87 lines
2.4 KiB
C#
87 lines
2.4 KiB
C#
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namespace OECS;
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/// <summary>
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/// Manages entity ID allocation and recycling.
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///
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/// Uses a free-list for recycled IDs and a bump allocator for new IDs.
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/// Each ID slot tracks a version byte that increments on recycle to
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/// prevent use-after-free bugs.
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/// </summary>
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internal class EntityAllocator
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{
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private const int DefaultCapacity = 1024;
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private const uint FirstValidId = 2; // 0 = Null, 1 = Singleton
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private byte[] _versions;
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private readonly Stack<uint> _freeIds;
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private uint _nextId;
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public EntityAllocator(int initialCapacity = DefaultCapacity)
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{
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_versions = new byte[initialCapacity];
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_freeIds = new Stack<uint>();
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_nextId = FirstValidId;
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}
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/// <summary>
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/// Allocates a new entity. Reuses a free ID if available, otherwise
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/// bumps the allocator.
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/// </summary>
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public Entity Allocate()
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{
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uint id;
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if (_freeIds.Count > 0)
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{
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id = _freeIds.Pop();
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}
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else
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{
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id = _nextId++;
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EnsureCapacity(id);
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// First time this ID is used — start at version 1.
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// 0 means "never alive" and is used by the sentinel.
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_versions[id] = 1;
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}
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return new Entity(id, _versions[id]);
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}
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/// <summary>
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/// Frees an entity ID, making it available for reuse.
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/// Increments the version to invalidate any outstanding references.
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/// </summary>
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public void Free(Entity entity)
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{
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uint id = entity.Id;
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EnsureCapacity(id);
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// Increment version (wrap at 255 back to 1; 0 means "never alive").
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_versions[id]++;
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if (_versions[id] == 0)
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_versions[id] = 1;
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_freeIds.Push(id);
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}
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/// <summary>
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/// Returns true if the entity's version matches the current version for its ID.
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/// </summary>
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public bool IsAlive(Entity entity)
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{
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uint id = entity.Id;
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if (entity.IsNull) return false;
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if (id == 1) return true; // Singleton is always alive.
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if (id >= _versions.Length) return false;
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return _versions[id] == entity.Version && _versions[id] != 0;
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}
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private void EnsureCapacity(uint id)
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{
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if (id >= _versions.Length)
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{
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int newSize = Math.Max((int)id + 1, _versions.Length * 2);
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Array.Resize(ref _versions, newSize);
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}
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}
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}
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