perf(number): skip the rebuild for tokens without underscores

Assisted-by: GLM 5.3

CHANGELOG.md

@@ -47,6 +47,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   the input is validated UTF-8 up front, so the conversion pass and its four
   bytes per rune were pure overhead. Parsing a large array-of-tables document
   runs about a fifth faster and allocates about half the memory.
+- Numeric tokens without underscores skip the normalising rebuild: digits are
+  validated in place in `joinDigits`, and a float whose token is already
+  clean goes to `strconv.ParseFloat` directly. One allocation per integer
+  atom and two per float atom disappear.
 
 ### Fixed
 

number.go

@@ -94,28 +94,43 @@ func decodeFloat(tok string) (any, error) {
 	if err := checkNoLeadingZero(ip); err != nil {
 		return nil, err
 	}
-	build := sign + ip
 
+	fp := ""
 	if hasDot {
-		fp, err := joinDigits(frac, isDecDigit)
-		if err != nil {
+		if fp, err = joinDigits(frac, isDecDigit); err != nil {
 			return nil, err
 		}
-		build += "." + fp
 	}
 	// The ABNF requires at least one digit after the exponent marker, so a
 	// trailing e or E is an error even though strconv would accept it. The
 	// digits are a zero-prefixable integer, so leading zeros are fine here
 	// (the corpus holds valid cases such as 1e06 and 0e00).
+	esign, ed := "", ""
 	if hasExp {
-		esign, edigits := splitSign(exp)
-		ed, err := joinDigits(edigits, isDecDigit)
-		if err != nil {
+		var digits string
+		esign, digits = splitSign(exp)
+		if ed, err = joinDigits(digits, isDecDigit); err != nil {
 			return nil, err
 		}
-		build += "e" + esign + ed
 	}
 
+	// The checks above validated the token's shape, and every character a
+	// valid token may carry is one strconv.ParseFloat accepts in place, so
+	// only a token with underscores needs the stripped rebuild.
+	if !strings.ContainsRune(tok, '_') {
+		f, err := strconv.ParseFloat(tok, 64)
+		if err != nil {
+			return nil, fmt.Errorf("invalid float %q", tok)
+		}
+		return f, nil
+	}
+	build := sign + ip
+	if hasDot {
+		build += "." + fp
+	}
+	if hasExp {
+		build += "e" + esign + ed
+	}
 	f, err := strconv.ParseFloat(build, 64)
 	if err != nil {
 		return nil, fmt.Errorf("invalid float %q", tok)
@@ -125,11 +140,20 @@ func decodeFloat(tok string) (any, error) {
 
 // joinDigits validates that every rune is a digit (per isDigit) and that each
 // underscore sits between two digits, returning the digits with underscores
-// removed.
+// removed. A token without underscores, the common case, is validated in
+// place and returned without a copy.
 func joinDigits(s string, isDigit func(byte) bool) (string, error) {
 	if s == "" {
 		return "", fmt.Errorf("number is missing digits")
 	}
+	if !strings.ContainsRune(s, '_') {
+		for i := range len(s) {
+			if !isDigit(s[i]) {
+				return "", fmt.Errorf("invalid character %q in number", string(s[i]))
+			}
+		}
+		return s, nil
+	}
 	var b strings.Builder
 	for i := range len(s) {
 		c := s[i]