strconv: accept underscores in ParseInt, ParseUint, ParseFloat

This CL modifies ParseInt, ParseUint, and ParseFloat
to accept digit-separating underscores in their arguments.
For ParseInt and ParseUint, the underscores are only
allowed when base == 0.

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

For #28493.

Change-Id: I057ca2539d89314643f591ba8144c3ea7126651c
Reviewed-on: https://go-review.googlesource.com/c/160243
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

4 files changed, 193 insertions, 11 deletions

diff --git a/src/strconv/atof.go b/src/strconv/atof.go
index 3ced3c7167..504b9613fb 100644
--- a/src/strconv/atof.go
+++ b/src/strconv/atof.go
@@ -83,6 +83,9 @@ func (b *decimal) set(s string) (ok bool) {
 	sawdigits := false
 	for ; i < len(s); i++ {
 		switch {
+		case s[i] == '_':
+			// underscoreOK already called
+			continue
 		case s[i] == '.':
 			if sawdot {
 				return
@@ -135,7 +138,11 @@ func (b *decimal) set(s string) (ok bool) {
 			return
 		}
 		e := 0
-		for ; i < len(s) && '0' <= s[i] && s[i] <= '9'; i++ {
+		for ; i < len(s) && ('0' <= s[i] && s[i] <= '9' || s[i] == '_'); i++ {
+			if s[i] == '_' {
+				// underscoreOK already called
+				continue
+			}
 			if e < 10000 {
 				e = e*10 + int(s[i]) - '0'
 			}
@@ -187,6 +194,10 @@ func readFloat(s string) (mantissa uint64, exp int, neg, trunc, hex, ok bool) {
 	dp := 0
 	for ; i < len(s); i++ {
 		switch c := s[i]; true {
+		case c == '_':
+			// underscoreOK already called
+			continue
+
 		case c == '.':
 			if sawdot {
 				return
@@ -258,7 +269,11 @@ func readFloat(s string) (mantissa uint64, exp int, neg, trunc, hex, ok bool) {
 			return
 		}
 		e := 0
-		for ; i < len(s) && '0' <= s[i] && s[i] <= '9'; i++ {
+		for ; i < len(s) && ('0' <= s[i] && s[i] <= '9' || s[i] == '_'); i++ {
+			if s[i] == '_' {
+				// underscoreOK already called
+				continue
+			}
 			if e < 10000 {
 				e = e*10 + int(s[i]) - '0'
 			}
@@ -640,6 +655,9 @@ func atof64(s string) (f float64, err error) {
 // away from the largest floating point number of the given size,
 // ParseFloat returns f = ±Inf, err.Err = ErrRange.
 func ParseFloat(s string, bitSize int) (float64, error) {
+	if !underscoreOK(s) {
+		return 0, syntaxError(fnParseFloat, s)
+	}
 	if bitSize == 32 {
 		f, err := atof32(s)
 		return float64(f), err
diff --git a/src/strconv/atof_test.go b/src/strconv/atof_test.go
index f67386a000..abe6c64466 100644
--- a/src/strconv/atof_test.go
+++ b/src/strconv/atof_test.go
@@ -47,6 +47,9 @@ var atoftests = []atofTest{
 	{"0x1p0", "1", nil},
 	{"0x1p1", "2", nil},
 	{"0x1p-1", "0.5", nil},
+	{"0x1ep-1", "15", nil},
+	{"-0x1ep-1", "-15", nil},
+	{"-0x1_ep-1", "-15", nil},
 	{"0x1p-200", "6.223015277861142e-61", nil},
 	{"0x1p200", "1.6069380442589903e+60", nil},
 	{"0x1fFe2.p0", "131042", nil},
@@ -299,6 +302,40 @@ var atoftests = []atofTest{
 	// Round to even (up).
 	{"1.00000000000000033306690738754696212708950042724609375", "1.0000000000000004", nil},
 	{"0x1.00000000000018p0", "1.0000000000000004", nil},
+
+	// Underscores.
+	{"1_23.50_0_0e+1_2", "1.235e+14", nil},
+	{"-_123.5e+12", "0", ErrSyntax},
+	{"+_123.5e+12", "0", ErrSyntax},
+	{"_123.5e+12", "0", ErrSyntax},
+	{"1__23.5e+12", "0", ErrSyntax},
+	{"123_.5e+12", "0", ErrSyntax},
+	{"123._5e+12", "0", ErrSyntax},
+	{"123.5_e+12", "0", ErrSyntax},
+	{"123.5__0e+12", "0", ErrSyntax},
+	{"123.5e_+12", "0", ErrSyntax},
+	{"123.5e+_12", "0", ErrSyntax},
+	{"123.5e_-12", "0", ErrSyntax},
+	{"123.5e-_12", "0", ErrSyntax},
+	{"123.5e+1__2", "0", ErrSyntax},
+	{"123.5e+12_", "0", ErrSyntax},
+
+	{"0x_1_2.3_4_5p+1_2", "74565", nil},
+	{"-_0x12.345p+12", "0", ErrSyntax},
+	{"+_0x12.345p+12", "0", ErrSyntax},
+	{"_0x12.345p+12", "0", ErrSyntax},
+	{"0x__12.345p+12", "0", ErrSyntax},
+	{"0x1__2.345p+12", "0", ErrSyntax},
+	{"0x12_.345p+12", "0", ErrSyntax},
+	{"0x12._345p+12", "0", ErrSyntax},
+	{"0x12.3__45p+12", "0", ErrSyntax},
+	{"0x12.345_p+12", "0", ErrSyntax},
+	{"0x12.345p_+12", "0", ErrSyntax},
+	{"0x12.345p+_12", "0", ErrSyntax},
+	{"0x12.345p_-12", "0", ErrSyntax},
+	{"0x12.345p-_12", "0", ErrSyntax},
+	{"0x12.345p+1__2", "0", ErrSyntax},
+	{"0x12.345p+12_", "0", ErrSyntax},
 }
 
