math/big: Always print exponent sign when using 'p' exponent for Floats.

Float.Format supports the 'b' and 'p' format, both of which print
a binary ('p') exponent. The 'b' format always printed a sign ('+'
or '-') for the exponent; the 'p' format only printed a negative
sign for the exponent. This change makes the two consistent. It
also makes the 'p' format easier to read if the exponent is >= 0.

Also:
- Comments added elsewhere.

Change-Id: Ifd2e01bdafb3043345972ca22a90248d055bd29b
Reviewed-on: https://go-review.googlesource.com/10359
Reviewed-by: Alan Donovan <adonovan@google.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/math/big/floatconv.go b/src/math/big/floatconv.go
index b929d1202c..5ab75e9031 100644
--- a/src/math/big/floatconv.go
+++ b/src/math/big/floatconv.go
@@ -67,6 +67,7 @@ func (z *Float) SetString(s string) (*Float, bool) {
 // defined if an error is reported.
 //
 // BUG(gri) The Float.Scan signature conflicts with Scan(s fmt.ScanState, ch rune) error.
+//          (https://github.com/golang/go/issues/10938)
 func (z *Float) Scan(r io.ByteScanner, base int) (f *Float, b int, err error) {
 	prec := z.prec
 	if prec == 0 {
@@ -268,6 +269,8 @@ func ParseFloat(s string, base int, prec uint, mode RoundingMode) (f *Float, b i
 // The prec value is ignored for the 'b' or 'p' format.
 //
 // BUG(gri) Float.Format does not accept negative precisions.
+// BUG(gri) The Float.Format signature conflicts with Format(f fmt.State, c rune).
+//          (https://github.com/golang/go/issues/10938)
 func (x *Float) Format(format byte, prec int) string {
 	const extra = 10 // TODO(gri) determine a good/better value here
 	return string(x.Append(make([]byte, 0, prec+extra), format, prec))
@@ -369,5 +372,8 @@ func (x *Float) pstring(buf []byte) []byte {
 	buf = append(buf, "0x."...)
 	buf = append(buf, strings.TrimRight(x.mant.hexString(), "0")...)
 	buf = append(buf, 'p')
+	if x.exp >= 0 {
+		buf = append(buf, '+')
+	}
 	return strconv.AppendInt(buf, int64(x.exp), 10)
 }