encoding/json: optimize isValidNumber functionjson-isValidNumber

We use an index instead of reslicing the string because reslicing a string
updates both the data and length fields instead of just the one index
variable.

Benchmark result,

name             old time/op    new time/op    delta
NumberIsValid-8  19.0ns ± 0%    14.5ns ± 1%  -23.70%  (p=0.008 n=5+5)

name             old alloc/op   new alloc/op   delta
NumberIsValid-8   0.00B          0.00B          ~     (all equal)

name             old allocs/op  new allocs/op  delta
NumberIsValid-8    0.00           0.00          ~     (all equal)

Change-Id: I4698c5db134998f83ff47fb3add6a04ba6ec3aa0

1 files changed, 28 insertions, 22 deletions

diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index 9d59b0ff2b..da936173da 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -652,14 +652,20 @@ func isValidNumber(s string) bool {
 	// See https://tools.ietf.org/html/rfc7159#section-6
 	// and https://www.json.org/img/number.png
 
-	if s == "" {
+	if len(s) == 0 {
 		return false
 	}
 
+	// We use an index instead of reslicing the string because
+	// reslicing a string updates both the data and length fields
+	// instead of just the one index variable.
+
+	var i int
+
 	// Optional -
-	if s[0] == '-' {
-		s = s[1:]
-		if s == "" {
+	if s[i] == '-' {
+		i++
+		if len(s) == i {
 			return false
 		}
 	}
@@ -669,41 +675,41 @@ func isValidNumber(s string) bool {
 	default:
 		return false
 
-	case s[0] == '0':
-		s = s[1:]
+	case s[i] == '0':
+		i++
 
-	case '1' <= s[0] && s[0] <= '9':
-		s = s[1:]
-		for len(s) > 0 && '0' <= s[0] && s[0] <= '9' {
-			s = s[1:]
+	case '1' <= s[i] && s[i] <= '9':
+		i++
+		for len(s) > i && '0' <= s[i] && s[i] <= '9' {
+			i++
 		}
 	}
 
 	// . 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:]
+	if len(s)-i >= 2 && s[i] == '.' && '0' <= s[i+1] && s[i+1] <= '9' {
+		i += 2
+		for len(s) > i && '0' <= s[i] && s[i] <= '9' {
+			i++
 		}
 	}
 
 	// 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 == "" {
+	if len(s)-i >= 2 && (s[i] == 'e' || s[i] == 'E') {
+		i++
+		if s[i] == '+' || s[i] == '-' {
+			i++
+			if len(s) == i {
 				return false
 			}
 		}
-		for len(s) > 0 && '0' <= s[0] && s[0] <= '9' {
-			s = s[1:]
+		for len(s) > i && '0' <= s[i] && s[i] <= '9' {
+			i++
 		}
 	}
 
 	// Make sure we are at the end.
-	return s == ""
+	return len(s) == i
 }
 
 func interfaceEncoder(e *encodeState, v reflect.Value, opts encOpts) {