exact},
 	} {
 		x := makeFloat(test.x)
-		res, acc := x.Int()
+		res, acc := x.Int(nil)
 		got := "nil"
 		if res != nil {
 			got = res.String()
@@ -723,6 +723,15 @@ func TestFloatInt(t *testing.T) {
 			t.Errorf("%s: got %s (%s); want %s (%s)", test.x, got, acc, test.want, test.acc)
 		}
 	}
+
+	// check that supplied *Int is used
+	for _, f := range []string{"0", "1", "-1", "1234"} {
+		x := makeFloat(f)
+		i := new(Int)
+		if res, _ := x.Int(i); res != i {
+			t.Errorf("(%s).Int is not using supplied *Int", f)
+		}
+	}
 }
 
 func TestFloatRat(t *testing.T) {
@@ -765,7 +774,7 @@ func TestFloatRat(t *testing.T) {
 	}
 
 	// check that supplied *Rat is used
-	for _, f := range []string{"0", "1"} {
+	for _, f := range []string{"0", "1", "-1", "1234"} {
 		x := makeFloat(f)
 		r := new(Rat)
 		if res := x.Rat(r); res != r {