// Remote-control navigation.
//
// webOS TVs have no tab key and no pointer worth designing for, so focus moves
// geometrically: pressing Right picks the nearest focusable element whose
// centre lies to the right, biased towards ones on the same row. Anything with
// the `focusable` class joins in, which keeps the markup free of tab indices.

(function (global) {
  'use strict';

  var KEY = {
    LEFT: 37, UP: 38, RIGHT: 39, DOWN: 40,
    ENTER: 13, BACK: 461, ESCAPE: 27,
    RED: 403, GREEN: 404, YELLOW: 405, BLUE: 406,
  };

  function visible(el) {
    if (el.disabled || el.classList.contains('hidden')) {
      return false;
    }
    var rect = el.getBoundingClientRect();
    return rect.width > 0 && rect.height > 0;
  }

  function candidates() {
    var all = document.querySelectorAll('.focusable');
    var out = [];
    for (var i = 0; i < all.length; i++) {
      if (visible(all[i])) {
        out.push(all[i]);
      }
    }
    return out;
  }

  function centre(el) {
    var r = el.getBoundingClientRect();
    return { x: r.left + r.width / 2, y: r.top + r.height / 2, rect: r };
  }

  // Minimum travel before a neighbour counts as being in that direction, so
  // elements that merely overlap slightly do not steal focus.
  var MIN_TRAVEL = 4;
  // How much drifting off-axis costs. High enough that a row of buttons is
  // traversed in order rather than diagonally.
  var ACROSS_PENALTY = 3;

  function move(direction) {
    var current = document.activeElement;
    var list = candidates();
    if (!list.length) {
      return;
    }
    if (!current || !current.classList || !current.classList.contains('focusable')) {
      focus(list[0]);
      return;
    }

    var from = centre(current);
    var best = null;
    var bestScore = Infinity;

    for (var i = 0; i < list.length; i++) {
      if (list[i] === current) {
        continue;
      }
      var to = centre(list[i]);
      var dx = to.x - from.x;
      var dy = to.y - from.y;

      var along;
      var across;
      if (direction === 'right') {
        along = dx;
        across = dy;
      } else if (direction === 'left') {
        along = -dx;
        across = dy;
      } else if (direction === 'down') {
        along = dy;
        across = dx;
      } else {
        along = -dy;
        across = dx;
      }

      if (along < MIN_TRAVEL) {
        continue;
      }

      var s = along + Math.abs(across) * ACROSS_PENALTY;
      if (s < bestScore) {
        bestScore = s;
        best = list[i];
      }
    }

    if (best) {
      focus(best);
    }
  }

  function focus(el) {
    if (!el) {
      return;
    }
    el.focus();
    // Keep the focused control clear of the sticky header.
    if (el.scrollIntoView) {
      var rect = el.getBoundingClientRect();
      if (rect.top < 120 || rect.bottom > global.innerHeight - 40) {
        el.scrollIntoView({ block: 'center' });
      }
    }
  }

  function focusFirstIn(container) {
    if (!container) {
      return;
    }
    var list = container.querySelectorAll('.focusable');
    for (var i = 0; i < list.length; i++) {
      if (visible(list[i])) {
        focus(list[i]);
        return;
      }
    }
  }

  var backHandler = null;

  document.addEventListener('keydown', function (e) {
    switch (e.keyCode) {
    case KEY.LEFT:
      move('left');
      break;
    case KEY.RIGHT:
      move('right');
      break;
    case KEY.UP:
      move('up');
      break;
    case KEY.DOWN:
      move('down');
      break;
    case KEY.BACK:
    case KEY.ESCAPE:
      if (backHandler && backHandler()) {
        break;
      }
      // Nothing wanted the Back press: leave the app the way the platform
      // expects rather than trapping the user inside it.
      if (global.webOS && global.webOS.platformBack) {
        global.webOS.platformBack();
      } else {
        global.close();
      }
      break;
    default:
      return;
    }
    e.preventDefault();
  });

  global.Nav = {
    KEY: KEY,
    focus: focus,
    focusFirstIn: focusFirstIn,
    onBack: function (fn) { backHandler = fn; },
  };
})(window);