自定义类 SparseList 怎么样：
- 一个列表，允许您通过将缺失值设置为 null 来删除项目（或者对于 SparseValueList，一个特殊的值，如 -1（如 Dr ABT 的解决方案）、0（默认值 (T)）、或 int.MinValue, 或 uint.MaxValue,) 和
- 然后维护已删除索引列表（堆栈）。然后，当您需要添加到列表中时，它会从堆栈中弹出一个已删除的索引并在那里添加值。 （对于多线程，也许 ConcurrentQueue 会是另一个想法。）
-
枚举器可以跳过已删除的项目（以支持 foreach）
-
可以在枚举期间从集合中删除项目！ （我必须经常这样做，所以这很好。）
- 索引器提供对包含空值的列表的原始访问。所以如果您使用 for(;;) 循环，请准备好过滤掉空值。
- 如果/当您想删除所有空值时，请调用 Compact()
- 如果在游戏中从不调用 compact，我担心会遍历大量空值。因此，对于紧凑的实验性替代方案，请参阅 SparseListCleaningEnumerator: auto-shrink the list every time it is enumerated, 至少对于单线程情况：当 MoveNext 离开一个项目时，它会查看堆栈以查看索引是否较低并且如果是这样，它将项目分配给较低的索引，将其从当前位置删除，这将缩小列表。平衡可能需要多次迭代并在优化之前涉及多次移动，除非堆栈被排序列表替换，或者堆栈偶尔排序。如果最后一个值为 null，这将不起作用，因为索引将被埋在空闲索引堆栈中（用排序的东西替换堆栈可以避免这种情况）。
我实现了这个（尚未测试），但我存储的是对我的游戏实体的实际引用而不是 id，因此您需要以某种方式将其调整为 int 或 Nullable。 （好的，以确保我回答您的问题的 int/uint 要求，我还添加了一个略有不同的 SparseValueList，使用 default(T) 而不是 null。这意味着您不能在列表中使用 0。）您也许可以如果您认为不需要，请取出版本控制 - 大多数游戏可能不需要。
/// <summary>
/// Specifying null as value has unspecified results.
/// CopyTo may contain nulls.
/// </summary>
/// <typeparam name="T"></typeparam>
public class SparseList<T> : IList<T>
where T : class
{
int version = 0;
List<T> list = new List<T>();
Stack<int> freeIndices = new Stack<int>();
public int Capacity { get { return list.Capacity; } set { list.Capacity = value; } }
public void Compact()
{
var sortedIndices = freeIndices.ToList();
foreach (var i in sortedIndices.OrderBy(x => x).Reverse())
{
list.RemoveAt(i);
}
freeIndices.Clear();
list.Capacity = list.Count;
version++; // breaks open enumerators
}
public int IndexOf(T item)
{
return list.IndexOf(item);
}
/// <summary>
/// Slow (forces a compact), not recommended
/// </summary>
/// <param name="index"></param>
/// <param name="item"></param>
public void Insert(int index, T item)
{
// One idea: remove index from freeIndices if it's in there. Stack doesn't support this though.
Compact(); // breaks the freeIndices list, so apply it before insert
list.Insert(index, item);
version++; // breaks open enumerators
}
public void RemoveAt(int index)
{
if (index == Count - 1) { list.RemoveAt(index); }
else { list[index] = null; freeIndices.Push(index); }
//version++; // Don't increment version for removals
}
public T this[int index]
{
get
{
return list[index];
}
set
{
if (value == null) throw new ArgumentNullException();
list[index] = value;
}
}
public void Add(T item)
{
if (item == null) throw new ArgumentNullException();
if (freeIndices.Count == 0) { list.Add(item); return; }
list[freeIndices.Pop()] = item;
//version++; // Don't increment version for additions? It could result in missing the new value, but shouldn't break open enumerators
}
public void Clear()
{
list.Clear();
freeIndices.Clear();
version++;
}
public bool Contains(T item)
{
if (item == null) return false;
return list.Contains(item);
}
/// <summary>
/// Result may contain nulls
/// </summary>
/// <param name="array"></param>
/// <param name="arrayIndex"></param>
public void CopyTo(T[] array, int arrayIndex)
{
list.CopyTo(array, arrayIndex);
}
//public void CopyNonNullTo(T[] array, int arrayIndex)
//{
//}
/// <summary>
/// Use this for iterating via for loop.
