using System;
using System.Collections.Generic;
using System.IO;
using System.Numerics;
using System.Runtime.InteropServices;
using System.Security.Cryptography;

namespace PKHeX.Core;

/// <summary>
/// MemeCrypto V2 - The Next Generation
/// </summary>
/// <remarks>
/// A variant of <see cref="SaveFile"/> encryption and obfuscation used in <see cref="GameVersion.SWSH"/> and future in-house titles.
/// <br> Individual save blocks are stored in a hash map, with some object-type details prefixing the block's raw data. </br>
/// <br> Once the raw save file data is dumped, the binary is hashed with SHA256 using a static Intro salt and static Outro salt. </br>
/// <br> With the hash computed, the data is encrypted with a repeating irregular-sized static xor cipher. </br>
/// </remarks>
public static class SwishCrypto
{
    private const int SIZE_HASH = 0x20;

    private static ReadOnlySpan<byte> IntroHashBytes => new byte[]
    {
        0x9E, 0xC9, 0x9C, 0xD7, 0x0E, 0xD3, 0x3C, 0x44, 0xFB, 0x93, 0x03, 0xDC, 0xEB, 0x39, 0xB4, 0x2A,
        0x19, 0x47, 0xE9, 0x63, 0x4B, 0xA2, 0x33, 0x44, 0x16, 0xBF, 0x82, 0xA2, 0xBA, 0x63, 0x55, 0xB6,
        0x3D, 0x9D, 0xF2, 0x4B, 0x5F, 0x7B, 0x6A, 0xB2, 0x62, 0x1D, 0xC2, 0x1B, 0x68, 0xE5, 0xC8, 0xB5,
        0x3A, 0x05, 0x90, 0x00, 0xE8, 0xA8, 0x10, 0x3D, 0xE2, 0xEC, 0xF0, 0x0C, 0xB2, 0xED, 0x4F, 0x6D,
    };

    private static ReadOnlySpan<byte> OutroHashBytes => new byte[]
    {
        0xD6, 0xC0, 0x1C, 0x59, 0x8B, 0xC8, 0xB8, 0xCB, 0x46, 0xE1, 0x53, 0xFC, 0x82, 0x8C, 0x75, 0x75,
        0x13, 0xE0, 0x45, 0xDF, 0x32, 0x69, 0x3C, 0x75, 0xF0, 0x59, 0xF8, 0xD9, 0xA2, 0x5F, 0xB2, 0x17,
        0xE0, 0x80, 0x52, 0xDB, 0xEA, 0x89, 0x73, 0x99, 0x75, 0x79, 0xAF, 0xCB, 0x2E, 0x80, 0x07, 0xE6,
        0xF1, 0x26, 0xE0, 0x03, 0x0A, 0xE6, 0x6F, 0xF6, 0x41, 0xBF, 0x7E, 0x59, 0xC2, 0xAE, 0x55, 0xFD,
    };

    private static ReadOnlySpan<byte> StaticXorpad => new byte[]
    {
        0xA0, 0x92, 0xD1, 0x06, 0x07, 0xDB, 0x32, 0xA1, 0xAE, 0x01, 0xF5, 0xC5, 0x1E, 0x84, 0x4F, 0xE3,
        0x53, 0xCA, 0x37, 0xF4, 0xA7, 0xB0, 0x4D, 0xA0, 0x18, 0xB7, 0xC2, 0x97, 0xDA, 0x5F, 0x53, 0x2B,
        0x75, 0xFA, 0x48, 0x16, 0xF8, 0xD4, 0x8A, 0x6F, 0x61, 0x05, 0xF4, 0xE2, 0xFD, 0x04, 0xB5, 0xA3,
        0x0F, 0xFC, 0x44, 0x92, 0xCB, 0x32, 0xE6, 0x1B, 0xB9, 0xB1, 0x2E, 0x01, 0xB0, 0x56, 0x53, 0x36,
        0xD2, 0xD1, 0x50, 0x3D, 0xDE, 0x5B, 0x2E, 0x0E, 0x52, 0xFD, 0xDF, 0x2F, 0x7B, 0xCA, 0x63, 0x50,
        0xA4, 0x67, 0x5D, 0x23, 0x17, 0xC0, 0x52, 0xE1, 0xA6, 0x30, 0x7C, 0x2B, 0xB6, 0x70, 0x36, 0x5B,
        0x2A, 0x27, 0x69, 0x33, 0xF5, 0x63, 0x7B, 0x36, 0x3F, 0x26, 0x9B, 0xA3, 0xED, 0x7A, 0x53, 0x00,
        0xA4, 0x48, 0xB3, 0x50, 0x9E, 0x14, 0xA0, 0x52, 0xDE, 0x7E, 0x10, 0x2B, 0x1B, 0x77, 0x6E, 0, // aligned to 0x80
    };