 var atof32tests = []atofTest{
diff --git a/src/strconv/atoi.go b/src/strconv/atoi.go
index 186c9b3f86..ecbc9f4bbe 100644
--- a/src/strconv/atoi.go
+++ b/src/strconv/atoi.go
@@ -58,10 +58,12 @@ const maxUint64 = 1<<64 - 1
 func ParseUint(s string, base int, bitSize int) (uint64, error) {
 	const fnParseUint = "ParseUint"
 
-	if len(s) == 0 {
+	if s == "" || !underscoreOK(s) {
 		return 0, syntaxError(fnParseUint, s)
 	}
 
+	base0 := base == 0
+
 	s0 := s
 	switch {
 	case 2 <= base && base <= 36:
@@ -70,7 +72,7 @@ func ParseUint(s string, base int, bitSize int) (uint64, error) {
 	case base == 0:
 		// Look for octal, hex prefix.
 		switch {
-		case s[0] == '0' && len(s) > 1 && (s[1] == 'x' || s[1] == 'X'):
+		case s[0] == '0' && len(s) >= 3 && lower(s[1]) == 'x':
 			if len(s) < 3 {
 				return 0, syntaxError(fnParseUint, s0)
 			}
@@ -111,12 +113,13 @@ func ParseUint(s string, base int, bitSize int) (uint64, error) {
 	for _, c := range []byte(s) {
 		var d byte
 		switch {
+		case c == '_' && base0:
+			// underscoreOK already called
+			continue
 		case '0' <= c && c <= '9':
 			d = c - '0'
-		case 'a' <= c && c <= 'z':
-			d = c - 'a' + 10
-		case 'A' <= c && c <= 'Z':
-			d = c - 'A' + 10
+		case 'a' <= lower(c) && lower(c) <= 'z':
+			d = lower(c) - 'a' + 10
 		default:
 			return 0, syntaxError(fnParseUint, s0)
 		}
@@ -164,8 +167,7 @@ func ParseUint(s string, base int, bitSize int) (uint64, error) {
 func ParseInt(s string, base int, bitSize int) (i int64, err error) {
 	const fnParseInt = "ParseInt"
 
-	// Empty string bad.
-	if len(s) == 0 {
+	if s == "" {
 		return 0, syntaxError(fnParseInt, s)
 	}
 
