using System;
using System.Collections.Generic;
using System.Linq;
using System.Numerics;
using System.Text;

namespace Ra3.BattleNet.Database.Utils;

public static class ChineseEncoding
{
    private const int EncodedChineseTotalLength = 120;
    private const int EncodedChineseChecksumLength = 3;
    private const int EncodedChineseContentLength = EncodedChineseTotalLength - EncodedChineseChecksumLength;
    private const int ChineseCodeSize = 13;
    private const int ChineseCodeMask = 0b1111111111111;
    private const int AsciiSectionSize = 0x80;
    private const int Gb2312EucOffset = 0xA0;
    private const int Gb2312RowWidth = 94;
    private const int Gb2312LastRow = 87;
    private const int Gb2312FirstUnassignedSectionBegin = 10;
    private const int Gb2312FirstUnassignedSectionSize = 6;
    private const int Gb2312SecondUnassignedSectionBegin = 88;
    private const int Base64CodeSize = 6;
    private const int Base64CodeMask = 0b111111;
    private const string Base64Table = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";

    public static string DecodeChineseFromBase64(string original)
    {
        BigInteger bits = 0;
        int index = 0;
        foreach (var c in original)
        {
            BigInteger value = Base64Table.IndexOf(c);
            if (value == -1)
            {
                throw new Exception("Invalid base64 string");
            }
            bits = bits | (value << index);
            index += Base64CodeSize;
        }
        BigInteger checksum = 0;
        var result = new List<byte>();
        for (var bitIndex = 0; bitIndex < EncodedChineseContentLength; bitIndex += ChineseCodeSize)
        {
            int value = (int)((bits >> bitIndex) & ChineseCodeMask);
            checksum += value;
            if (value == 0)
            {
                break;
            }
            if (value < AsciiSectionSize)
            {
                if (value < 0x20 || value == 0x7F)
                {
                    throw new Exception("Invalid ASCII");
                }
                result.Add((byte)value);
            }
            else
            {
                value -= AsciiSectionSize;
                var index1 = value / Gb2312RowWidth + 1;
                var index2 = value % Gb2312RowWidth + 1;
                if (index1 >= Gb2312FirstUnassignedSectionBegin)
                {
                    index1 += Gb2312FirstUnassignedSectionSize;
                }
                if (index1 >= Gb2312SecondUnassignedSectionBegin)
                {
                    throw new Exception("Invalid first byte");
                }
                if (index2 > Gb2312RowWidth)
                {
                    throw new Exception("Invalid second byte");
                }
                index1 += Gb2312EucOffset;
                index2 += Gb2312EucOffset;
                result.Add((byte)index1);
                result.Add((byte)index2);
            }
        }
        if ((bits >> EncodedChineseContentLength) != checksum % 8)
        {
            throw new Exception("Invalid checksum");
        }
        Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
        return Encoding.GetEncoding(936).GetString(result.ToArray());
    }

    public static string EncodeChineseToBase64(string original)
    {
        BigInteger bits = 0;
        int index = 0;
        Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
        var bytes = Encoding.GetEncoding(936).GetBytes(original);
        BigInteger checksum = 0;
        for (var i = 0; i < bytes.Length;)
        {
            BigInteger c = bytes[i];
            if (c < AsciiSectionSize)
            {
                if (c < 0x20 || c == 0x7F)
                {
                    throw new Exception("Invalid ASCII");
                }
                bits = bits | (c << index);
                index += ChineseCodeSize;
                checksum += c;
                i++;
            }
            else
            {
                if (c <= Gb2312EucOffset || c > (Gb2312EucOffset + Gb2312LastRow))
                {
                    throw new Exception("Invalid first byte");
                }
                BigInteger c2 = bytes[i + 1];
                if (c2 <= Gb2312EucOffset || c2 > (Gb2312EucOffset + Gb2312RowWidth))
                {
                    throw new Exception("Invalid second byte");
                }
                var index1 = c - Gb2312EucOffset;
                if (index1 >= Gb2312FirstUnassignedSectionBegin)
                {
                    var offset = index1 - Gb2312FirstUnassignedSectionBegin;
                    if (offset < Gb2312FirstUnassignedSectionSize)
                    {
                        throw new Exception("AA-AF user defined zone not supported");
                    }
                    index1 -= Gb2312FirstUnassignedSectionSize;
                }
                var index2 = c2 - Gb2312EucOffset;
                if (index2 > Gb2312RowWidth)
                {
                    throw new Exception("Invalid second byte");
                }
                var value = (index1 - 1) * Gb2312RowWidth + (index2 - 1);
                value = AsciiSectionSize + value;
                bits = bits | (value << index);
                index += ChineseCodeSize;
                checksum += value;
                i += 2;
            }
        }
        bits = bits | ((checksum % 8) << EncodedChineseContentLength);
        var result = "";
        var bitsIndex = 0;
        while (bitsIndex < EncodedChineseTotalLength)
        {
            int v = (int)((bits >> bitsIndex) & Base64CodeMask);
            result += Base64Table[v];
            bitsIndex += Base64CodeSize;
        }
        return result;
    }

    public static string GetPrettyName(string name)
    {
        if (name.Length != 20)
        {
            return name;
        }
        foreach (var c in name)
        {
            if (!Base64Table.Contains(c))
            {
                return name;
            }
        }
        try
        {
            return DecodeChineseFromBase64(name);
        }
        catch
        {
            return name;
        }
    }
}