var Base62 = (function() {

  var chars = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
  var base = chars.length;
  var decoder = chars.split('').reduce(function(memo, c, i) {
    memo[c] = i;
    return memo;
  }, {});
  
  var encode = function(value) {
    if (typeof(value) !== 'number') {
      value = parseInt(value);
    }   
    if (Math.floor(value) !== value) {
      throw new Error('Base62.encode called with value=' + value 
        + ', should only be called with integers');
    }   
    
    var encoded = ''; 
    do {
      var m = value % base;
      encoded = chars[m] + encoded;
      value = (value - m) / base;
    } while (value > 0); 
    
    return encoded;
  };  
  
  var decode = function(encoded) {
    var len = encoded.length;
    var value = 0;
    for(var i = 0; i < len; i++) {
      value += decoder[encoded[i]] * Math.pow(base, len - i - 1); 
    }   
    return value;
  };  
  
  return {
    encode: encode,
    decode: decode
  };  
}());