using System;
using System.Buffers.Binary;

namespace AcDream.Core.Net.Messages;

/// <summary>
/// Inbound skill update <c>GameMessage</c> for the local player
/// (<c>0x02DD</c>) — the server's authoritative answer to RaiseSkill /
/// TrainSkill / specialize / untrain / reset. A standalone GameMessage
/// like <see cref="PrivateUpdateVital"/>, NOT a <c>0xF7B0</c> GameEvent.
///
/// <para>
/// Campaign CA slice CA2 (#431). NOTE the opcode-neighborhood trap this
/// slice also corrected: 0x02DD is <b>PrivateUpdateSkill</b>, not
/// PrivateUpdatePropertyString (which is 0x02D5) — ACE
/// <c>GameMessageOpcode.cs:21,29</c>. The related ranks-only
/// <c>PrivateUpdateSkillLevel (0x02DF)</c> has NO producer anywhere in
/// ACE (verified 2026-08-24) and is deliberately not parsed.
/// </para>
///
/// <para>
/// Wire layout — three-source agreement (ACE
/// <c>GameMessagePrivateUpdateSkill.cs:8-24</c>; Chorizite
/// <c>Skill.generated.cs:22-86</c>; holtburger
/// <c>player/types.rs:71-131</c> with the golden fixture at
/// <c>:279-293</c> confirming <c>adjustPP=1</c> on real captures), full
/// citations in
/// <c>docs/research/2026-08-24-advancement-wire-and-recompute.md</c> §2.8:
/// </para>
/// <code>
/// PrivateUpdateSkill (0x02DD):
///     u32 opcode = 0x02DD
///     u8  sequence          // ByteSequence, per-skill counter
///     u32 skillId           // Skill enum ordinal
///     u16 ranks             // LevelFromPP — ushort on the wire!
///     u16 adjustPP          // hardcoded 1 by ACE on every send
///     u32 advancementClass  // SkillAdvancementClass (1=Untrained/2=Trained/3=Specialized)
///     u32 xp                // ExperienceSpent / PP
///     u32 init              // InitLevel
///     u32 resistance        // ResistanceAtLastCheck
///     f64 lastUsedTime
/// </code>
/// </summary>
public static class PrivateUpdateSkill
{
    public const uint Opcode = 0x02DDu;

    /// <summary>Parsed skill update. Ranks widened from the wire's u16.</summary>
    public readonly record struct Parsed(
        byte Sequence,
        uint SkillId,
        uint Ranks,
        ushort AdjustPP,
        uint AdvancementClass,
        uint Xp,
        uint Init,
        uint Resistance,
        double LastUsed);

    /// <summary>
    /// Parse a raw <c>PrivateUpdateSkill (0x02DD)</c> body. Returns
    /// <c>null</c> on opcode mismatch or truncation.
    /// </summary>
    public static Parsed? TryParse(ReadOnlySpan<byte> body)
    {
        // 4 (opcode) + 1 (seq) + 4 + 2 + 2 + 4*4 + 8 = 37 bytes minimum.
        if (body.Length < 37) return null;
        uint opcode = BinaryPrimitives.ReadUInt32LittleEndian(body);
        if (opcode != Opcode) return null;

        int pos = 4;
        byte seq        = body[pos]; pos += 1;
        uint skillId    = BinaryPrimitives.ReadUInt32LittleEndian(body[pos..]); pos += 4;
        ushort ranks    = BinaryPrimitives.ReadUInt16LittleEndian(body[pos..]); pos += 2;
        ushort adjustPP = BinaryPrimitives.ReadUInt16LittleEndian(body[pos..]); pos += 2;
        uint sac        = BinaryPrimitives.ReadUInt32LittleEndian(body[pos..]); pos += 4;
        uint xp         = BinaryPrimitives.ReadUInt32LittleEndian(body[pos..]); pos += 4;
        uint init       = BinaryPrimitives.ReadUInt32LittleEndian(body[pos..]); pos += 4;
        uint resistance = BinaryPrimitives.ReadUInt32LittleEndian(body[pos..]); pos += 4;
        double lastUsed = BitConverter.Int64BitsToDouble(
            BinaryPrimitives.ReadInt64LittleEndian(body[pos..]));
        return new Parsed(
            seq, skillId, ranks, adjustPP, sac, xp, init, resistance, lastUsed);
    }
}