using System;
using System.Buffers.Binary;

namespace AcDream.Core.Net.Messages;

/// <summary>
/// Inbound vital-update <c>GameMessage</c>s for the local player.
/// These do NOT ride the <c>0xF7B0</c> GameEvent envelope — they're
/// standalone GameMessages dispatched the same way as
/// <see cref="HearSpeech"/> / <c>CreateObject</c> / <c>UpdateMotion</c>.
///
/// <para>
/// Wire format reference: holtburger
/// <c>crates/holtburger-protocol/src/messages/player/types.rs</c>
/// <c>UpdateVital</c> + <c>UpdateVitalCurrent</c>. ACE producers:
/// <c>GameMessagePrivateUpdateVital</c> +
/// <c>GameMessagePrivateUpdateAttribute2ndLevel</c>. The "sequence"
/// field is a <b>single byte</b> per ACE
/// <c>ByteSequence.NextBytes</c>; not 4 bytes despite some ACE writer
/// signatures looking like <c>uint</c>.
/// </para>
///
/// <para>Wire layouts (private flavour, no object_guid):</para>
/// <code>
/// PrivateUpdateVital (0x02E7):
///     u32 opcode = 0x02E7
///     u8  sequence
///     u32 vital      // ACE Vital enum (1=MaxHealth..6=Mana)
///     u32 ranks
///     u32 start      // StartingValue
///     u32 xp         // ExperienceSpent
///     u32 current
/// </code>
/// <code>
/// PrivateUpdateVitalCurrent (0x02E9) — current-only delta (regen / drain):
///     u32 opcode = 0x02E9
///     u8  sequence
///     u32 vital
///     u32 current
/// </code>
///
/// <para>
/// Vital ID semantics live in
/// <see cref="AcDream.Core.Player.LocalPlayerState.VitalIdToKind"/>.
/// </para>
/// </summary>
public static class PrivateUpdateVital
{
    public const uint FullOpcode    = 0x02E7u;
    public const uint CurrentOpcode = 0x02E9u;

    /// <summary>Parsed full-update message.</summary>
    public readonly record struct ParsedFull(
        byte Sequence,
        uint VitalId,
        uint Ranks,
        uint Start,
        uint Xp,
        uint Current);

    /// <summary>Parsed current-only delta.</summary>
    public readonly record struct ParsedCurrent(
        byte Sequence,
        uint VitalId,
        uint Current);

    /// <summary>
    /// Parse a raw <c>PrivateUpdateVital (0x02E7)</c> body. Returns
    /// <c>null</c> if opcode mismatch or truncated.
    /// </summary>
    public static ParsedFull? TryParseFull(ReadOnlySpan<byte> body)
    {
        // 4 (opcode) + 1 (seq) + 5 * 4 (uints) = 25 bytes minimum.
        if (body.Length < 25) return null;
        uint opcode = BinaryPrimitives.ReadUInt32LittleEndian(body);
        if (opcode != FullOpcode) return null;

        int pos = 4;
        byte seq      = body[pos]; pos += 1;
        uint vital    = BinaryPrimitives.ReadUInt32LittleEndian(body[pos..]); pos += 4;
        uint ranks    = BinaryPrimitives.ReadUInt32LittleEndian(body[pos..]); pos += 4;
        uint start    = BinaryPrimitives.ReadUInt32LittleEndian(body[pos..]); pos += 4;
        uint xp       = BinaryPrimitives.ReadUInt32LittleEndian(body[pos..]); pos += 4;
        uint current  = BinaryPrimitives.ReadUInt32LittleEndian(body[pos..]);
        return new ParsedFull(seq, vital, ranks, start, xp, current);
    }

    /// <summary>
    /// Parse a raw <c>PrivateUpdateVitalCurrent (0x02E9)</c> body. Returns
    /// <c>null</c> if opcode mismatch or truncated.
    /// </summary>
    public static ParsedCurrent? TryParseCurrent(ReadOnlySpan<byte> body)
    {
        // 4 (opcode) + 1 (seq) + 2 * 4 (uints) = 13 bytes minimum.
        if (body.Length < 13) return null;
        uint opcode = BinaryPrimitives.ReadUInt32LittleEndian(body);
        if (opcode != CurrentOpcode) return null;

        int pos = 4;
        byte seq     = body[pos]; pos += 1;
        uint vital   = BinaryPrimitives.ReadUInt32LittleEndian(body[pos..]); pos += 4;
        uint current = BinaryPrimitives.ReadUInt32LittleEndian(body[pos..]);
        return new ParsedCurrent(seq, vital, current);
    }
}