UnityECS/LocalPackages/com.nimin.tinyecs/Runtime/EntityGenerator.cs

using System;
using Unity.Collections;
using Unity.Mathematics;

namespace GameCore.TinyECS
{
    public struct EntityGenerator<TEnum> : IDisposable
        where TEnum : unmanaged, IConvertible
    {
        public readonly TEnum entityType;
        private NativeList<int> indexToGeneration;
        private int beginSearchIndex;
        private int freeCount;

        public EntityGenerator(TEnum type, int capacity, AllocatorManager.AllocatorHandle allocator)
        {
            indexToGeneration = new(capacity, allocator) { 0 };
            beginSearchIndex = 1;
            freeCount = 0;
            entityType = type;
        }

        public void Dispose()
        {
            indexToGeneration.Dispose();
        }

        public Entity<TEnum> Create()
        {
            if (freeCount == 0)
            {
                var index = indexToGeneration.Length;
                indexToGeneration.Add(1);
                return new(entityType, index, 1);
            }

            var count = indexToGeneration.Length;
            for (var i = beginSearchIndex; i < count; ++i)
            {
                var oldGeneration = indexToGeneration[i];
                if (oldGeneration >= 0)
                    continue;
                beginSearchIndex = i + 1;
                freeCount--;
                var generation = 1 - oldGeneration;
                indexToGeneration[i] = generation;

                return new(entityType, i, generation);
            }

            throw new Exception("Impossible");
        }

        public void Delete(Entity<TEnum> entity)
        {
            var generation = indexToGeneration[entity.index];
            if (generation != entity.generation)
            {
                throw new Exception($"Invalid entity: {entity}");
            }

            indexToGeneration[entity.index] = -generation;
            freeCount++;
            beginSearchIndex = math.min(beginSearchIndex, entity.index);
        }

        public bool IsValid(Entity<TEnum> entity)
        {
            var generation = indexToGeneration[entity.index];
            return generation == entity.generation;
        }

        public void Clear()
        {
            indexToGeneration.Resize(1, NativeArrayOptions.UninitializedMemory);
            beginSearchIndex = 1;
            freeCount = 0;
        }
    }
}