/// </summary>
public int Count { get { return list.Count; } }
/// <summary>
/// Don't use this for for loops! Use Count.
/// </summary>
public int NonNullCount
{
get { return list.Count - freeIndices.Count; }
}
public bool IsReadOnly
{
get { return false; }
}
public bool Remove(T item)
{
int i = list.IndexOf(item);
if (i < 0) return false;
if (i == list.Count - 1)
{
// Could throw . Could add check in
list.RemoveAt(i);
}
else
{
list[i] = null;
freeIndices.Push(i);
}
//version++; // Don't increment version for removals
return true;
}
public IEnumerator<T> GetEnumerator()
{
return new SparseListEnumerator(this);
}
private class SparseListEnumerator : IEnumerator<T>, IRemovingEnumerator
{
SparseList<T> list;
int version;
int index = -1;
public SparseListEnumerator(SparseList<T> list)
{
this.list = list;
this.version = list.version;
//while (Current == null && MoveNext()) ;
}
public T Current
{
get {
if (index >= list.Count) return null; // Supports removing last items of collection without throwing on Enumerator access
return list[index];
}
}
public void Dispose()
{
list = null;
}
object IEnumerator.Current
{
get { return Current; }
}
public bool MoveNext()
{
do
{
if (version != list.version) { throw new InvalidOperationException("Collection modified"); }
index++;
return index < list.Count;
} while (Current == null);
}
public void Reset()
{
index = -1;
version = list.version;
}
/// <summary>
/// Accessing Current after RemoveCurrent may throw a NullReferenceException or return null.
/// </summary>
public void RemoveCurrent()
{
list.RemoveAt(index);
}
}
private class SparseListCleaningEnumerator : IEnumerator<T>, IRemovingEnumerator
{
SparseList<T> list;
int version;
int index = -1;
public SparseListCleaningEnumerator(SparseList<T> list)
{
this.list = list;
this.version = list.version;
//while (Current == null && MoveNext()) ;
}
public T Current
{
get
{
if (index >= list.Count) return null; // Supports removing last items of collection without throwing on Enumerator access
return list[index];
}
}
public void Dispose()
{
list = null;
}
object IEnumerator.Current
{
get { return Current; }
}
public bool MoveNext()
{
do
{
if (version != list.version) { throw new InvalidOperationException("Collection modified"); }
if (index > 0
&& Current != null // only works for values that are set, otherwise the index is buried in the free index stack somewhere
)
{
int freeIndex = list.freeIndices.Peek();
if (freeIndex < index)
{
list.freeIndices.Pop();
list[freeIndex] = list[index];
list.RemoveAt(index);
}
}
index++;
return index < list.Count;
} while (Current == null);
}
public void Reset()
{
index = -1;
version = list.version;
}
/// <summary>
/// Accessing Current after RemoveCurrent may throw a NullReferenceException or return null.
/// </summary>
public void RemoveCurrent()
{
list.RemoveAt(index);
}
}
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}
/// <summary>
/// Like SparseList but Supports value types, using default(T) in place of null.
/// This means values of default(T) are not permitted as values in the collection.
/// CopyTo may contain default(T).
/// TODO: Use EqualityComparer<T>.Default instead of default(T).Equals()
/// </summary>
/// <typeparam name="T"></typeparam>
public class SparseValueList<T> : IList<T>
{
int version = 0;
List<T> list = new List<T>();
Stack<int> freeIndices = new Stack<int>();
public int Capacity { get { return list.Capacity; } set { list.Capacity = value; } }
public void Compact()
{
var sortedIndices = freeIndices.ToList();
foreach (var i in sortedIndices.OrderBy(x => x).Reverse())
{
list.RemoveAt(i);
}
freeIndices.Clear();
list.Capacity = list.Count;
version++; // breaks open enumerators
}
public int IndexOf(T item)
{
return list.IndexOf(item);
}
/// <summary>
/// Slow (forces a compact), not recommended
/// </summary>
/// <param name="index"></param>
/// <param name="item"></param>
public void Insert(int index, T item)
{
// One idea: remove index from freeIndices if it's in there. Stack doesn't support this though.