@@ -236,10 +238,60 @@ func Atoi(s string) (int, error) {
 		return n, nil
 	}
 
-	// Slow path for invalid or big integers.
+	// Slow path for invalid, big, or underscored integers.
 	i64, err := ParseInt(s, 10, 0)
 	if nerr, ok := err.(*NumError); ok {
 		nerr.Func = fnAtoi
 	}
 	return int(i64), err
 }
+
+// underscoreOK reports whether the underscores in s are allowed.
+// Checking them in this one function lets all the parsers skip over them simply.
+// Underscore must appear only between digits or between a base prefix and a digit.
+func underscoreOK(s string) bool {
+	// saw tracks the last character (class) we saw:
+	// ^ for beginning of number,
+	// 0 for a digit or base prefix,
+	// _ for an underscore,
+	// ! for none of the above.
+	saw := '^'
+	i := 0
+
+	// Optional sign.
+	if len(s) >= 1 && (s[0] == '-' || s[0] == '+') {
+		s = s[1:]
+	}
+
+	// Optional base prefix.
+	hex := false
+	if len(s) >= 2 && s[0] == '0' && (lower(s[1]) == 'b' || lower(s[1]) == 'o' || lower(s[1]) == 'x') {
+		i = 2
+		saw = '0' // base prefix counts as a digit for "underscore as digit separator"
+		hex = lower(s[1]) == 'x'
+	}
+
+	// Number proper.
+	for ; i < len(s); i++ {
+		// Digits are always okay.
+		if '0' <= s[i] && s[i] <= '9' || hex && 'a' <= lower(s[i]) && lower(s[i]) <= 'f' {
+			saw = '0'
+			continue
+		}
+		// Underscore must follow digit.
+		if s[i] == '_' {
+			if saw != '0' {
+				return false
+			}
+			saw = '_'
+			continue
+		}
+		// Underscore must also be followed by digit.
+		if saw == '_' {
+			return false
+		}
+		// Saw non-digit, non-underscore.
+		saw = '!'
+	}
+	return saw != '_'
+}
diff --git a/src/strconv/atoi_test.go b/src/strconv/atoi_test.go
index e2f505a665..ec0542abf8 100644
--- a/src/strconv/atoi_test.go
+++ b/src/strconv/atoi_test.go
@@ -29,6 +29,10 @@ var parseUint64Tests = []parseUint64Test{
 	{"18446744073709551615", 1<<64 - 1, nil},
 	{"18446744073709551616", 1<<64 - 1, ErrRange},
 	{"18446744073709551620", 1<<64 - 1, ErrRange},
+	{"1_2_3_4_5", 0, ErrSyntax}, // base=10 so no underscores allowed
+	{"_12345", 0, ErrSyntax},
+	{"1__2345", 0, ErrSyntax},
+	{"12345_", 0, ErrSyntax},
 }
 
 type parseUint64BaseTest struct {
@@ -61,6 +65,42 @@ var parseUint64BaseTests = []parseUint64BaseTest{
 	{"01777777777777777777778", 0, 0, ErrSyntax},
 	{"02000000000000000000000", 0, 1<<64 - 1, ErrRange},
 	{"0200000000000000000000", 0, 1 << 61, nil},
+
+	// underscores allowed with base == 0 only
+	{"1_2_3_4_5", 0, 12345, nil},
+	{"_12345", 0, 0, ErrSyntax},
+	{"1__2345", 0, 0, ErrSyntax},
+	{"12345_", 0, 0, ErrSyntax},
+
+	{"1_2_3_4_5", 10, 0, ErrSyntax},
+	{"_12345", 10, 0, ErrSyntax},
+	{"1__2345", 10, 0, ErrSyntax},
+	{"12345_", 10, 0, ErrSyntax},
+
+	{"0x_1_2_3_4_5", 0, 0x12345, nil},
+	{"_0x12345", 0, 0, ErrSyntax},
+	{"0x__12345", 0, 0, ErrSyntax},
+	{"0x1__2345", 0, 0, ErrSyntax},
+	{"0x1234__5", 0, 0, ErrSyntax},
+	{"0x12345_", 0, 0, ErrSyntax},
+
+	{"1_2_3_4_5", 16, 0, ErrSyntax},
+	{"_12345", 16, 0, ErrSyntax},
+	{"1__2345", 16, 0, ErrSyntax},
+	{"1234__5", 16, 0, ErrSyntax},
+	{"12345_", 16, 0, ErrSyntax},
+
+	{"0_1_2_3_4_5", 0, 012345, nil},
+	{"_012345", 0, 0, ErrSyntax},
+	{"0__12345", 0, 0, ErrSyntax},
+	{"01234__5", 0, 0, ErrSyntax},
+	{"012345_", 0, 0, ErrSyntax},
+
+	{"0_1_2_3_4_5", 8, 0, ErrSyntax},
+	{"_012345", 8, 0, ErrSyntax},
+	{"0__12345", 8, 0, ErrSyntax},
+	{"01234__5", 8, 0, ErrSyntax},
+	{"012345_", 8, 0, ErrSyntax},
 }
 
