const b64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
const b64re = /^(?:[A-Za-z\d+/]{4})*?(?:[A-Za-z\d+/]{2}(?:==)?|[A-Za-z\d+/]{3}=?)?$/;

export function weBtoa(string) {
  string = String(string);
  let bitmap, a, b, c, result = "", i = 0, rest = string.length % 3;
  for (; i < string.length;) {
    if ((a = string.charCodeAt(i++)) > 255 ||
        (b = string.charCodeAt(i++)) > 255 ||
        (c = string.charCodeAt(i++)) > 255) {
      throw new TypeError("Failed to execute 'btoa' on 'Window': The string to be encoded contains characters outside of the Latin1 range.");
    }
    bitmap = (a << 16) | (b << 8) | c;
    result += b64.charAt((bitmap >> 18) & 63)
            + b64.charAt((bitmap >> 12) & 63)
            + b64.charAt((bitmap >> 6) & 63)
            + b64.charAt(bitmap & 63);
  }
  return rest ? result.slice(0, rest - 3) + "===".substring(rest) : result;
}

export function weAtob(string) {
  string = String(string).replace(/[\t\n\f\r ]+/g, "");
  if (!b64re.test(string)) {
    throw new TypeError("Failed to execute 'atob' on 'Window': The string to be decoded is not correctly encoded.");
  }
  string += "==".slice(2 - (string.length & 3));
  let bitmap, result = "", r1, r2, i = 0;
  for (; i < string.length;) {
    bitmap = (b64.indexOf(string.charAt(i++)) << 18) |
             (b64.indexOf(string.charAt(i++)) << 12) |
             ((r1 = b64.indexOf(string.charAt(i++))) << 6) |
             (r2 = b64.indexOf(string.charAt(i++)));
    result += r1 === 64
      ? String.fromCharCode((bitmap >> 16) & 255)
      : r2 === 64
        ? String.fromCharCode((bitmap >> 16) & 255, (bitmap >> 8) & 255)
        : String.fromCharCode((bitmap >> 16) & 255, (bitmap >> 8) & 255, bitmap & 255);
  }
  return result;
}

function b64DecodeUnicode(str) {
  return decodeURIComponent(
    weAtob(str).replace(/(.)/g, (p) => {
      let code = p.charCodeAt(0).toString(16).toUpperCase();
      if (code.length < 2) code = "0" + code;
      return "%" + code;
    })
  );
}

function base64_url_decode(str) {
  let output = str.replace(/-/g, "+").replace(/_/g, "/");
  switch (output.length % 4) {
    case 0: break;
    case 2: output += "=="; break;
    case 3: output += "="; break;
    default: throw new Error("Illegal base64url string!");
  }
  try {
    return b64DecodeUnicode(output);
  } catch (err) {
    return weAtob(output);
  }
}

export default function jwt(token, options = {}) {
  if (typeof token !== "string") {
    throw new Error("Invalid token specified");
  }
  const pos = options.header === true ? 0 : 1;
  try {
    return JSON.parse(base64_url_decode(token.split(".")[pos]));
  } catch (e) {
    throw new Error("Invalid token specified: " + e.message);
  }
}