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. Entity-level changes (Added/Removed)
/// take precedence over component-level changes for the same entity.
/// </summary>
internal class ChangeSet
{
    private readonly List<EntityChange> _changes = new();
    private readonly HashSet<(Entity Entity, ChangeKind Kind, Type? ComponentType)> _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.Add(key))
        {
            _changes.Add(change);
        }
    }
}