math/big: fix test for denormalized inputs and enable more test cases
Also: removed unnecessary BUG comment (was fixed).
Change-Id: I8f11fbcb4e30a19ec5a25df742b3e25e2ee7f846
Reviewed-on: https://go-review.googlesource.com/14923
Reviewed-by: Alan Donovan <adonovan@google.com>
diff --git a/src/math/big/floatconv_test.go b/src/math/big/floatconv_test.go
index a29f8a1..cea8f82 100644
--- a/src/math/big/floatconv_test.go
+++ b/src/math/big/floatconv_test.go
@@ -254,11 +254,10 @@
{above1e23, 'f', -1, "100000000000000010000000"},
{above1e23, 'g', -1, "1.0000000000000001e+23"},
- // TODO(gri) track down why these don't work yet
- // {5e-304/1e20, 'g', -1, "5e-324"},
- // {-5e-304/1e20, 'g', -1, "-5e-324"},
- // {fdiv(5e-304, 1e20), 'g', -1, "5e-324"}, // avoid constant arithmetic
- // {fdiv(-5e-304, 1e20), 'g', -1, "-5e-324"}, // avoid constant arithmetic
+ {5e-304 / 1e20, 'g', -1, "5e-324"},
+ {-5e-304 / 1e20, 'g', -1, "-5e-324"},
+ {fdiv(5e-304, 1e20), 'g', -1, "5e-324"}, // avoid constant arithmetic
+ {fdiv(-5e-304, 1e20), 'g', -1, "-5e-324"}, // avoid constant arithmetic
{32, 'g', -1, "32"},
{32, 'g', 0, "3e+01"},
@@ -292,10 +291,16 @@
{383260575764816448, 'f', 0, "383260575764816448"},
{383260575764816448, 'g', -1, "3.8326057576481645e+17"},
} {
- f := new(Float).SetFloat64(test.x)
+ // The test cases are from the strconv package which tests float64 values.
+ // When formatting values with prec = -1 (shortest representation),
+ // the actually available mantissa precision matters.
+ // For denormalized values, that precision is < 53 (SetFloat64 default).
+ // Compute and set the actual precision explicitly.
+ f := new(Float).SetPrec(actualPrec(test.x)).SetFloat64(test.x)
got := f.Text(test.format, test.prec)
if got != test.want {
t.Errorf("%v: got %s; want %s", test, got, test.want)
+ continue
}
if test.format == 'b' && test.x == 0 {
@@ -313,6 +318,15 @@
}
}
+// actualPrec returns the number of actually used mantissa bits.
+func actualPrec(x float64) uint {
+ if bits := math.Float64bits(x); x != 0 && bits&(0x7ff<<52) == 0 {
+ // x is denormalized
+ return 64 - nlz64(bits&(1<<52-1))
+ }
+ return 53
+}
+
func TestFloatText(t *testing.T) {
for _, test := range []struct {
x string
diff --git a/src/math/big/ftoa.go b/src/math/big/ftoa.go
index 5c1d31e..506a6cb9 100644
--- a/src/math/big/ftoa.go
+++ b/src/math/big/ftoa.go
@@ -37,7 +37,8 @@
// printed by the 'e', 'E', 'f', 'g', and 'G' formats. For 'e', 'E', and 'f'
// it is the number of digits after the decimal point. For 'g' and 'G' it is
// the total number of digits. A negative precision selects the smallest
-// number of digits necessary to identify the value x uniquely.
+// number of decimal digits necessary to identify the value x uniquely using
+// x.Prec() mantissa bits.
// The prec value is ignored for the 'b' or 'p' format.
func (x *Float) Text(format byte, prec int) string {
const extra = 10 // TODO(gri) determine a good/better value here
@@ -381,10 +382,6 @@
// '+' and ' ' for sign control, '0' for space or zero padding,
// and '-' for left or right justification. See the fmt package
// for details.
-//
-// BUG(gri) A missing precision for the 'g' format, or a negative
-// (via '*') precision is not yet supported. Instead the
-// default precision (6) is used in that case (issue #10991).
func (x *Float) Format(s fmt.State, format rune) {
prec, hasPrec := s.Precision()
if !hasPrec {