 type parseInt64Test struct {
@@ -87,6 +127,11 @@ var parseInt64Tests = []parseInt64Test{
 	{"-9223372036854775808", -1 << 63, nil},
 	{"9223372036854775809", 1<<63 - 1, ErrRange},
 	{"-9223372036854775809", -1 << 63, ErrRange},
+	{"-1_2_3_4_5", 0, ErrSyntax}, // base=10 so no underscores allowed
+	{"-_12345", 0, ErrSyntax},
+	{"_12345", 0, ErrSyntax},
+	{"1__2345", 0, ErrSyntax},
+	{"12345_", 0, ErrSyntax},
 }
 
 type parseInt64BaseTest struct {
@@ -144,6 +189,26 @@ var parseInt64BaseTests = []parseInt64BaseTest{
 	{"10", 16, 16, nil},
 	{"-123456789abcdef", 16, -0x123456789abcdef, nil},
 	{"7fffffffffffffff", 16, 1<<63 - 1, nil},
+
+	// underscores
+	{"-0x_1_2_3_4_5", 0, -0x12345, nil},
+	{"0x_1_2_3_4_5", 0, 0x12345, nil},
+	{"-_0x12345", 0, 0, ErrSyntax},
+	{"_-0x12345", 0, 0, ErrSyntax},
+	{"_0x12345", 0, 0, ErrSyntax},
+	{"0x__12345", 0, 0, ErrSyntax},
+	{"0x1__2345", 0, 0, ErrSyntax},
+	{"0x1234__5", 0, 0, ErrSyntax},
+	{"0x12345_", 0, 0, ErrSyntax},
+
+	{"-0_1_2_3_4_5", 0, -012345, nil}, // octal
+	{"0_1_2_3_4_5", 0, 012345, nil},   // octal
+	{"-_012345", 0, 0, ErrSyntax},
+	{"_-012345", 0, 0, ErrSyntax},
+	{"_012345", 0, 0, ErrSyntax},
+	{"0__12345", 0, 0, ErrSyntax},
+	{"01234__5", 0, 0, ErrSyntax},
+	{"012345_", 0, 0, ErrSyntax},
 }
 
 type parseUint32Test struct {
@@ -162,6 +227,11 @@ var parseUint32Tests = []parseUint32Test{
 	{"987654321", 987654321, nil},
 	{"4294967295", 1<<32 - 1, nil},
 	{"4294967296", 1<<32 - 1, ErrRange},
+	{"1_2_3_4_5", 0, ErrSyntax}, // base=10 so no underscores allowed
+	{"_12345", 0, ErrSyntax},
+	{"_12345", 0, ErrSyntax},
+	{"1__2345", 0, ErrSyntax},
+	{"12345_", 0, ErrSyntax},
 }
 
 type parseInt32Test struct {
@@ -190,6 +260,11 @@ var parseInt32Tests = []parseInt32Test{
 	{"-2147483648", -1 << 31, nil},
 	{"2147483649", 1<<31 - 1, ErrRange},
 	{"-2147483649", -1 << 31, ErrRange},
+	{"-1_2_3_4_5", 0, ErrSyntax}, // base=10 so no underscores allowed
+	{"-_12345", 0, ErrSyntax},
+	{"_12345", 0, ErrSyntax},
+	{"1__2345", 0, ErrSyntax},
+	{"12345_", 0, ErrSyntax},
 }
 
 type numErrorTest struct {