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math/big: fix Add, Sub when receiver aliases 2nd operand

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Fixes #20490

Change-Id: I9cfa604f9ff94df779cb9b4cbbd706258fc473ac
Reviewed-on: https://go-review.googlesource.com/44150
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Alberto Donizetti 2017-05-25 11:50:40 +02:00
parent 673fdea5e7
commit 1948b7f806
2 changed files with 39 additions and 2 deletions
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13
src/math/big/float.go
View File

@ -1439,8 +1439,16 @@ func (z *Float) Add(x, y *Float) *Float {
if x.form == finite && y.form == finite {
// x + y (common case)
// Below we set z.neg = x.neg, and when z aliases y this will
// change the y operand's sign. This is fine, because if an
// operand aliases the receiver it'll be overwritten, but we still
// want the original x.neg and y.neg values when we evaluate
// x.neg != y.neg, so we need to save y.neg before setting z.neg.
yneg := y.neg
z.neg = x.neg
if x.neg == y.neg {
if x.neg == yneg {
// x + y == x + y
// (-x) + (-y) == -(x + y)
z.uadd(x, y)
@ -1502,8 +1510,9 @@ func (z *Float) Sub(x, y *Float) *Float {
if x.form == finite && y.form == finite {
// x - y (common case)
yneg := y.neg
z.neg = x.neg
if x.neg != y.neg {
if x.neg != yneg {
// x - (-y) == x + y
// (-x) - y == -(x + y)
z.uadd(x, y)

28
src/math/big/float_test.go
View File

@ -1325,6 +1325,34 @@ func TestFloatAdd64(t *testing.T) {
}
}
func TestIssue20490(t *testing.T) {
var tests = []struct {
a, b float64
}{
{4, 1},
{-4, 1},
{4, -1},
{-4, -1},
}
for _, test := range tests {
a, b := NewFloat(test.a), NewFloat(test.b)
diff := new(Float).Sub(a, b)
b.Sub(a, b)
if b.Cmp(diff) != 0 {
t.Errorf("got %g - %g = %g; want %g\n", a, NewFloat(test.b), b, diff)
}
b = NewFloat(test.b)
sum := new(Float).Add(a, b)
b.Add(a, b)
if b.Cmp(sum) != 0 {
t.Errorf("got %g + %g = %g; want %g\n", a, NewFloat(test.b), b, sum)
}
}
}
// TestFloatMul tests Float.Mul/Quo by comparing the result of a "manual"
// multiplication/division of arguments represented by Bits values with the
// respective Float multiplication/division for a variety of precisions