Revert "Use strconv and strings to parse version string."

This reverts commit 517a7be269.

version.go

@@ -2,8 +2,6 @@ package main
 
 import (
 	"fmt"
-	"strconv"
-	"strings"
 )
 
 type Version struct {
@@ -57,28 +55,46 @@ func parseVersion(s string) (Version, bool) {
 		return InvalidVersion, false
 	}
 	v := Version{-1, -1, -1}
-
-	parts := strings.Split(s[1:], ".")
-	if len(parts) == 0 || len(parts) > 3 {
+	i := 1
+	v.Major, i = parseVersionPart(s, i)
+	if i < 0 {
 		return InvalidVersion, false
 	}
-	for i, part := range parts {
-		if len(part) == 0 || part[0] == '0' {
-			return InvalidVersion, false
-		}
-		num, err := strconv.Atoi(part)
-		if err != nil {
-			return InvalidVersion, false
+	if i == len(s) {
+		return v, true
+	}
+	v.Minor, i = parseVersionPart(s, i)
+	if i < 0 {
+		return InvalidVersion, false
+	}
+	if i == len(s) {
+		return v, true
+	}
+	v.Patch, i = parseVersionPart(s, i)
+	if i < 0 || i < len(s) {
+		return InvalidVersion, false
+	}
+	return v, true
+}
+
+func parseVersionPart(s string, i int) (part int, newi int) {
+	dot := i
+	for dot < len(s) && s[dot] != '.' {
+		dot++
+	}
+	if dot == i || dot-i > 1 && s[i] == '0' {
+		return -1, -1
+	}
+	for i < len(s) {
+		if s[i] < '0' || s[i] > '9' {
+			return -1, -1
 		}
-		switch i {
-		case 0:
-			v.Major = num
-		case 1:
-			v.Minor = num
-		case 2:
-			v.Patch = num
+		part *= 10
+		part += int(s[i] - '0')
+		i++
+		if i+1 < len(s) && s[i] == '.' {
+			return part, i+1
 		}
 	}
-
-	return v, true
+	return part, i
 }