module.exports = (function() {

  'use strict';

  /*
    ---------------
    variables
    ---------------
  */

  // array of actively pressed keys
  var activeKeys = [];

  // cache document.body
  var body;

  // boolean: true if touch buffer timer is running
  var buffer = false;

  // the last used input type
  var currentInput = null;

  // form input types
  var formInputs = [
    'button',
    'input',
    'select',
    'textarea'
  ];

  // detect version of mouse wheel event to use
  // via https://developer.mozilla.org/en-US/docs/Web/Events/wheel
  var mouseWheel = detectWheel();

  // list of modifier keys commonly used with the mouse and
  // can be safely ignored to prevent false keyboard detection
  var ignoreMap = [
    16, // shift
    17, // control
    18, // alt
    91, // Windows key / left Apple cmd
    93  // Windows menu / right Apple cmd
  ];

  // mapping of events to input types
  var inputMap = {
    'keydown': 'keyboard',
    'keyup': 'keyboard',
    'mousedown': 'mouse',
    'mousemove': 'mouse',
    'MSPointerDown': 'pointer',
    'MSPointerMove': 'pointer',
    'pointerdown': 'pointer',
    'pointermove': 'pointer',
    'touchstart': 'touch'
  };

  // add correct mouse wheel event mapping to `inputMap`
  inputMap[detectWheel()] = 'mouse';

  // array of all used input types
  var inputTypes = [];

  // mapping of key codes to a common name
  var keyMap = {
    9: 'tab',
    13: 'enter',
    16: 'shift',
    27: 'esc',
    32: 'space',
    37: 'left',
    38: 'up',
    39: 'right',
    40: 'down'
  };

  // map of IE 10 pointer events
  var pointerMap = {
    2: 'touch',
    3: 'touch', // treat pen like touch
    4: 'mouse'
  };

  // touch buffer timer
  var timer;


  /*
    ---------------
    functions
    ---------------
  */

  // allows events that are also triggered to be filtered out for `touchstart`
  function eventBuffer(event) {
    clearTimer();
    setInput(event);

    buffer = true;
    timer = window.setTimeout(function() {
      buffer = false;
    }, 650);
  }

  function bufferedEvent(event) {
    if (!buffer) setInput(event);
  }

  function clearTimer() {
    window.clearTimeout(timer);
  }

  function setInput(event) {
    var eventKey = key(event);
    var value = inputMap[event.type];
    if (value === 'pointer') value = pointerType(event);

    // don't do anything if the value matches the input type already set
    if (currentInput !== value) {
      var activeElement = document.activeElement.nodeName.toLowerCase();

      if (
        (
          // only if the user flag to allow input switching
          // while interacting with form fields isn't set
          !body.hasAttribute('data-whatinput-formswitching') &&

          // support for legacy keyword
          !body.hasAttribute('data-whatinput-formtyping') &&

          // only if currentInput has a value
          currentInput &&

          formInputs.indexOf(activeElement) > -1
        ) || (
          // ignore modifier keys
          ignoreMap.indexOf(eventKey) > -1
        )
      ) {
        // ignore keyboard typing and do nothing
      } else {
        switchInput(value);
      }
    }

    if (value === 'keyboard') logKeys(eventKey);
  }

  function switchInput(string) {
    currentInput = string;
    body.setAttribute('data-whatinput', currentInput);

    if (inputTypes.indexOf(currentInput) === -1) inputTypes.push(currentInput);
  }

  function key(event) {
    return (event.keyCode) ? event.keyCode : event.which;
  }

  function target(event) {
    return event.target || event.srcElement;
  }

  function pointerType(event) {
    if (typeof event.pointerType === 'number') {
      return pointerMap[event.pointerType];
    } else {
      return (event.pointerType === 'pen') ? 'touch' : event.pointerType; // treat pen like touch
    }
  }

  // keyboard logging
  function logKeys(eventKey) {
    if (activeKeys.indexOf(keyMap[eventKey]) === -1 && keyMap[eventKey]) activeKeys.push(keyMap[eventKey]);
  }

  function unLogKeys(event) {
    var eventKey = key(event);
    var arrayPos = activeKeys.indexOf(keyMap[eventKey]);

    if (arrayPos !== -1) activeKeys.splice(arrayPos, 1);
  }

  function bindEvents() {
    body = document.body;

    // pointer events (mouse, pen, touch)
    if (window.PointerEvent) {
      body.addEventListener('pointerdown', bufferedEvent);
      body.addEventListener('pointermove', bufferedEvent);
    } else if (window.MSPointerEvent) {
      body.addEventListener('MSPointerDown', bufferedEvent);
      body.addEventListener('MSPointerMove', bufferedEvent);
    } else {

      // mouse events
      body.addEventListener('mousedown', bufferedEvent);
      body.addEventListener('mousemove', bufferedEvent);

      // touch events
      if ('ontouchstart' in window) {
        body.addEventListener('touchstart', eventBuffer);
      }
    }

    // mouse wheel
    body.addEventListener(mouseWheel, bufferedEvent);

    // keyboard events
    body.addEventListener('keydown', eventBuffer);
    body.addEventListener('keyup', eventBuffer);
    document.addEventListener('keyup', unLogKeys);
  }


  /*
    ---------------
    utilities
    ---------------
  */

  // detect version of mouse wheel event to use
  // via https://developer.mozilla.org/en-US/docs/Web/Events/wheel
  function detectWheel() {
    return mouseWheel = 'onwheel' in document.createElement('div') ?
      'wheel' : // Modern browsers support "wheel"

      document.onmousewheel !== undefined ?
        'mousewheel' : // Webkit and IE support at least "mousewheel"
        'DOMMouseScroll'; // let's assume that remaining browsers are older Firefox
  }


  /*
    ---------------
    init

    don't start script unless browser cuts the mustard,
    also passes if polyfills are used
    ---------------
  */

  if (
    'addEventListener' in window &&
    Array.prototype.indexOf
  ) {

    // if the dom is already ready already (script was placed at bottom of <body>)
    if (document.body) {
      bindEvents();

    // otherwise wait for the dom to load (script was placed in the <head>)
    } else {
      document.addEventListener('DOMContentLoaded', bindEvents);
    }
  }


  /*
    ---------------
    api
    ---------------
  */

  return {

    // returns string: the current input type
    ask: function() { return currentInput; },

    // returns array: currently pressed keys
    keys: function() { return activeKeys; },

    // returns array: all the detected input types
    types: function() { return inputTypes; },

    // accepts string: manually set the input type
    set: switchInput
  };

}());
