using System;

namespace DownKyi.Core.BiliApi.BiliUtils
{
    public static class BvId
    {
        private const string tableStr = "fZodR9XQDSUm21yCkr6zBqiveYah8bt4xsWpHnJE7jL5VG3guMTKNPAwcF"; //码表
        private static readonly char[] table = tableStr.ToCharArray();

        private static readonly char[] tr = new char[124]; //反查码表
        private const ulong Xor = 177451812; //固定异或值
        private const ulong add = 8728348608; //固定加法值
        private static readonly int[] s = { 11, 10, 3, 8, 4, 6 }; //位置编码表

        static BvId()
        {
            Tr_init();
        }

        //初始化反查码表
        private static void Tr_init()
        {
            for (int i = 0; i < 58; i++)
                tr[table[i]] = (char)i;
        }

        /// <summary>
        /// bvid转avid
        /// </summary>
        /// <param name="bvid"></param>
        /// <returns></returns>
        public static ulong Bv2Av(string bvid)
        {
            char[] bv = bvid.ToCharArray();

            ulong r = 0;
            ulong av;
            for (int i = 0; i < 6; i++)
                r += tr[bv[s[i]]] * (ulong)Math.Pow(58, i);
            av = (r - add) ^ Xor;
            return av;
        }

        /// <summary>
        /// avid转bvid
        /// </summary>
        /// <param name="av"></param>
        /// <returns></returns>
        public static string Av2Bv(ulong av)
        {
            //编码结果
            string res = "BV1  4 1 7  ";
            char[] result = res.ToCharArray();

            av = (av ^ Xor) + add;
            for (int i = 0; i < 6; i++)
                result[s[i]] = table[av / (ulong)Math.Pow(58, i) % 58];
            string bv = new string(result);
            return bv;
        }

    }
}