http://theblindprophet.com/

gistfile1.js

function invert(move) {
  switch (move) {
    case 'up': return 'down';
    case 'down': return 'up';
    case 'left': return 'right';
    case 'right': return 'left';
    default: return move;
  }
}

function orthogonal(move) {
  switch (move) {
    case 'up': return 'left';
    case 'down': return 'left';
    case 'left': return 'up';
    case 'right': return 'up';
    default: return move;
  }
}

var previousMove;
var previousDistance;
var blacklist = {};
var state = 0;

function tick(distance) {
  if (distance === 0) {
    return '';
  }

  var move;
  switch (state) {
    case 0:
      move = 'up';
      state = 1;
      break;
    case 1:
      if (distance < previousDistance) {
        move = previousMove;
      } else {
        blacklist[previousMove] = true;
        move = invert(previousMove);
        state = 2;
      }
      break;
    case 2:
      move = orthogonal(previousMove);
      if (move in blacklist) {
        move = invert(move);
      }
      state = 3;
      break;
    case 3:
      if (distance < previousDistance) {
        move = previousMove;
        state = 1;
      } else {
        move = invert(previousMove);
        state = 4;
      }
      break;
    case 4:
      move = previousMove;
      state = 1;
      break;
    default:
  }

  previousDistance = distance;
  previousMove = move;

  return move;
}