fmt: format hex floats and complexes

This CL modifies fmt's printer to implement %x and %X
for formatting floating-point data (floats and complexes)
in standard hexadecimal notation.

See golang.org/design/19308-number-literals for background.

For #29008.
Vet update is #29986.

Change-Id: If2842a11631bc393a1ebcf6914ed07658652af5a
Reviewed-on: https://go-review.googlesource.com/c/160245
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

4 files changed, 39 insertions, 6 deletions

diff --git a/src/fmt/doc.go b/src/fmt/doc.go
index 3b657f3681..c349f8e321 100644
--- a/src/fmt/doc.go
+++ b/src/fmt/doc.go
@@ -40,6 +40,8 @@
 		%F	synonym for %f
 		%g	%e for large exponents, %f otherwise. Precision is discussed below.
 		%G	%E for large exponents, %F otherwise
+		%x	hexadecimal notation (with decimal power of two exponent), e.g. -0x1.23abcp+20
+		%X	upper-case hexadecimal notation, e.g. -0X1.23ABCP+20
 	String and slice of bytes (treated equivalently with these verbs):
 		%s	the uninterpreted bytes of the string or slice
 		%q	a double-quoted string safely escaped with Go syntax
diff --git a/src/fmt/fmt_test.go b/src/fmt/fmt_test.go
index 068c2620a8..2d10c7a841 100644
--- a/src/fmt/fmt_test.go
+++ b/src/fmt/fmt_test.go
@@ -413,6 +413,8 @@ var fmtTests = []struct {
 	// floats
 	{"%+.3e", 0.0, "+0.000e+00"},
 	{"%+.3e", 1.0, "+1.000e+00"},
+	{"%+.3x", 0.0, "+0x0.000p+00"},
+	{"%+.3x", 1.0, "+0x1.000p+00"},
 	{"%+.3f", -1.0, "-1.000"},
 	{"%+.3F", -1.0, "-1.000"},
 	{"%+.3F", float32(-1.0), "-1.000"},
@@ -428,6 +430,8 @@ var fmtTests = []struct {
 	{"%+10.2f", -1.0, "     -1.00"},
 	{"% .3E", -1.0, "-1.000E+00"},
 	{"% .3e", 1.0, " 1.000e+00"},
+	{"% .3X", -1.0, "-0X1.000P+00"},
+	{"% .3x", 1.0, " 0x1.000p+00"},
 	{"%+.3g", 0.0, "+0"},
 	{"%+.3g", 1.0, "+1"},
 	{"%+.3g", -1.0, "-1"},
@@ -445,17 +449,21 @@ var fmtTests = []struct {
 	{"%#g", 1000000.0, "1.00000e+06"},
 	{"%#.0f", 1.0, "1."},
 	{"%#.0e", 1.0, "1.e+00"},
+	{"%#.0x", 1.0, "0x1.p+00"},
 	{"%#.0g", 1.0, "1."},
 	{"%#.0g", 1100000.0, "1.e+06"},
 	{"%#.4f", 1.0, "1.0000"},
 	{"%#.4e", 1.0, "1.0000e+00"},
+	{"%#.4x", 1.0, "0x1.0000p+00"},
 	{"%#.4g", 1.0, "1.000"},
 	{"%#.4g", 100000.0, "1.000e+05"},
 	{"%#.0f", 123.0, "123."},
 	{"%#.0e", 123.0, "1.e+02"},
+	{"%#.0x", 123.0, "0x1.p+07"},
 	{"%#.0g", 123.0, "1.e+02"},
 	{"%#.4f", 123.0, "123.0000"},
 	{"%#.4e", 123.0, "1.2300e+02"},
+	{"%#.4x", 123.0, "0x1.ec00p+06"},
 	{"%#.4g", 123.0, "123.0"},
 	{"%#.4g", 123000.0, "1.230e+05"},
 	{"%#9.4g", 1.0, "    1.000"},
@@ -474,17 +482,23 @@ var fmtTests = []struct {
 	{"%20f", posInf, "                +Inf"},
 	{"% 20F", posInf, "                 Inf"},
 	{"% 20e", negInf, "                -Inf"},
+	{"% 20x", negInf, "                -Inf"},
 	{"%+20E", negInf, "                -Inf"},
+	{"%+20X", negInf, "                -Inf"},
 	{"% +20g", negInf, "                -Inf"},
 	{"%+-20G", posInf, "+Inf                "},
 	{"%20e", NaN, "                 NaN"},
+	{"%20x", NaN, "                 NaN"},
 	{"% +20E", NaN, "                +NaN"},
+	{"% +20X", NaN, "                +NaN"},
 	{"% -20g", NaN, " NaN                "},
 	{"%+-20G", NaN, "+NaN                "},
 	// Zero padding does not apply to infinities and NaN.
 	{"%+020e", posInf, "                +Inf"},
+	{"%+020x", posInf, "                +Inf"},
 	{"%-020f", negInf, "-Inf                "},
 	{"%-020E", NaN, "NaN                 "},
+	{"%-020X", NaN, "NaN                 "},
 
