math/big: bug in AndNot(x,y) for x>0,y<0.

The comment says to use (y-1), but then we did add(y.abs, natOne).  We meant sub.

Fixes #9609

Change-Id: I4fe4783326ca082c05588310a0af7895a48fc779
Reviewed-on: https://go-review.googlesource.com/2961
Reviewed-by: Robert Griesemer <gri@golang.org>

diff --git a/src/math/big/int.go b/src/math/big/int.go
index b6c7070d9d974fdc2fbbbd84c0f1be7fff6b981d..38297707436e845a9b72a888dd2bc054e35b18be 100644 (file)
--- a/src/math/big/int.go
+++ b/src/math/big/int.go
@@ -890,7 +890,7 @@ func (z *Int) AndNot(x, y *Int) *Int {
        }
 
        // x &^ (-y) == x &^ ^(y-1) == x & (y-1)
-       y1 := nat(nil).add(y.abs, natOne)
+       y1 := nat(nil).sub(y.abs, natOne)
        z.abs = z.abs.and(x.abs, y1)
        z.neg = false
        return z

diff --git a/src/math/big/int_test.go b/src/math/big/int_test.go
index af3af910e9a366b318b3d75a7bdfa8102168f14d..520fcb31d5802ad89601d2370d59f1c8d6d2095f 100644 (file)
--- a/src/math/big/int_test.go
+++ b/src/math/big/int_test.go
@@ -1216,6 +1216,7 @@ var bitwiseTests = []struct {
        {"-0x01", "-0x01", "-0x01", "-0x01", "0x00", "0x00"},
        {"0x07", "0x08", "0x00", "0x0f", "0x0f", "0x07"},
        {"0x05", "0x0f", "0x05", "0x0f", "0x0a", "0x00"},
+       {"0xff", "-0x0a", "0xf6", "-0x01", "-0xf7", "0x09"},
        {"0x013ff6", "0x9a4e", "0x1a46", "0x01bffe", "0x01a5b8", "0x0125b0"},
        {"-0x013ff6", "0x9a4e", "0x800a", "-0x0125b2", "-0x01a5bc", "-0x01c000"},
        {"-0x013ff6", "-0x9a4e", "-0x01bffe", "-0x1a46", "0x01a5b8", "0x8008"},