    public static void CryptStaticXorpadBytes(Span<byte> data)
    {
        // Apply the xorpad over each chunk of xorpad-sized spans.
        // This is 30x as fast as a single loop with a modulus operation (benchmarked; modulo is slower).
        // Marshal as a vectorized operation to speed up the process.
        // Due to the xorpad being extended 0x7F->0x80, if len%7F==0, we miss the last vectored xor.
        // Subtract 1 from the data size in the event that the length is an even multiple, to get one less iteration.
        var xp = StaticXorpad;
        var xp64 = MemoryMarshal.Cast<byte, Vector<ulong>>(xp);
        var size = xp.Length - 1;
        int iterations = (data.Length - 1) / size;
        do
        {
            var slice = MemoryMarshal.Cast<byte, Vector<ulong>>(data[..xp.Length]);
            for (int i = slice.Length - 1; i >= 0; i--)
                slice[i] ^= xp64[i];
            data = data[size..];
        } while (--iterations != 0);
        // Xor the remainder.
        for (int i = data.Length - 1; i >= 0; i--)
            data[i] ^= xp[i];
    }

    private static void ComputeHash(ReadOnlySpan<byte> data, Span<byte> hash)
    {
        using var h = IncrementalHash.CreateHash(HashAlgorithmName.SHA256);
        h.AppendData(IntroHashBytes);
        h.AppendData(data);
        h.AppendData(OutroHashBytes);
        h.TryGetCurrentHash(hash, out _);
    }

    /// <summary>
    /// Checks if the file is a rough example of a save file.
    /// </summary>
    /// <param name="data">Encrypted save data</param>
    /// <returns>True if hash matches</returns>
    public static bool GetIsHashValid(ReadOnlySpan<byte> data)
    {
        Span<byte> computed = stackalloc byte[SIZE_HASH];
        ComputeHash(data[..^SIZE_HASH], computed);
        var stored = data[^computed.Length..];
        return computed.SequenceEqual(stored);
    }

    /// <summary>
    /// Decrypts the save data in-place, then unpacks the blocks.
    /// </summary>
    /// <param name="data">Encrypted save data</param>
    /// <returns>Decrypted blocks.</returns>
    /// <remarks>
    /// Hash is assumed to be valid before calling this method.
    /// </remarks>
    public static IReadOnlyList<SCBlock> Decrypt(Span<byte> data)
    {
        // ignore hash
        var payload = data[..^SIZE_HASH];
        CryptStaticXorpadBytes(payload);
        return ReadBlocks(payload);
    }

    private const int BlockDataRatioEstimate1 = 777; // bytes per block, on average (generous)
    private const int BlockDataRatioEstimate2 = 555; // bytes per block, on average (stingy)

    private static IReadOnlyList<SCBlock> ReadBlocks(ReadOnlySpan<byte> data)
    {
        var result = new List<SCBlock>(data.Length / BlockDataRatioEstimate2);
        int offset = 0;
        while (offset < data.Length)
        {
            var block = SCBlock.ReadFromOffset(data, ref offset);
            result.Add(block);
        }

        return result;
    }

    /// <summary>
    /// Tries to encrypt the save data.
    /// </summary>
    /// <param name="blocks">Decrypted save data</param>
    /// <returns>Encrypted save data.</returns>
    public static byte[] Encrypt(IReadOnlyList<SCBlock> blocks)
    {
        var result = GetDecryptedRawData(blocks);
        var span = result.AsSpan();
        var payload = span[..^SIZE_HASH];
        CryptStaticXorpadBytes(payload);
        ComputeHash(payload, span[^SIZE_HASH..]);
        return result;
    }

    /// <summary>
    /// Tries to encrypt the save data.
    /// </summary>
    /// <returns>Raw save data without the final xorpad layer.</returns>
    public static byte[] GetDecryptedRawData(IReadOnlyList<SCBlock> blocks)
    {
        using var ms = new MemoryStream(blocks.Count * BlockDataRatioEstimate1);
        using var bw = new BinaryWriter(ms);
        foreach (var block in blocks)
            block.WriteBlock(bw);

        var result = new byte[ms.Position + SIZE_HASH];
        var payload = result.AsSpan()[..^SIZE_HASH];
        ms.Position = 0;
        ms.ReadExactly(payload);
        return result;
    }
}