using System.Collections.Generic;

namespace AcDream.Core.Physics.Motion;

/// <summary>
/// One entry of a <see cref="MotionState"/> chain — retail's
/// <c>MotionList</c> node (12 bytes: motion + speed_mod + next;
/// acclient.h, r2-motiontable-decomp.md §14). One struct, two independent
/// chain disciplines: the modifier chain is a PUSH-FRONT STACK, the action
/// chain a TAIL-APPEND FIFO.
/// </summary>
public sealed class MotionEntry
{
    public uint Motion;
    public float SpeedMod = 1f;

    public MotionEntry(uint motion, float speedMod)
    {
        Motion = motion;
        SpeedMod = speedMod;
    }
}

/// <summary>
/// R2-Q1 — verbatim port of retail's <c>MotionState</c> (acclient.h:31081;
/// r2-motiontable-decomp.md §13): the motion-table-facing state block —
/// current style / substate / substate_mod plus the modifier stack and the
/// action FIFO that <c>CMotionTable::GetObjectSequence</c>,
/// <c>StopSequenceMotion</c>, <c>re_modify</c>, and
/// <c>MotionTableManager::AnimationDone</c> read and write.
/// </summary>
public sealed class MotionState
{
    /// <summary>Current style/stance command word (0 = unset; ctor 0x00525fd0).</summary>
    public uint Style;

    /// <summary>Current base substate command word (0 = unset).</summary>
    public uint Substate;

    /// <summary>Speed modifier of the base substate (default 1.0).</summary>
    public float SubstateMod = 1f;

    private readonly LinkedList<MotionEntry> _modifiers = new(); // modifier_head — push-front stack
    private readonly LinkedList<MotionEntry> _actions = new();   // action_head/tail — FIFO

    public MotionState()
    {
    }

    /// <summary>
    /// Deep copy (retail copy ctor 0x00526790; Q0-pins A4-#5): both chains
    /// CLONED — <c>re_modify</c>'s snapshot is a termination bound, never
    /// shared state.
    /// </summary>
    public MotionState(MotionState other)
    {
        Style = other.Style;
        Substate = other.Substate;
        SubstateMod = other.SubstateMod;
        foreach (var m in other._modifiers)
            _modifiers.AddLast(new MotionEntry(m.Motion, m.SpeedMod));
        foreach (var a in other._actions)
            _actions.AddLast(new MotionEntry(a.Motion, a.SpeedMod));
    }

    /// <summary>Modifier chain in retail order (newest first — push-front).</summary>
    public IEnumerable<MotionEntry> Modifiers => _modifiers;

    /// <summary>Action FIFO in retail order (oldest first).</summary>
    public IEnumerable<MotionEntry> Actions => _actions;

    /// <summary><c>add_modifier_no_check</c> (0x00525ff0): unconditional
    /// push-front.</summary>
    public void AddModifierNoCheck(uint motion, float speedMod)
        => _modifiers.AddFirst(new MotionEntry(motion, speedMod));

    /// <summary>
    /// <c>add_modifier</c> (0x00526340): refuse when the motion is already
    /// a modifier (caller must stop-then-re-add to refresh) or already IS
    /// the current base substate.
    /// </summary>
    public bool AddModifier(uint motion, float speedMod)
    {
        for (var n = _modifiers.First; n is not null; n = n.Next)
            if (n.Value.Motion == motion)
                return false;

        if (Substate == motion)
            return false;

        AddModifierNoCheck(motion, speedMod);
        return true;
    }

    /// <summary><c>remove_modifier</c> (0x00526040) — by node identity
    /// (retail's node+predecessor pair collapses in a managed list).</summary>
    public void RemoveModifier(MotionEntry entry)
    {
        _modifiers.Remove(entry);
    }

    /// <summary><c>clear_modifiers</c> (0x00526070).</summary>
    public void ClearModifiers() => _modifiers.Clear();

    /// <summary><c>add_action</c> (0x005260a0): tail append.</summary>
    public void AddAction(uint motion, float speedMod)
        => _actions.AddLast(new MotionEntry(motion, speedMod));

    /// <summary><c>remove_action_head</c> (0x00526120): pop the FIFO head,
    /// returning its motion id (0 when empty).</summary>
    public uint RemoveActionHead()
    {
        var head = _actions.First;
        if (head is null)
            return 0;
        _actions.RemoveFirst();
        return head.Value.Motion;
    }

    /// <summary><c>clear_actions</c> (0x005260f0).</summary>
    public void ClearActions() => _actions.Clear();
}