function buildMatches(pool) {
  var matches = [];
  for (var index = 0; index < pool.length; index += 2) {
    matches.push({ left: pool[index], right: pool[index + 1] || null, winner: pool[index + 1] ? null : pool[index] });
  }
  return matches;
}

function create(candidates) {
  var pool = candidates.slice();
  return { round: 1, pool: pool, matches: buildMatches(pool), eliminated: [], decisions: [] };
}

function current(state) {
  var match = state.matches.filter(function (item) { return item.right && !item.winner; })[0];
  return match ? [match.left, match.right] : null;
}

function advance(state) {
  if (state.matches.some(function (item) { return !item.winner; })) return false;
  state.pool = state.matches.map(function (item) { return item.winner; });
  if (state.pool.length === 1) return true;
  state.round += 1;
  state.matches = buildMatches(state.pool);
  return false;
}

function chooseAt(state, matchIndex, winner) {
  var match = state.matches[matchIndex];
  if (!match || !match.right || match.winner) throw new Error('invalid bracket match');
  if (winner.id !== match.left.id && winner.id !== match.right.id) throw new Error('winner is not in match');
  var loser = match.left.id === winner.id ? match.right : match.left;
  match.winner = winner;
  state.eliminated.unshift(loser);
  state.decisions.push({ round: state.round, left_id: match.left.id, right_id: match.right.id, winner_id: winner.id });
  return advance(state);
}

function choose(state, winner) {
  var index = state.matches.findIndex(function (item) { return item.right && !item.winner; });
  return chooseAt(state, index, winner);
}

function ranking(state) {
  return state.pool.concat(state.eliminated).map(function (item) { return item.id; });
}

module.exports = { create: create, current: current, choose: choose, chooseAt: chooseAt, ranking: ranking };