fastfloat: localize handling of invalid number without integer and fr…

…actional parts

This is a follow-up for 6f52d1b

fastfloat/parse.go

@@ -215,7 +215,9 @@ func ParseBestEffort(s string) float64 {
 
 	// the integer part might be elided to remain compliant
 	// with https://go.dev/ref/spec#Floating-point_literals
-	intPartElided := s[i] == '.'
+	if s[i] == '.' && (i+1 >= uint(len(s)) || s[i+1] < '0' || s[i+1] > '9') {
+		return 0
+	}
 
 	d := uint64(0)
 	j := i
@@ -236,7 +238,7 @@ func ParseBestEffort(s string) float64 {
 		}
 		break
 	}
-	if i <= j && !intPartElided {
+	if i <= j && s[i] != '.' {
 		s = s[i:]
 		if strings.HasPrefix(s, "+") {
 			s = s[1:]
@@ -267,9 +269,6 @@ func ParseBestEffort(s string) float64 {
 		// Parse fractional part.
 		i++
 		if i >= uint(len(s)) {
-			if intPartElided {
-				return 0
-			}
 			// the fractional part may be elided to remain compliant
 			// with https://go.dev/ref/spec#Floating-point_literals
 			return f
@@ -305,9 +304,6 @@ func ParseBestEffort(s string) float64 {
 		}
 	}
 	if s[i] == 'e' || s[i] == 'E' {
-		if intPartElided {
-			return 0
-		}
 		// Parse exponent part.
 		i++
 		if i >= uint(len(s)) {
@@ -377,7 +373,9 @@ func Parse(s string) (float64, error) {
 
 	// the integer part might be elided to remain compliant
 	// with https://go.dev/ref/spec#Floating-point_literals
-	intPartElided := s[i] == '.'
+	if s[i] == '.' && (i+1 >= uint(len(s)) || s[i+1] < '0' || s[i+1] > '9') {
+		return 0, fmt.Errorf("missing integer and fractional part in %q", s)
+	}
 
 	d := uint64(0)
 	j := i
@@ -398,7 +396,7 @@ func Parse(s string) (float64, error) {
 		}
 		break
 	}
-	if i <= j && !intPartElided {
+	if i <= j && s[i] != '.' {
 		ss := s[i:]
 		if strings.HasPrefix(ss, "+") {
 			ss = ss[1:]
@@ -429,9 +427,6 @@ func Parse(s string) (float64, error) {
 		// Parse fractional part.
 		i++
 		if i >= uint(len(s)) {
-			if intPartElided {
-				return 0, fmt.Errorf("cannot parse integer or fractional part in %q", s)
-			}
 			// the fractional part might be elided to remain compliant
 			// with https://go.dev/ref/spec#Floating-point_literals
 			return f, nil
@@ -467,9 +462,6 @@ func Parse(s string) (float64, error) {
 		}
 	}
 	if s[i] == 'e' || s[i] == 'E' {
-		if intPartElided {
-			return 0, fmt.Errorf("cannot parse integer part in %q", s)
-		}
 		// Parse exponent part.
 		i++
 		if i >= uint(len(s)) {

fastfloat/parse_test.go

@@ -451,6 +451,8 @@ func TestParseSuccess(t *testing.T) {
 	f("-123.456E-10", -123.456e-10)
 	f("1.e4", 1.e4)
 	f("-1.E-10", -1.e-10)
+	f(".1e3", 100)
+	f("-.12e3", -120)
 
 	// inf and nan
 	f("12345678909123456789012e45678", math.Inf(1))

parser_test.go

@@ -35,6 +35,9 @@ func TestParseRawNumber(t *testing.T) {
 		f("-12.345E+67 tail", "-12.345E+67", " tail")
 		f("-12.345E-67,tail", "-12.345E-67", ",tail")
 		f("-1234567.8e+90tail", "-1234567.8e+90", "tail")
+		f("12.tail", "12.", "tail")
+		f(".2tail", ".2", "tail")
+		f("-.2tail", "-.2", "tail")
 		f("NaN", "NaN", "")
 		f("nantail", "nan", "tail")
 		f("inf", "inf", "")