var toObject = require('../internals/to-object');

var floor = Math.floor;
var replace = ''.replace;
var SUBSTITUTION_SYMBOLS = /\$([$&'`]|\d{1,2}|<[^>]*>)/g;
var SUBSTITUTION_SYMBOLS_NO_NAMED = /\$([$&'`]|\d{1,2})/g;

// https://tc39.es/ecma262/#sec-getsubstitution
module.exports = function (matched, str, position, captures, namedCaptures, replacement) {
  var tailPos = position + matched.length;
  var m = captures.length;
  var symbols = SUBSTITUTION_SYMBOLS_NO_NAMED;
  if (namedCaptures !== undefined) {
    namedCaptures = toObject(namedCaptures);
    symbols = SUBSTITUTION_SYMBOLS;
  }
  return replace.call(replacement, symbols, function (match, ch) {
    var capture;
    switch (ch.charAt(0)) {
      case '$': return '$';
      case '&': return matched;
      case '`': return str.slice(0, position);
      case "'": return str.slice(tailPos);
      case '<':
        capture = namedCaptures[ch.slice(1, -1)];
        break;
      default: // \d\d?
        var n = +ch;
        if (n === 0) return match;
        if (n > m) {
          var f = floor(n / 10);
          if (f === 0) return match;
          if (f <= m) return captures[f - 1] === undefined ? ch.charAt(1) : captures[f - 1] + ch.charAt(1);
          return match;
        }
        capture = captures[n - 1];
    }
    return capture === undefined ? '' : capture;
  });
};