math/big: fix %b format so it matches strconf %b format for non-zero values

(For zero values the strconv %b format prints the bias-adjusted exponent;
there's no bias in Float.)

Change-Id: I6f4dda9c3a50d02eac375cfe2c927c1540aae865
Reviewed-on: https://go-review.googlesource.com/3841Reviewed-by: Alan Donovan <adonovan@google.com>

src/math/big/floatconv.go

@@ -205,12 +205,6 @@ func (x *Float) String() string {
 	return x.Format('p', 0)
 }
 
-// TODO(gri) The 'b' and 'p' formats have different meanings here than
-// in strconv: in strconv, the printed exponent is the biased (hardware)
-// exponent; here it is the unbiased exponent. Decide what to do.
-// (a strconv 'p' formatted float value can only be interpreted correctly
-// if the bias is known; i.e., we must know if it's a 32bit or 64bit number).
-
 // bstring appends the string of x in the format ["-"] mantissa "p" exponent
 // with a decimal mantissa and a binary exponent, or ["-"] "0" if x is zero,
 // and returns the extended buffer.
@@ -233,7 +227,11 @@ func (x *Float) bstring(buf []byte) []byte {
 	}
 	buf = append(buf, m.decimalString()...)
 	buf = append(buf, 'p')
-	return strconv.AppendInt(buf, int64(x.exp), 10)
+	e := int64(x.exp) - int64(x.prec)
+	if e >= 0 {
+		buf = append(buf, '+')
+	}
+	return strconv.AppendInt(buf, e, 10)
 }
 
 // pstring appends the string of x in the format ["-"] "0x." mantissa "p" exponent

src/math/big/floatconv_test.go

@@ -9,10 +9,11 @@ import (
 	"testing"
 )
 
-var floatSetFloat64StringTests = []struct {
+func TestFloatSetFloat64String(t *testing.T) {
+	for _, test := range []struct {
 		s string
 		x float64
-}{
+	}{
 		{"0", 0},
 		{"-0", -0},
 		{"+0", 0},
@@ -54,10 +55,7 @@ var floatSetFloat64StringTests = []struct {
 
 		{".0000000000000000000000000000000000000001", 1e-40},
 		{"+10000000000000000000000000000000000000000e-0", 1e40},
-}
-
-func TestFloatSetFloat64String(t *testing.T) {
-	for _, test := range floatSetFloat64StringTests {
+	} {
 		var x Float
 		x.prec = 53 // TODO(gri) find better solution
 		_, ok := x.SetString(test.s)
@@ -82,8 +80,9 @@ func TestFloatFormat(t *testing.T) {
 	}{
 		{"0", 'b', 0, "0"},
 		{"-0", 'b', 0, "-0"},
-		{"1.0", 'b', 0, "4503599627370496p1"},
-		{"-1.0", 'b', 0, "-4503599627370496p1"},
+		{"1.0", 'b', 0, "4503599627370496p-52"},
+		{"-1.0", 'b', 0, "-4503599627370496p-52"},
+		{"4503599627370496", 'b', 0, "4503599627370496p+0"},
 
 		{"0", 'p', 0, "0"},
 		{"-0", 'p', 0, "-0"},
@@ -95,14 +94,21 @@ func TestFloatFormat(t *testing.T) {
 			t.Error(err)
 			continue
 		}
+
 		f := new(Float).SetFloat64(f64)
 		got := f.Format(test.format, test.prec)
 		if got != test.want {
-			t.Errorf("%v: got %s", test, got)
+			t.Errorf("%v: got %s; want %s", test, got, test.want)
 		}
-		if test.format == 'b' || test.format == 'p' {
-			continue // 'b', 'p' format not supported or different in strconv.Format
+
+		if test.format == 'b' && f64 == 0 {
+			continue // 'b' format in strconv.Float requires knowledge of bias for 0.0
 		}
+		if test.format == 'p' {
+			continue // 'p' format not supported in strconv.Format
+		}
+
+		// verify that Float format matches strconv format
 		want := strconv.FormatFloat(f64, test.format, test.prec, 64)
 		if got != want {
 			t.Errorf("%v: got %s; want %s", test, got, want)