/*
 * Copyright (c) 2013, Stefan Brand <seiichiro@seiichiro0185.org>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice, this 
 *    list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright notice, this
 *    list of conditions and the following disclaimer in the documentation and/or other 
 *    materials provided with the distribution.
 * 
 * 3. The names of the contributors may not be used to endorse or promote products 
 *    derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" 
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, 
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE 
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; 
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING 
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, 
 * EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

.import "./sha.js" as SHA

// *** Helper Functions *** //

// Decimal to HEX
function dec2hex(s) { return (s < 15.5 ? '0' : '') + Math.round(s).toString(16); }

// HEX to Decimal
function hex2dec(s) { return parseInt(s, 16); }

// Convert Base32-secret to HEX Value
function base32tohex(base32) {
  var base32chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";
  var bits = "";
  var hex = "";

  for (var i = 0; i < base32.length; i++) {
    var val = base32chars.indexOf(base32.charAt(i).toUpperCase());
    bits += leftpad(val.toString(2), 5, '0');
  }

  for (var i = 0; i+4 <= bits.length; i+=4) {
    var chunk = bits.substr(i, 4);
    hex = hex + parseInt(chunk, 2).toString(16) ;
  }
  return hex;
}

// Pad Strings to given length
function leftpad(str, len, pad) {
  if (len + 1 >= str.length) {
      str = Array(len + 1 - str.length).join(pad) + str;
  }
  return str;
}

// *** Main Function *** //

// Calculate an OTP-Value from the given secret
// Parameter is the secret key in Base32-notation
function calcOTP(secret) {
  // Convert the key to HEX
  var key = base32tohex(secret);
  // Get current Time in UNIX Timestamp format (Seconds since 01.01.1970 00:00 UTC)
  var epoch = Math.round(new Date().getTime() / 1000.0);
  // Get last full 30 / 60 Seconds and convert to HEX
  var time = leftpad(dec2hex(Math.floor(epoch / 30)), 16, '0');

  // Calculate the SHA-1 HMAC Value from time and key
  var hmacObj = new SHA.jsSHA(time, 'HEX');
  var hmac = hmacObj.getHMAC(key, 'HEX', 'SHA-1', "HEX");

  // Finally convert the HMAC-Value to the corresponding 6-digit token
  var offset = hex2dec(hmac.substring(hmac.length - 1));

  var otp = (hex2dec(hmac.substr(offset * 2, 8)) & hex2dec('7fffffff')) + '';
  otp = (otp).substr(otp.length - 6, 6);

  // return the calculated token
  return otp;
}