Compact(); // breaks the freeIndices list, so apply it before insert
list.Insert(index, item);
version++; // breaks open enumerators
}
public void RemoveAt(int index)
{
if (index == Count - 1) { list.RemoveAt(index); }
else { list[index] = default(T); freeIndices.Push(index); }
//version++; // Don't increment version for removals
}
public T this[int index]
{
get
{
return list[index];
}
set
{
if (default(T).Equals(value)) throw new ArgumentNullException();
list[index] = value;
}
}
public void Add(T item)
{
if (default(T).Equals(item)) throw new ArgumentNullException();
if (freeIndices.Count == 0) { list.Add(item); return; }
list[freeIndices.Pop()] = item;
//version++; // Don't increment version for additions? It could result in missing the new value, but shouldn't break open enumerators
}
public void Clear()
{
list.Clear();
freeIndices.Clear();
version++;
}
public bool Contains(T item)
{
if (default(T).Equals(item)) return false;
return list.Contains(item);
}
/// <summary>
/// Result may contain default(T)'s
/// </summary>
/// <param name="array"></param>
/// <param name="arrayIndex"></param>
public void CopyTo(T[] array, int arrayIndex)
{
list.CopyTo(array, arrayIndex);
}
//public void CopyNonNullTo(T[] array, int arrayIndex)
//{
//}
/// <summary>
/// Use this for iterating via for loop.
/// </summary>
public int Count { get { return list.Count; } }
/// <summary>
/// Don't use this for for loops! Use Count.
/// </summary>
public int NonNullCount
{
get { return list.Count - freeIndices.Count; }
}
public bool IsReadOnly
{
get { return false; }
}
public bool Remove(T item)
{
int i = list.IndexOf(item);
if (i < 0) return false;
if (i == list.Count - 1)
{
// Could throw . Could add check in
list.RemoveAt(i);
}
else
{
list[i] = default(T);
freeIndices.Push(i);
}
//version++; // Don't increment version for removals
return true;
}
public IEnumerator<T> GetEnumerator()
{
return new SparseValueListEnumerator(this);
}
private class SparseValueListEnumerator : IEnumerator<T>, IRemovingEnumerator
{
SparseValueList<T> list;
int version;
int index = -1;
public SparseValueListEnumerator(SparseValueList<T> list)
{
this.list = list;
this.version = list.version;
//while (Current == default(T) && MoveNext()) ;
}
public T Current
{
get
{
if (index >= list.Count) return default(T); // Supports removing last items of collection without throwing on Enumerator access
return list[index];
}
}
public void Dispose()
{
list = null;
}
object IEnumerator.Current
{
get { return Current; }
}
public bool MoveNext()
{
do
{
if (version != list.version) { throw new InvalidOperationException("Collection modified"); }
index++;
return index < list.Count;
} while (default(T).Equals(Current));
}
public void Reset()
{
index = -1;
version = list.version;
}
/// <summary>
/// Accessing Current after RemoveCurrent may throw a NullReferenceException or return default(T).
/// </summary>
public void RemoveCurrent()
{
list.RemoveAt(index);
}
}
private class SparseValueListCleaningEnumerator : IEnumerator<T>, IRemovingEnumerator
{
SparseValueList<T> list;
int version;
int index = -1;
public SparseValueListCleaningEnumerator(SparseValueList<T> list)
{
this.list = list;
this.version = list.version;
while (default(T).Equals(Current) && MoveNext()) ;
}
public T Current
{
get
{
if (index >= list.Count) return default(T); // Supports removing last items of collection without throwing on Enumerator access
return list[index];
}
}
public void Dispose()
{
list = null;
}
object IEnumerator.Current
{
get { return Current; }
}
public bool MoveNext()
{
do
{
if (version != list.version) { throw new InvalidOperationException("Collection modified"); }
if (index > 0
&& (!default(T).Equals(Current)) // only works for values that are set, otherwise the index might be buried in the stack somewhere
)
{
int freeIndex = list.freeIndices.Peek();
if (freeIndex < index)
{
list.freeIndices.Pop();
list[freeIndex] = list[index];
list.RemoveAt(index);
}
}
index++;
return index < list.Count;
} while (default(T).Equals(Current));
}
public void Reset()
{
index = -1;
version = list.version;
}
/// <summary>
/// Accessing Current after RemoveCurrent may throw a NullReferenceException or return default(T).
/// </summary>
public void RemoveCurrent()
{
list.RemoveAt(index);
}
}
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}