using System.Text;

namespace WxAgent.Core;

public sealed record WcdbKeyCandidate(string EncKey, string? SaltHint);

public sealed class WcdbKeyCandidateScanner
{
    private const int KeyHexLength = 64;
    private const int SaltHexLength = 32;

    private readonly StringBuilder _keyPrefix = new(KeyHexLength);
    private readonly char[] _saltSuffix = new char[SaltHexLength];
    private readonly HashSet<string> _seen = new(StringComparer.Ordinal);
    private long _runLength;
    private readonly bool _utf16;
    private bool _hasLowByte;
    private byte _lowByte;

    public WcdbKeyCandidateScanner(bool utf16 = false) => _utf16 = utf16;

    public IReadOnlyList<WcdbKeyCandidate> Feed(ReadOnlySpan<byte> bytes, bool finalBlock = false)
    {
        var found = new List<WcdbKeyCandidate>();
        foreach (var rawValue in bytes)
        {
            var value = rawValue;
            if (_utf16)
            {
                if (!_hasLowByte)
                {
                    _lowByte = value;
                    _hasLowByte = true;
                    continue;
                }
                _hasLowByte = false;
                if (value != 0)
                {
                    CompleteRun(found);
                    continue;
                }
                value = _lowByte;
            }
            if (IsAsciiHex(value))
            {
                AppendHex(value);
            }
            else
            {
                CompleteRun(found);
            }
        }

        if (finalBlock)
        {
            CompleteRun(found);
            _hasLowByte = false;
        }

        return found;
    }

    private void AppendHex(byte value)
    {
        var character = char.ToLowerInvariant((char)value);
        if (_runLength < KeyHexLength)
        {
            _keyPrefix.Append(character);
        }

        _saltSuffix[_runLength % SaltHexLength] = character;
        _runLength++;
    }

    private void CompleteRun(List<WcdbKeyCandidate> found)
    {
        if (_runLength >= KeyHexLength)
        {
            var key = _keyPrefix.ToString();
            var salt = _runLength >= KeyHexLength + SaltHexLength ? GetSaltSuffix() : null;
            if (_seen.Add(key + ":" + salt))
            {
                found.Add(new WcdbKeyCandidate(key, salt));
            }
        }

        _keyPrefix.Clear();
        _runLength = 0;
    }

    private string GetSaltSuffix()
    {
        var salt = new char[SaltHexLength];
        var start = (int)(_runLength % SaltHexLength);
        for (var index = 0; index < salt.Length; index++)
        {
            salt[index] = _saltSuffix[(start + index) % SaltHexLength];
        }

        return new string(salt);
    }

    private static bool IsAsciiHex(byte value) =>
        value is >= (byte)'0' and <= (byte)'9' or
        >= (byte)'a' and <= (byte)'f' or
        >= (byte)'A' and <= (byte)'F';
}