using System.Buffers.Binary;

namespace WebDrop.Core;

/// <summary>帧封装的 26 字节明文头部,见 protocol.md §4。负载由服务端原样保存,从不解析。</summary>
public readonly record struct Envelope(FrameFlags Flags, Id16 ItemId, uint FrameIndex, int PayloadLength)
{
    public const int HeaderSize = 26;
    public const byte Version = 1;

    public int TotalLength => HeaderSize + PayloadLength;

    /// <summary>头部格式合法、保留位为 0、长度与 payload_len 严格一致。</summary>
    public static bool TryParse(ReadOnlySpan<byte> data, out Envelope envelope)
    {
        envelope = default;
        if (data.Length < HeaderSize || data[0] != Version) return false;

        byte flags = data[1];
        if ((flags & 0xF0) != 0) return false;

        uint payloadLen = BinaryPrimitives.ReadUInt32BigEndian(data[22..]);
        if (payloadLen > int.MaxValue || data.Length != HeaderSize + (int)payloadLen) return false;

        envelope = new Envelope(
            (FrameFlags)flags,
            Id16.FromBytes(data.Slice(2, Id16.Size)),
            BinaryPrimitives.ReadUInt32BigEndian(data[18..]),
            (int)payloadLen);
        return true;
    }

    public bool IsFinal => (Flags & FrameFlags.Final) != 0;
    public bool IsGated => (Flags & FrameFlags.Gated) != 0;
    public bool IsSnapshot => (Flags & FrameFlags.Snapshot) != 0;
    public bool IsEnc => (Flags & FrameFlags.Enc) != 0;

    /// <summary>条目级标志:同一条目的所有帧必须一致。</summary>
    public FrameFlags ItemFlags => Flags & (FrameFlags.Snapshot | FrameFlags.Gated);
}

public static class PayloadTiers
{
    public const int TagSize = 16;
    private static readonly int[] Tiers = [1024, 4096, 16384, 65536, 262144, 1048576];

    /// <summary>enc 负载 = 档位明文 + 16 字节 GCM 标签,服务端据此强制 padding。</summary>
    public static bool IsValidEncPayloadLength(int payloadLength) =>
        payloadLength > TagSize && Array.IndexOf(Tiers, payloadLength - TagSize) >= 0;
}