 	// complex values
 	{"%.f", 0i, "(0+0i)"},
@@ -492,23 +506,29 @@ var fmtTests = []struct {
 	{"%+.f", 0i, "(+0+0i)"},
 	{"% +.f", 0i, "(+0+0i)"},
 	{"%+.3e", 0i, "(+0.000e+00+0.000e+00i)"},
+	{"%+.3x", 0i, "(+0x0.000p+00+0x0.000p+00i)"},
 	{"%+.3f", 0i, "(+0.000+0.000i)"},
 	{"%+.3g", 0i, "(+0+0i)"},
 	{"%+.3e", 1 + 2i, "(+1.000e+00+2.000e+00i)"},
+	{"%+.3x", 1 + 2i, "(+0x1.000p+00+0x1.000p+01i)"},
 	{"%+.3f", 1 + 2i, "(+1.000+2.000i)"},
 	{"%+.3g", 1 + 2i, "(+1+2i)"},
 	{"%.3e", 0i, "(0.000e+00+0.000e+00i)"},
+	{"%.3x", 0i, "(0x0.000p+00+0x0.000p+00i)"},
 	{"%.3f", 0i, "(0.000+0.000i)"},
 	{"%.3F", 0i, "(0.000+0.000i)"},
 	{"%.3F", complex64(0i), "(0.000+0.000i)"},
 	{"%.3g", 0i, "(0+0i)"},
 	{"%.3e", 1 + 2i, "(1.000e+00+2.000e+00i)"},
+	{"%.3x", 1 + 2i, "(0x1.000p+00+0x1.000p+01i)"},
 	{"%.3f", 1 + 2i, "(1.000+2.000i)"},
 	{"%.3g", 1 + 2i, "(1+2i)"},
 	{"%.3e", -1 - 2i, "(-1.000e+00-2.000e+00i)"},
+	{"%.3x", -1 - 2i, "(-0x1.000p+00-0x1.000p+01i)"},
 	{"%.3f", -1 - 2i, "(-1.000-2.000i)"},
 	{"%.3g", -1 - 2i, "(-1-2i)"},
 	{"% .3E", -1 - 2i, "(-1.000E+00-2.000E+00i)"},
+	{"% .3X", -1 - 2i, "(-0X1.000P+00-0X1.000P+01i)"},
 	{"%+.3g", 1 + 2i, "(+1+2i)"},
 	{"%+.3g", complex64(1 + 2i), "(+1+2i)"},
 	{"%#g", 1 + 2i, "(1.00000+2.00000i)"},
@@ -517,11 +537,13 @@ var fmtTests = []struct {
 	{"%#g", -1e10 - 1.11e100i, "(-1.00000e+10-1.11000e+100i)"},
 	{"%#.0f", 1.23 + 1.0i, "(1.+1.i)"},
 	{"%#.0e", 1.23 + 1.0i, "(1.e+00+1.e+00i)"},
+	{"%#.0x", 1.23 + 1.0i, "(0x1.p+00+0x1.p+00i)"},
 	{"%#.0g", 1.23 + 1.0i, "(1.+1.i)"},
 	{"%#.0g", 0 + 100000i, "(0.+1.e+05i)"},
 	{"%#.0g", 1230000 + 0i, "(1.e+06+0.i)"},
 	{"%#.4f", 1 + 1.23i, "(1.0000+1.2300i)"},
 	{"%#.4e", 123 + 1i, "(1.2300e+02+1.0000e+00i)"},
+	{"%#.4x", 123 + 1i, "(0x1.ec00p+06+0x1.0000p+00i)"},
 	{"%#.4g", 123 + 1.23i, "(123.0+1.230i)"},
 	{"%#12.5g", 0 + 100000i, "(      0.0000 +1.0000e+05i)"},
 	{"%#12.5g", 1230000 - 0i, "(  1.2300e+06     +0.0000i)"},
@@ -541,7 +563,9 @@ var fmtTests = []struct {
 	{"% f", complex(negInf, negInf), "(-Inf-Infi)"},
 	{"% f", complex(NaN, NaN), "( NaN+NaNi)"},
 	{"%8e", complex(posInf, posInf), "(    +Inf    +Infi)"},
+	{"%8x", complex(posInf, posInf), "(    +Inf    +Infi)"},
 	{"% 8E", complex(posInf, posInf), "(     Inf    +Infi)"},
+	{"% 8X", complex(posInf, posInf), "(     Inf    +Infi)"},
 	{"%+8f", complex(negInf, negInf), "(    -Inf    -Infi)"},
 	{"% +8g", complex(negInf, negInf), "(    -Inf    -Infi)"},
 	{"% -8G", complex(NaN, NaN), "( NaN    +NaN    i)"},
diff --git a/src/fmt/format.go b/src/fmt/format.go
index d6da8aed1e..6d93908095 100644
--- a/src/fmt/format.go
+++ b/src/fmt/format.go
@@ -510,7 +510,7 @@ func (f *fmt) fmtFloat(v float64, size int, verb rune, prec int) {
 	if f.sharp && verb != 'b' {
 		digits := 0
 		switch verb {
-		case 'v', 'g', 'G':
+		case 'v', 'g', 'G', 'x':
 			digits = prec
 			// If no precision is set explicitly use a precision of 6.
 			if digits == -1 {
@@ -519,8 +519,8 @@ func (f *fmt) fmtFloat(v float64, size int, verb rune, prec int) {
 		}
 
