#nullable enable
using System;
using System.Buffers.Binary;
using System.Collections.Generic;
using System.Linq;
using System.Text;


public static class ImageSignatureRequest
{



    const int HeaderScanLimit = 0x100;

    const int MinimumRecords = 32;




    const int PrefixProbe = 8;

    const int PackingProbe = 1024;

    const int MaxLiteralLength = 1 << 20;

    static readonly UTF8Encoding StrictUtf8 = new UTF8Encoding(false, true);


    public static string? DetectKeyId(byte[]? metadata)
    {
        if (metadata == null || metadata.Length < 24 ||
            BinaryPrimitives.ReadUInt32LittleEndian(metadata) != 0xFAB11BAF ||
            BinaryPrimitives.ReadInt32LittleEndian(metadata.AsSpan(4)) != 27)
            return null;
        int table = BinaryPrimitives.ReadInt32LittleEndian(metadata.AsSpan(8));
        int size = BinaryPrimitives.ReadInt32LittleEndian(metadata.AsSpan(12));
        int data = BinaryPrimitives.ReadInt32LittleEndian(metadata.AsSpan(16));
        int dataSize = BinaryPrimitives.ReadInt32LittleEndian(metadata.AsSpan(20));
        if (table < 24 || size < 0 || size % 8 != 0 || (long)table + size > metadata.Length ||
            data < 24 || dataSize < 0 || (long)data + dataSize > metadata.Length)
            return null;

        return ScanKeyTails(metadata, table, size / 8, data, dataSize);
    }



    public static string? DetectKeyIdFromGameMetadata(byte[]? metadata)
    {
        if (metadata == null || metadata.Length < 24) return null;
        if (BinaryPrimitives.ReadUInt32LittleEndian(metadata) == 0xFAB11BAF &&
            BinaryPrimitives.ReadInt32LittleEndian(metadata.AsSpan(4)) == 27)
            return DetectKeyId(metadata);
        return DetectKeyIdByHeaderScan(metadata);
    }




    static string? ScanKeyTails(byte[] metadata, int tableOffset, int recordCount, int dataOffset, int dataSize)
    {
        string? key = null;
        for (int record = 0; record < recordCount; record++)
        {
            var (length, index) = Record(metadata, tableOffset, record);
            if (length < 0 || index < 0 || (long)index + length > dataSize) return null;
            if (length != 8 && length != 18) continue;
            var literal = Encoding.UTF8.GetString(metadata, dataOffset + index, length);
            if (literal != ".rec.net" && literal != ".radicade.internal") continue;
            var candidate = "KEY:RSA:p1" + literal;
            if (key != null && key != candidate) return null;
            key = candidate;
        }
        return key;
    }




    static (int Length, int Index) Record(byte[] metadata, int tableOffset, int record)
    {
        int offset = tableOffset + 8 * record;
        return (BinaryPrimitives.ReadInt32LittleEndian(metadata.AsSpan(offset)),
                BinaryPrimitives.ReadInt32LittleEndian(metadata.AsSpan(offset + 4)));
    }

    public static string? DetectKeyIdByHeaderScan(byte[]? metadata)
    {
        if (metadata == null || metadata.Length < HeaderScanLimit + 8) return null;

        var fields = new List<int>();
        for (int offset = 0; offset + 4 <= HeaderScanLimit; offset += 4)
        {
            int value = BinaryPrimitives.ReadInt32LittleEndian(metadata.AsSpan(offset));
            if (value > 0 && value <= metadata.Length && !fields.Contains(value)) fields.Add(value);
        }

        var tables = new List<(int Offset, int Count)>();
        foreach (var tableOffset in fields)
        foreach (var tableSize in fields)
        {
            if (tableSize % 8 != 0 || tableSize / 8 < MinimumRecords) continue;
            if (tableOffset < HeaderScanLimit || (long)tableOffset + tableSize > metadata.Length) continue;
            tables.Add((tableOffset, tableSize / 8));
        }

        var slices = new List<(int Offset, int Size)>();
        foreach (var dataOffset in fields)
        foreach (var dataSize in fields)
        {
            if (dataSize <= 0 || dataOffset < HeaderScanLimit) continue;
            if ((long)dataOffset + dataSize > metadata.Length) continue;
            slices.Add((dataOffset, dataSize));
        }



        tables.Sort((a, b) => b.Count.CompareTo(a.Count));

        foreach (var (tableOffset, count) in tables)
        foreach (var (dataOffset, dataSize) in slices)
        {
            if (!LooksLikeLiteralTable(metadata, tableOffset, count, dataOffset, dataSize, PrefixProbe)) continue;
            if (!SampleDecodesAsText(metadata, tableOffset, count, dataOffset, dataSize)) continue;
            if (!LooksLikeLiteralTable(metadata, tableOffset, count, dataOffset, dataSize, PackingProbe)) continue;
            var key = ScanKeyTails(metadata, tableOffset, count, dataOffset, dataSize);
            if (key != null) return key;
        }
        return null;
    }




    static bool LooksLikeLiteralTable(byte[] metadata, int tableOffset, int count, int dataOffset, int dataSize, int probe)
    {
        int examined = Math.Min(count, probe);
        int contiguous = 0;
        long running = 0;
        for (int record = 0; record < examined; record++)
        {
            var (length, index) = Record(metadata, tableOffset, record);
            if (length < 0 || length > MaxLiteralLength || index < 0 || (long)index + length > dataSize) return false;
            if (index == running) contiguous++;
            running = index + length;
        }
        return contiguous >= examined - Math.Max(1, examined / 100);
    }

    static bool SampleDecodesAsText(byte[] metadata, int tableOffset, int count, int dataOffset, int dataSize)
    {
        int step = Math.Max(1, count / 48);
        for (int record = 0; record < count; record += step)
        {
            var (length, index) = Record(metadata, tableOffset, record);
            if (length < 0 || index < 0 || (long)index + length > dataSize) return false;
            if (length == 0) continue;
            try
            {
                StrictUtf8.GetCharCount(metadata, dataOffset + index, length);
            }
            catch (ArgumentException)
            {
                return false;
            }
        }
        return true;
    }

    public static string AddKeyId(string url, string serverUrl, string? keyId)
    {
        if (string.IsNullOrEmpty(keyId) ||
            !Uri.TryCreate(url, UriKind.Absolute, out var request) ||
            !Uri.TryCreate(serverUrl, UriKind.Absolute, out var server) ||
            (server.Scheme != "http" && server.Scheme != "https") ||
            request.Scheme != server.Scheme || request.Authority != server.Authority ||
            !request.AbsolutePath.StartsWith(server.AbsolutePath.TrimEnd('/') + "/images/", StringComparison.OrdinalIgnoreCase))
            return url;


        var parameters = request.Query.TrimStart('?').Split('&', StringSplitOptions.RemoveEmptyEntries)
            .Where(p => !Uri.UnescapeDataString(p.Split('=')[0]).Equals("rcsig", StringComparison.OrdinalIgnoreCase))
            .Append("rcsig=" + Uri.EscapeDataString(keyId));
        return request.GetLeftPart(UriPartial.Path) + "?" + string.Join("&", parameters) + request.Fragment;
    }
}