function create(candidates) {
  return {
    round: 1,
    pool: candidates.slice(),
    pairIndex: 0,
    winners: [],
    eliminated: [],
    decisions: []
  };
}

function current(state) {
  if (state.pool.length === 1) return null;
  return [state.pool[state.pairIndex], state.pool[state.pairIndex + 1] || null];
}

function choose(state, winner) {
  var pair = current(state);
  if (!pair || !pair[1]) throw new Error('invalid bracket pair');
  var loser = pair[0].id === winner.id ? pair[1] : pair[0];
  state.winners.push(winner);
  state.eliminated.unshift(loser);
  state.decisions.push({ round: state.round, left_id: pair[0].id, right_id: pair[1].id, winner_id: winner.id });
  state.pairIndex += 2;
  if (state.pairIndex < state.pool.length) {
    if (!state.pool[state.pairIndex + 1]) {
      state.winners.push(state.pool[state.pairIndex]);
      state.pairIndex += 1;
    }
    if (state.pairIndex < state.pool.length) return false;
  }
  state.pool = state.winners;
  state.winners = [];
  state.pairIndex = 0;
  state.round += 1;
  return state.pool.length === 1;
}

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

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