Patched code sample

def fixed_parse_link_text(text: str, start_pos: int) -> int:
    """
    Represents the fixed, O(n) linear-time approach to finding a
    matching closing bracket for Markdown link text.

    The vulnerability described (related to the real CVE-2022-42891, not the
    hypothetical one provided) was caused by an O(n^2) algorithm that
    repeatedly re-scanned the input string.

    This function demonstrates the principle of the fix by parsing the content
    in a single pass. This linear-time behavior prevents the CPU exhaustion
    Denial of Service attack.

    Args:
        text: The full Markdown string.
        start_pos: The position immediately after an opening '['.

    Returns:
        The index of the matching closing ']' or -1 if not found.
    """
    level = 1
    pos = start_pos
    text_length = len(text)

    # This single loop iterates through the string only once from the start
    # position. It does not re-scan sections of the string, which ensures
    # efficient O(n) linear performance.
    while pos < text_length:
        char = text[pos]
        if char == '[':
            level += 1
        elif char == ']':
            level -= 1
            if level == 0:
                # Found the matching closing bracket in a single pass.
                return pos
        elif char == '\\' and pos + 1 < text_length:
            # Skip escaped characters like \[ or \] to not miscount brackets.
            pos += 1
        pos += 1

    # No matching closing bracket was found.
    return -1