using System;
using System.Collections.Generic;
using System.Numerics;

namespace AcDream.Core.Audio;

/// <summary>
/// One ambient firing, as the scheduler hands it to the audio backend.
/// </summary>
/// <param name="Instance">The instance that fired (its descriptor names the slot).</param>
/// <param name="Volume">The volume to play at — crossfaded for a continuous bed.</param>
/// <param name="Position">
/// The world position, or null for a continuous bed — retail plays those through
/// <c>PlayAmbientSoundFromCenter</c>, with no position, no pan and no distance
/// attenuation: a stereo bed centred on the listener.
/// </param>
public readonly record struct AmbientSoundFiring(
    AmbientSoundInstance Instance,
    float Volume,
    Vector3? Position);

/// <summary>
/// Retail's ambient playback engine: a min-heap of ABSOLUTE deadlines, drained
/// once per frame, where each pop plays a one-shot and immediately re-arms
/// itself. There is no looping voice anywhere in retail's audio path — a
/// "continuous" ambient is a one-shot re-fired every <c>min_rate</c> seconds,
/// which is why an <c>AL_LOOPING</c> port sounds wrong (no re-randomised table
/// pick, no re-rolled crossfade volume, no gap).
///
/// <para>
/// Ports <c>Ambient::UpdatePlayQueue</c> @ <c>0x551A50</c>,
/// <c>Ambient::Play</c> @ <c>0x5517A0</c> and <c>Ambient::UseTime</c> @
/// <c>0x551880</c>.
/// </para>
/// </summary>
public sealed class AmbientSoundScheduler
{
    private readonly PriorityQueue<AmbientSoundInstance, double> _queue = new();
    private readonly List<AmbientSoundInstance> _instances = [];
    private readonly ISoundRandom _rng;

    public AmbientSoundScheduler(ISoundRandom? rng = null) => _rng = rng ?? new SoundRandom();

    /// <summary>Every live instance, in accumulation order.</summary>
    public IReadOnlyList<AmbientSoundInstance> Instances => _instances;

    /// <summary>Instances currently holding a deadline. Diagnostic use.</summary>
    public int QueuedCount => _queue.Count;

    /// <summary>Sum of every instance's weight — the crossfade denominator.</summary>
    public float TotalSoundCount { get; private set; }

    /// <summary>
    /// Begin a rebuild. <c>Ambient::InitSounds</c> @ <c>0x5515D0</c> resets EVERY
    /// existing instance's counters before the accumulation pass — skipping that
    /// leaves stale bearings and probabilities alive indefinitely, because
    /// <c>IntermitSound::UpdateSound</c> never clears them itself.
    /// </summary>
    public void BeginRebuild()
    {
        foreach (AmbientSoundInstance instance in _instances)
            instance.ResetCount();
        TotalSoundCount = 0f;
    }

    /// <summary>
    /// Register an instance for this world, if it is not already tracked. Called
    /// while walking the contributing land cells.
    /// </summary>
    public AmbientSoundInstance Track(AmbientSoundDescriptor descriptor, uint soundTableDid)
    {
        foreach (AmbientSoundInstance existing in _instances)
        {
            if (existing.Descriptor == descriptor && existing.SoundTableDid == soundTableDid)
                return existing;
        }

        var created = new AmbientSoundInstance(descriptor, soundTableDid);
        _instances.Add(created);
        return created;
    }

    /// <summary>
    /// Contribute one land cell to every ambient its sound table authors —
    /// <c>Ambient::AddSound</c> @ <c>0x551610</c>.
    ///
    /// <para>
    /// The cell's weight lands in <see cref="TotalSoundCount"/> exactly ONCE,
    /// however many descriptors the table carries. Retail adds <c>w</c> to
    /// <c>total_sound_count</c> before looping the descriptors, so with an
    /// N-entry table the per-instance counts sum to N × the denominator — a
    /// deliberate retail property, not an oversight. Adding the weight per
    /// descriptor instead divides every bed's crossfade by N, which pushes a
    /// typical authored volume under the 0.03 audibility floor and silences it.
    /// </para>
    /// </summary>
    public void ContributeCell<TTable>(
        Vector3 offset,
        TTable table,
        Func<TTable, int, AmbientSoundDescriptor> descriptorAt)
        where TTable : DatReaderWriter.Types.AmbientSTBDesc
    {
        ArgumentNullException.ThrowIfNull(table);
        ArgumentNullException.ThrowIfNull(descriptorAt);

        float weight = AmbientSoundConstants.CalcWeight(offset);
        if (weight <= 0f)
            return;

