(function() {
  let activeService = null;
  let batchedTransitions = new Map();
  function nop() {}
  function interpret(machine) {
    let listeners = new Set();
    let status = 'not-started';
    let state = machine.initialState;
    function capture(fn) {
      return function (...args) {
        activeService = service;
        let ret = fn(...args);
        activeService = null;
        let fifo = batchedTransitions.get(service) || [];
        let next = fifo.shift();
        if (next) { service.send(next); }
        return ret;
      };
    }
    const service = {
      subscribe: capture(function (listener) {
        listeners.add(listener);
        listener(state);
        return { unsubscribe: () => listeners.delete(listener) };
      }),
      send: capture(function (event) {
        state = machine.transition(state, event);
        state.actions.forEach(({ exec = nop }) => exec(state.context, event));
        listeners.forEach(listener => listener(state, event));
        return service;
      }),
      start: capture(function () {
        status = 'running';
        state.actions.forEach(({ exec = nop }) => exec(state.context, '#init'));
        return service;
      }),
      stop() {
        status = 'stopped';
        listeners.clear();
        return service;
      },
      get status() { return status; },
    };
    return service;
  };
  interpret.send = function send(event) {
    return function() {
      let fifo = batchedTransitions.get(activeService) || [];
      fifo.push(event);
      batchedTransitions.set(activeService, fifo);
    };
  };
  return interpret;
})();