// Constructs a List with a given initial capacity. The list is
// initially empty, but will have room for the given number of elements
// before any reallocations are required.
//
public LowLevelList(int capacity)
{
if (capacity < 0)
{
throw new ArgumentOutOfRangeException("capacity");
}
Contract.EndContractBlock();
if (capacity == 0)
{
_items = Array_.Empty <T>();
}
else
{
_items = new T[capacity];
}
}
// Constructs a List, copying the contents of the given collection. The
// size and capacity of the new list will both be equal to the size of the
// given collection.
//
public LowLevelList(IEnumerable <T> collection)
{
if (collection == null)
{
throw new ArgumentNullException("collection");
}
Contract.EndContractBlock();
ICollection <T> c = collection as ICollection <T>;
if (c != null)
{
int count = c.Count;
if (count == 0)
{
_items = Array_.Empty <T>();
}
else
{
_items = new T[count];
c.CopyTo(_items, 0);
_size = count;
}
}
else
{
_size = 0;
_items = Array_.Empty <T>();
// This enumerable could be empty. Let Add allocate a new array, if needed.
// Note it will also go to _defaultCapacity first, not 1, then 2, etc.
using (IEnumerator <T> en = collection.GetEnumerator())
{
while (en.MoveNext())
{
Add(en.Current);
}
}
}
}
// Constructs a List. The list is initially empty and has a capacity
// of zero. Upon adding the first element to the list the capacity is
// increased to 16, and then increased in multiples of two as required.
public LowLevelList()
{
_items = Array_.Empty <T>();
}
/// <summary>Converts an enumerable to an array using the same logic as does List{T}.</summary>
/// <param name="length">The number of items stored in the resulting array, 0-indexed.</param>
/// <returns>
/// The resulting array. The length of the array may be greater than <paramref name="length"/>,
/// which is the actual number of elements in the array.
/// </returns>
internal static T[] ToArray <T>(IEnumerable <T> source, out int length)
{
T[] arr;
int count = 0;
ICollection <T> ic = source as ICollection <T>;
if (ic != null)
{
count = ic.Count;
if (count == 0)
{
arr = Array_.Empty <T>();
}
else
{
// Allocate an array of the desired size, then copy the elements into it. Note that this has the same
// issue regarding concurrency as other existing collections like List<T>. If the collection size
// concurrently changes between the array allocation and the CopyTo, we could end up either getting an
// exception from overrunning the array (if the size went up) or we could end up not filling as many
// items as 'count' suggests (if the size went down). This is only an issue for concurrent collections
// that implement ICollection<T>, which as of .NET 4.6 is just ConcurrentDictionary<TKey, TValue>.
arr = new T[count];
ic.CopyTo(arr, 0);
}
}
else
{
arr = Array_.Empty <T>();
foreach (var item in source)
{
if (count == arr.Length)
{
// MaxArrayLength is defined in Array.MaxArrayLength and in gchelpers in CoreCLR.
// It represents the maximum number of elements that can be in an array where
// the size of the element is greater than one byte; a separate, slightly larger constant,
// is used when the size of the element is one.
const int MaxArrayLength = 0x7FEFFFFF;
// This is the same growth logic as in List<T>:
// If the array is currently empty, we make it a default size. Otherwise, we attempt to
// double the size of the array. Doubling will overflow once the size of the array reaches
// 2^30, since doubling to 2^31 is 1 larger than Int32.MaxValue. In that case, we instead
// constrain the length to be MaxArrayLength (this overflow check works because of of the
// cast to uint). Because a slightly larger constant is used when T is one byte in size, we
// could then end up in a situation where arr.Length is MaxArrayLength or slightly larger, such
// that we constrain newLength to be MaxArrayLength but the needed number of elements is actually
// larger than that. For that case, we then ensure that the newLength is large enough to hold
// the desired capacity. This does mean that in the very rare case where we've grown to such a
// large size, each new element added after MaxArrayLength will end up doing a resize.
const int DefaultCapacity = 4;
int newLength = count == 0 ? DefaultCapacity : count * 2;
if ((uint)newLength > MaxArrayLength)
{
newLength = MaxArrayLength;
}
if (newLength < count + 1)
{
newLength = count + 1;
}
Array.Resize(ref arr, newLength);
}
arr[count++] = item;
}
}
length = count;
return(arr);
}
