rewrite placeholder parser

The old parser logic was quite complicated and does not support nested placeholder evaluation for defaults, as does not support counting closing braces.

I assume the given approach using two loops and switch statements is more readable, and changes braces matching behavior to support nested defaulting provided in another commit.

Signed-off-by: Jens Erat <[email protected]>

replacer.go

@@ -141,46 +141,55 @@ func (r *Replacer) replace(input, empty string,
 	// iterate the input to find each placeholder
 	var lastWriteCursor int
 
-	// fail fast if too many placeholders are unclosed
-	var unclosedCount int
-
-scan:
+	//scan:
 	for i := 0; i < len(input); i++ {
-		// check for escaped braces
-		if i > 0 && input[i-1] == phEscape && (input[i] == phClose || input[i] == phOpen) {
-			sb.WriteString(input[lastWriteCursor : i-1])
+		switch input[i] {
+		case phOpen:
+			// process possible placeholder in remaining loop (not drop into default)
+		case phEscape:
+			// escape character at the end of the input or next character not a brace or escape character
+			if i+1 == len(input) || (input[i+1] != phOpen && input[i+1] != phClose && input[i+1] != phEscape) {
+				continue
+			}
+			// if there's anything to copy (until the escape character), do so
+			if i > lastWriteCursor {
+				sb.WriteString(input[lastWriteCursor:i])
+			}
+			// skip handling escaped character, get it copied with the next special character
+			i++
 			lastWriteCursor = i
 			continue
-		}
-
-		if input[i] != phOpen {
+		default:
+			// just copy anything else
 			continue
 		}
 
 		// our iterator is now on an unescaped open brace (start of placeholder)
 
-		// too many unclosed placeholders in absolutely ridiculous input can be extremely slow (issue #4170)
-		if unclosedCount > 100 {
-			return "", fmt.Errorf("too many unclosed placeholders")
+		end := 0
+		bracesLevel := 0
+		for j := i + 1; j < len(input); j++ {
+			switch input[j] {
+			case phOpen:
+				bracesLevel++
+			case phClose:
+				if bracesLevel > 0 {
+					bracesLevel--
+					continue
+				}
+				end = j
+				break
+			case phEscape:
+				// skip scaped character
+				j++
+			default:
+			}
 		}
-
-		// find the end of the placeholder
-		end := strings.Index(input[i:], string(phClose)) + i
-		if end < i {
-			unclosedCount++
+		// no matching closing brace found
+		if end == 0 {
 			continue
 		}
 
-		// if necessary look for the first closing brace that is not escaped
-		for end > 0 && end < len(input)-1 && input[end-1] == phEscape {
-			nextEnd := strings.Index(input[end+1:], string(phClose))
-			if nextEnd < 0 {
-				unclosedCount++
-				continue scan
-			}
-			end += nextEnd + 1
-		}
-
 		// write the substring from the last cursor to this point
 		sb.WriteString(input[lastWriteCursor:i])