math/big: fix aliasing error in Add, Sub

Also:
- make representation more flexible (no need to store trailing 0 digits to match precision)
- simplify rounding as a consequence
- minor related fixes

TBR adonovan

Change-Id: Ie91075990688b506d28371ec3b633b8267397ebb
Reviewed-on: https://go-review.googlesource.com/4841
Reviewed-by: Rob Pike <r@golang.org>

1 files changed, 14 insertions, 8 deletions

diff --git a/src/math/big/float_test.go b/src/math/big/float_test.go
index 2f804fa569..be2ac6ff06 100644
--- a/src/math/big/float_test.go
+++ b/src/math/big/float_test.go
@@ -729,14 +729,20 @@ func TestFloatNeg(t *testing.T) {
 }
 
 func TestFloatInc(t *testing.T) {
-	var x, one Float
-	// x.prec = 256 TODO(gri) This doesn't work at the moment
-	one.SetInt64(1)
-	for i := 0; i < 10; i++ {
-		x.Add(&x, &one)
-	}
-	if s := x.Format('g', 10); s != "10" {
-		t.Errorf("got %s; want 10", s)
+	const n = 10
+	for _, prec := range precList {
+		if 1<<prec < n {
+			continue // prec must be large enough to hold all numbers from 0 to n
+		}
+		var x, one Float
+		x.prec = prec
+		one.SetInt64(1)
+		for i := 0; i < n; i++ {
+			x.Add(&x, &one)
+		}
+		if x.Cmp(new(Float).SetInt64(n)) != 0 {
+			t.Errorf("prec = %d: got %s; want %d", prec, &x, n)
+		}
 	}
 }