'use strict';

module.exports = factory;

/* Factory to de-escape a value, based on a list at `key`
 * in `ctx`. */
function factory(ctx, key) {
  return unescape;

  /* De-escape a string using the expression at `key`
   * in `ctx`. */
  function unescape(value) {
    var prev = 0;
    var index = value.indexOf('\\');
    var escape = ctx[key];
    var queue = [];
    var character;

    while (index !== -1) {
      queue.push(value.slice(prev, index));
      prev = index + 1;
      character = value.charAt(prev);

      /* If the following character is not a valid escape,
       * add the slash. */
      if (!character || escape.indexOf(character) === -1) {
        queue.push('\\');
      }

      index = value.indexOf('\\', prev);
    }

    queue.push(value.slice(prev));

    return queue.join('');
  }
}