strconv: cleanup variable declarations in ParseUint

Move variable declarations closer to their first uses.

Use an additional string variable s0 for error reporting that references
the original input string. This allows the variable s to be modified.

Change-Id: I4725152490ca1dc10c1161ad8ad2f4ae8933493f
Reviewed-on: https://go-review.googlesource.com/55138
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/strconv/atoi.go

@@ -46,18 +46,16 @@ const maxUint64 = (1<<64 - 1)
 func ParseUint(s string, base int, bitSize int) (uint64, error) {
 	const fnParseUint = "ParseUint"
 
-	var n uint64
-	var cutoff, maxVal uint64
-
 	if bitSize == 0 {
 		bitSize = int(IntSize)
 	}
 
-	i := 0
-	switch {
-	case len(s) == 0:
+	if len(s) == 0 {
 		return 0, syntaxError(fnParseUint, s)
+	}
 
+	s0 := s
+	switch {
 	case 2 <= base && base <= 36:
 		// valid base; nothing to do
 
@@ -66,23 +64,24 @@ func ParseUint(s string, base int, bitSize int) (uint64, error) {
 		switch {
 		case s[0] == '0' && len(s) > 1 && (s[1] == 'x' || s[1] == 'X'):
 			if len(s) < 3 {
-				return 0, syntaxError(fnParseUint, s)
+				return 0, syntaxError(fnParseUint, s0)
 			}
 			base = 16
-			i = 2
+			s = s[2:]
 		case s[0] == '0':
 			base = 8
-			i = 1
+			s = s[1:]
 		default:
 			base = 10
 		}
 
 	default:
-		return 0, baseError(fnParseUint, s, base)
+		return 0, baseError(fnParseUint, s0, base)
 	}
 
 	// Cutoff is the smallest number such that cutoff*base > maxUint64.
 	// Use compile-time constants for common cases.
+	var cutoff uint64
 	switch base {
 	case 10:
 		cutoff = maxUint64/10 + 1
@@ -92,35 +91,36 @@ func ParseUint(s string, base int, bitSize int) (uint64, error) {
 		cutoff = maxUint64/uint64(base) + 1
 	}
 
-	maxVal = 1<<uint(bitSize) - 1
+	maxVal := uint64(1)<<uint(bitSize) - 1
 
-	for ; i < len(s); i++ {
-		var v byte
-		d := s[i]
+	var n uint64
+	for _, c := range []byte(s) {
+		var d byte
 		switch {
-		case '0' <= d && d <= '9':
-			v = d - '0'
-		case 'a' <= d && d <= 'z':
-			v = d - 'a' + 10
-		case 'A' <= d && d <= 'Z':
-			v = d - 'A' + 10
+		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
 		default:
-			return 0, syntaxError(fnParseUint, s)
+			return 0, syntaxError(fnParseUint, s0)
 		}
-		if v >= byte(base) {
-			return 0, syntaxError(fnParseUint, s)
+
+		if d >= byte(base) {
+			return 0, syntaxError(fnParseUint, s0)
 		}
 
 		if n >= cutoff {
 			// n*base overflows
-			return maxVal, rangeError(fnParseUint, s)
+			return maxVal, rangeError(fnParseUint, s0)
 		}
 		n *= uint64(base)
 
-		n1 := n + uint64(v)
+		n1 := n + uint64(d)
 		if n1 < n || n1 > maxVal {
 			// n+v overflows
-			return maxVal, rangeError(fnParseUint, s)
+			return maxVal, rangeError(fnParseUint, s0)
 		}
 		n = n1
 	}