math/big: permit passing of (possibly nil) *Float to MantExp to avoid allocation
Change-Id: Ia92eea833283f8b16fa09d4ca1c9cb3bc0eb18a2
Reviewed-on: https://go-review.googlesource.com/5870
Reviewed-by: Rob Pike <r@golang.org>
diff --git a/src/math/big/float.go b/src/math/big/float.go
index 015f164..60e9a2d 100644
--- a/src/math/big/float.go
+++ b/src/math/big/float.go
@@ -203,6 +203,8 @@
// It returns mant and exp satisfying x == mant × 2**exp, with
// the absolute value of mant satisfying 0.5 <= |mant| < 1.0.
// mant has the same precision and rounding mode as x.
+// If a non-nil *Float argument z is provided it is used to
+// store the result mant; otherwise a new Float is allocated.
//
// Special cases are:
//
@@ -210,11 +212,14 @@
// (±Inf).MantExp() = ±Inf, 0
//
// MantExp does not modify x; the result mant is a new Float.
-func (x *Float) MantExp() (mant *Float, exp int) {
- mant = new(Float).Copy(x)
+func (x *Float) MantExp(z *Float) (mant *Float, exp int) {
+ if z == nil {
+ z = new(Float)
+ }
+ mant = z.Copy(x)
if x.exp != infExp {
- mant.exp = 0
exp = int(x.exp)
+ mant.exp = 0 // after reading x.exp (x and mant may be aliases)
}
return
}
diff --git a/src/math/big/float_test.go b/src/math/big/float_test.go
index 00bb309..f7c243e 100644
--- a/src/math/big/float_test.go
+++ b/src/math/big/float_test.go
@@ -147,13 +147,24 @@
} {
x := makeFloat(test.x)
frac := makeFloat(test.frac)
- f, e := x.MantExp()
+ f, e := x.MantExp(nil)
if !feq(f, frac) || e != test.exp {
t.Errorf("%s.MantExp() = %s, %d; want %s, %d", test.x, f.Format('g', 10), e, test.frac, test.exp)
}
}
}
+func TestFloatMantExpAliasing(t *testing.T) {
+ x := makeFloat("0.5p10")
+ z := new(Float)
+ if m, _ := x.MantExp(z); m != z {
+ t.Fatalf("MantExp didn't use supplied *Float")
+ }
+ if _, e := x.MantExp(x); e != 10 {
+ t.Fatalf("MantExp aliasing error: got %d; want 10", e)
+ }
+}
+
func TestFloatSetMantExp(t *testing.T) {
for _, test := range []struct {
frac string
@@ -185,7 +196,7 @@
t.Errorf("SetMantExp(%s, %d) = %s; want %s", test.frac, test.exp, z.Format('g', 10), test.z)
}
// test inverse property
- if z.SetMantExp(want.MantExp()).Cmp(want) != 0 {
+ if z.SetMantExp(want.MantExp(nil)).Cmp(want) != 0 {
t.Errorf("Inverse property not satisfied: got %s; want %s", z.Format('g', 10), test.z)
}
}
