namespace OECS;

/// <summary>
/// Accumulates <see cref="EntityChange"/> entries during a system run.
///
/// Deduplication: if the same (entity, kind, componentType) change is marked
/// multiple times, only one entry is kept. However, if a structurally opposed
/// change arrives (Added vs Removed) for the same (entity, componentType),
/// the old entry is removed so observers see the full state transition.
/// </summary>
internal class ChangeSet
{
    private readonly List<EntityChange> _changes = new();
    private readonly Dictionary<(Entity Entity, ChangeKind Kind, Type? ComponentType), int> _dedup = new();

    /// <summary>
    /// All accumulated changes in insertion order.
    /// </summary>
    public IReadOnlyList<EntityChange> Changes => _changes;

    /// <summary>
    /// Number of changes in this set.
    /// </summary>
    public int Count => _changes.Count;

    /// <summary>
    /// Records that an entity was created.
    /// </summary>
    public void MarkEntityAdded(Entity entity)
    {
        TryAdd(new EntityChange(entity, ChangeKind.EntityAdded));
    }

    /// <summary>
    /// Records that an entity was destroyed.
    /// </summary>
    public void MarkEntityRemoved(Entity entity)
    {
        TryAdd(new EntityChange(entity, ChangeKind.EntityRemoved));
    }

    /// <summary>
    /// Records that a component was added to an entity.
    /// </summary>
    public void MarkComponentAdded(Entity entity, Type componentType)
    {
        TryAdd(new EntityChange(entity, ChangeKind.ComponentAdded, componentType));
    }

    /// <summary>
    /// Records that a component was removed from an entity.
    /// </summary>
    public void MarkComponentRemoved(Entity entity, Type componentType)
    {
        TryAdd(new EntityChange(entity, ChangeKind.ComponentRemoved, componentType));
    }

    /// <summary>
    /// Records that a component's value was modified.
    /// Must be called explicitly by user code via <see cref="World.MarkModified{T}"/>.
    /// </summary>
    public void MarkComponentModified(Entity entity, Type componentType)
    {
        TryAdd(new EntityChange(entity, ChangeKind.ComponentModified, componentType));
    }

    /// <summary>
    /// Clears all accumulated changes.
    /// </summary>
    public void Clear()
    {
        _changes.Clear();
        _dedup.Clear();
    }

    private void TryAdd(EntityChange change)
    {
        var key = (change.Entity, change.Kind, change.ComponentType);
        if (_dedup.ContainsKey(key))
        {
            // Same (entity, kind, componentType) already recorded — deduplicate.
            return;
        }

        // When a component is re-added after being removed within the same
        // batch, remove the old ComponentRemoved entry so observers see the
        // full Add → Remove → Add sequence.
        if (change.Kind == ChangeKind.ComponentAdded && change.ComponentType != null)
        {
            var removedKey = (change.Entity, ChangeKind.ComponentRemoved, change.ComponentType);
            if (_dedup.Remove(removedKey, out int oldIndex))
            {
                _changes.RemoveAt(oldIndex);
                // Adjust indices for all entries that shifted down.
                foreach (var k in _dedup.Keys.ToList())
                {
                    if (_dedup[k] > oldIndex)
                        _dedup[k]--;
                }
            }
        }

        _dedup[key] = _changes.Count;
        _changes.Add(change);
    }
}