math/big: handling of +/-Inf and zero precision, enable zero values

- clarified representation of +/-Inf
- only 0 and Inf values can have 0 precision
- a zero precision value used as result value takes the max precision
  of the arguments (to be fine-tuned for setters)
- the zero precision approach makes Float zero values possible
  (they represent +0)
- more tests

Missing: Filling in the blanks. More tests.

Change-Id: Ibb4f97e12e1f356c3085ce80f3464e97b82ac130
Reviewed-on: https://go-review.googlesource.com/4000
Reviewed-by: Alan Donovan <adonovan@google.com>

1 files changed, 14 insertions, 3 deletions

diff --git a/src/math/big/floatconv.go b/src/math/big/floatconv.go
index 7628e77d9a..e3611b234b 100644
--- a/src/math/big/floatconv.go
+++ b/src/math/big/floatconv.go
@@ -191,13 +191,26 @@ func (x *Float) Format(format byte, prec int) string {
 // Append appends the string form of the floating-point number x,
 // as generated by x.Format, to buf and returns the extended buffer.
 func (x *Float) Append(buf []byte, format byte, prec int) []byte {
-	// pick off simple cases
+	// TODO(gri) factor out handling of sign?
+
+	// Inf
+	if x.IsInf(0) {
+		var ch byte = '+'
+		if x.neg {
+			ch = '-'
+		}
+		buf = append(buf, ch)
+		return append(buf, "Inf"...)
+	}
+
+	// easy formats
 	switch format {
 	case 'b':
 		return x.bstring(buf)
 	case 'p':
 		return x.pstring(buf)
 	}
+
 	return x.bigFtoa(buf, format, prec)
 }
 
@@ -212,7 +225,6 @@ func (x *Float) String() string {
 // The mantissa is normalized such that is uses x.Precision() bits in binary
 // representation.
 func (x *Float) bstring(buf []byte) []byte {
-	// TODO(gri) handle Inf
 	if x.neg {
 		buf = append(buf, '-')
 	}
@@ -240,7 +252,6 @@ func (x *Float) bstring(buf []byte) []byte {
 // ad returns the extended buffer.
 // The mantissa is normalized such that 0.5 <= 0.mantissa < 1.0.
 func (x *Float) pstring(buf []byte) []byte {
-	// TODO(gri) handle Inf
 	if x.neg {
 		buf = append(buf, '-')
 	}