 		// Buffer pre-allocated with enough room for
-		// exponent notations of the form "e+123".
-		var tailBuf [5]byte
+		// exponent notations of the form "e+123" or "p-1023".
+		var tailBuf [6]byte
 		tail := tailBuf[:0]
 
 		hasDecimalPoint := false
@@ -529,9 +529,16 @@ func (f *fmt) fmtFloat(v float64, size int, verb rune, prec int) {
 			switch num[i] {
 			case '.':
 				hasDecimalPoint = true
-			case 'e', 'E':
+			case 'p', 'P':
 				tail = append(tail, num[i:]...)
 				num = num[:i]
+			case 'e', 'E':
+				if verb != 'x' && verb != 'X' {
+					tail = append(tail, num[i:]...)
+					num = num[:i]
+					break
+				}
+				fallthrough
 			default:
 				digits--
 			}
diff --git a/src/fmt/print.go b/src/fmt/print.go
index 42fcd8b979..9976b8d263 100644
--- a/src/fmt/print.go
+++ b/src/fmt/print.go
@@ -407,7 +407,7 @@ func (p *pp) fmtFloat(v float64, size int, verb rune) {
 	switch verb {
 	case 'v':
 		p.fmt.fmtFloat(v, size, 'g', -1)
-	case 'b', 'g', 'G':
+	case 'b', 'g', 'G', 'x', 'X':
 		p.fmt.fmtFloat(v, size, verb, -1)
 	case 'f', 'e', 'E':
 		p.fmt.fmtFloat(v, size, verb, 6)
@@ -425,7 +425,7 @@ func (p *pp) fmtComplex(v complex128, size int, verb rune) {
 	// Make sure any unsupported verbs are found before the
 	// calls to fmtFloat to not generate an incorrect error string.
 	switch verb {
-	case 'v', 'b', 'g', 'G', 'f', 'F', 'e', 'E':
+	case 'v', 'b', 'g', 'G', 'x', 'X', 'f', 'F', 'e', 'E':
 		oldPlus := p.fmt.plus
 		p.buf.WriteByte('(')
 		p.fmtFloat(real(v), size/2, verb)