encoding/json: remove unnecessary isValidNumber call

The decoder called this function to check numbers being decoded into a
json.Number. However, these can't be quoted as strings, so the tokenizer
has already verified they are valid JSON numbers.

Verified this by adding a test with such an input. As expected, it
produces a syntax error, not the fmt.Errorf - that line could never
execute.

Since the only remaining non-test caller of isvalidnumber is in
encode.go, move the function there.

This change should slightly reduce the amount of work when decoding into
json.Number, though that isn't very common nor part of any current
benchmarks.

Change-Id: I67a1723deb3d18d5b542d6dd35f3ae56a43f23eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/184817
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 60 insertions, 0 deletions

diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index 67412763d6..07d3098f1c 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -611,6 +611,66 @@ func stringEncoder(e *encodeState, v reflect.Value, opts encOpts) {
 	}
 }
 
+// isValidNumber reports whether s is a valid JSON number literal.
+func isValidNumber(s string) bool {
+	// This function implements the JSON numbers grammar.
+	// See https://tools.ietf.org/html/rfc7159#section-6
+	// and https://json.org/number.gif
+
+	if s == "" {
+		return false
+	}
+
+	// Optional -
+	if s[0] == '-' {
+		s = s[1:]
+		if s == "" {
+			return false
+		}
+	}
+
+	// Digits
+	switch {
+	default:
+		return false
+
+	case s[0] == '0':
+		s = s[1:]
+
+	case '1' <= s[0] && s[0] <= '9':
+		s = s[1:]
+		for len(s) > 0 && '0' <= s[0] && s[0] <= '9' {
+			s = s[1:]
+		}
+	}
+
+	// . followed by 1 or more digits.
+	if len(s) >= 2 && s[0] == '.' && '0' <= s[1] && s[1] <= '9' {
+		s = s[2:]
+		for len(s) > 0 && '0' <= s[0] && s[0] <= '9' {
+			s = s[1:]
+		}
+	}
+
+	// e or E followed by an optional - or + and
+	// 1 or more digits.
+	if len(s) >= 2 && (s[0] == 'e' || s[0] == 'E') {
+		s = s[1:]
+		if s[0] == '+' || s[0] == '-' {
+			s = s[1:]
+			if s == "" {
+				return false
+			}
+		}
+		for len(s) > 0 && '0' <= s[0] && s[0] <= '9' {
+			s = s[1:]
+		}
+	}
+
+	// Make sure we are at the end.
+	return s == ""
+}
+
 func interfaceEncoder(e *encodeState, v reflect.Value, opts encOpts) {
 	if v.IsNil() {
 		e.WriteString("null")