const ASSIGNMENT = 'assignment';
const ANSWER = 'answer';
const ALGORITHM_CHECKBOX = 'algorithm';
const MARKER_DISTINCT_CHARACTERS = 4;
const MESSAGE_DISTINCT_CHARACTERS = 14;
const NEWLINE_CHARACTER = '\n';

/**
 * Main function
 */
window.onload = function() {
  document.getElementById(ASSIGNMENT).addEventListener("input", calculateAnswer);
  document.getElementById(ALGORITHM_CHECKBOX).addEventListener("click", calculateAnswer);
}

/**
 * Listener function for input in assignment field.
 * @param event the onInput event 
 */
function calculateAnswer(event) {
  console.info("Calculating answer for input...");
  let assignment = document.getElementById(ASSIGNMENT).value;
  let distinctCharacters = (document.getElementById(ALGORITHM_CHECKBOX).checked)? MESSAGE_DISTINCT_CHARACTERS: MARKER_DISTINCT_CHARACTERS;
  let answer = algorithm(assignment, distinctCharacters);

  document.getElementById(ANSWER).innerText = answer;
}

/**
 * Calculate the answer to assignment.
 * @param assignment the input from the assignment.
 * @return string the answer
 */
function algorithm(assignment, distinctCharacters) {
    let signal = assignment.trim();
    let marker = assignment.substring(0,distinctCharacters);

    for(let i=distinctCharacters; i<signal.length; i++) {
      if(containsDistinctCharacters(marker)) {
        console.debug("Found marker after:" + i + " characters");
        return "Marker found after processing: " + i + " characters.";
      }

      marker = marker.substring(1, distinctCharacters) + assignment[i];
    }

    return "No marker found.";
}

/**
 * Check if string only contains distinct characters.
 * @param String string the string to check
 * @return true, if string contains only distinct characters
 */
function containsDistinctCharacters(string) {
  console.debug("Check marker: " + string);
  return new Set(string).size == string.length;
}