        AmbientDirection direction = AmbientSoundConstants.CalcDirection(offset);
        TotalSoundCount += weight;

        for (int i = 0; i < table.AmbientSounds.Count; i++)
        {
            AmbientSoundInstance instance = Track(descriptorAt(table, i), table.STBId);
            instance.AddTo(weight, offset, direction);
        }
    }

    /// <summary>
    /// Contribute one land cell's weight to a single instance. Test seam and the
    /// single-descriptor case; <see cref="ContributeCell{TTable}"/> is the
    /// production path and owns the shared denominator.
    /// </summary>
    public void Contribute(AmbientSoundInstance instance, Vector3 offset)
    {
        ArgumentNullException.ThrowIfNull(instance);

        float weight = AmbientSoundConstants.CalcWeight(offset);
        if (weight <= 0f)
            return;

        instance.AddTo(weight, offset, AmbientSoundConstants.CalcDirection(offset));
        TotalSoundCount += weight;
    }

    /// <summary>
    /// Finish a rebuild: recompute every instance's crossfade against the shared
    /// denominator, then arm any audible instance that is not already queued.
    ///
    /// <para>
    /// The <c>on_queue</c> guard is load-bearing in both directions. Re-arming
    /// unconditionally restarts every ambient on every 24 m crossing — audible as
    /// a machine-gun of one-shots. Never re-arming leaves a newly-audible ambient
    /// silent until the next rebuild.
    /// </para>
    /// </summary>
    public void EndRebuild(
        double now,
        ICollection<AmbientSoundFiring>? firings = null,
        Vector3 listenerPosition = default)
    {
        foreach (AmbientSoundInstance instance in _instances)
            instance.UpdateSound(TotalSoundCount);

        foreach (AmbientSoundInstance instance in _instances)
        {
            if (instance.OnQueue || !instance.CanHear())
                continue;

            // UpdatePlayQueue @ 0x551A50 calls Play for every on_queue == 0
            // sound, and Play @ 0x5517A0 PLAYS first and then re-arms. There is
            // no initial delay: an ambient that becomes audible at a crossing
            // sounds at the crossing, not one min_rate later.
            Fire(instance, now, firings, listenerPosition);
        }
    }

    /// <summary>
    /// Drain every deadline that has come due — <c>Ambient::UseTime</c>. Each pop
    /// either fires (and re-arms) or, if the instance can no longer be heard,
    /// leaves the queue entirely until a later rebuild re-arms it.
    /// </summary>
    public void Tick(double now, ICollection<AmbientSoundFiring> firings, Vector3 listenerPosition)
    {
        ArgumentNullException.ThrowIfNull(firings);

        // Ambient::UseTime @ 0x551880 breaks on !(key < cur_time) — STRICTLY
        // below. That is also what stops a descriptor authored with a zero rate
        // from re-arming at the same instant and spinning this loop forever.
        while (_queue.TryPeek(out _, out double deadline) && deadline < now)
        {
            AmbientSoundInstance instance = _queue.Dequeue();
            instance.OnQueue = false;

            // Ambient::Play @ 0x5517A0 — an inaudible instance drops off the
            // queue rather than re-arming.
            if (!instance.CanHear())
                continue;

            Fire(instance, now, firings, listenerPosition);
        }
    }

    /// <summary>
    /// Drop every instance and deadline — <c>Ambient::FlushSoundTables</c> /
    /// <c>ReleaseSoundTables</c>, reached from <c>CellManager::Reset</c>.
    /// </summary>
    public void Clear()
    {
        foreach (AmbientSoundInstance instance in _instances)
            instance.OnQueue = false;
        _instances.Clear();
        _queue.Clear();
        TotalSoundCount = 0f;
    }

    /// <summary>
    /// <c>Ambient::Play</c> @ <c>0x5517A0</c>: roll, emit if it fires, then
    /// re-arm at <c>cur_time + GetPlayInterval()</c> regardless.
    /// </summary>
    private void Fire(
        AmbientSoundInstance instance,
        double now,
        ICollection<AmbientSoundFiring>? firings,
        Vector3 listenerPosition)
    {
        if (firings is not null && instance.PlayNow(_rng))
        {
            Vector3? position =
                instance.TryGetSoundPosition(listenerPosition, _rng, out Vector3 resolved)
                    ? resolved
                    : null;
            firings.Add(new AmbientSoundFiring(instance, instance.GetVolume(), position));
        }

        Enqueue(instance, now);
    }

    private void Enqueue(AmbientSoundInstance instance, double now)
    {
        _queue.Enqueue(instance, now + instance.GetPlayInterval(_rng));
        instance.OnQueue = true;
    }
}