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using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace IPA.Utilities
{
/// <summary>
/// Extensions for <see cref="IEnumerable{T}"/> that don't currently exist in <c>System.Linq</c>.
/// </summary>
public static class EnumerableExtensions
{
/// <summary>
/// Adds a value to the beginning of the sequence.
/// </summary>
/// <typeparam name="T">the type of the elements of <paramref name="seq"/></typeparam>
/// <param name="seq">a sequence of values</param>
/// <param name="prep">the value to prepend to <paramref name="seq"/></param>
/// <returns>a new sequence beginning with <paramref name="prep"/></returns>
public static IEnumerable<T> Prepend<T>(this IEnumerable<T> seq, T prep)
=> new PrependEnumerable<T>(seq, prep);
private sealed class PrependEnumerable<T> : IEnumerable<T>
{
private readonly IEnumerable<T> rest;
private readonly T first;
public PrependEnumerable(IEnumerable<T> rest, T first)
{
this.rest = rest;
this.first = first;
}
public PrependEnumerator GetEnumerator() => new PrependEnumerator(this);
public struct PrependEnumerator : IEnumerator<T>
{
private readonly IEnumerator<T> restEnum;
private readonly PrependEnumerable<T> enumerable;
private int state;
internal PrependEnumerator(PrependEnumerable<T> enumerable)
{
this.enumerable = enumerable;
restEnum = enumerable.rest.GetEnumerator();
state = 0;
Current = default;
}
public T Current { get; private set; }
object IEnumerator.Current => Current;
public void Dispose() => restEnum.Dispose();
public bool MoveNext()
{
switch (state)
{
case 0:
Current = enumerable.first;
state++;
return true;
case 1:
if (!restEnum.MoveNext())
{
state = 2;
return false;
}
else
Current = restEnum.Current;
return true;
case 2:
default:
return false;
}
}
public void Reset()
{
restEnum.Reset();
state = 0;
}
}
IEnumerator<T> IEnumerable<T>.GetEnumerator() => new PrependEnumerator(this);
IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}
/// <summary>
/// Adds a value to the end of the sequence.
/// </summary>
/// <typeparam name="T">the type of the elements of <paramref name="seq"/></typeparam>
/// <param name="seq">a sequence of values</param>
/// <param name="app">the value to append to <paramref name="seq"/></param>
/// <returns>a new sequence ending with <paramref name="app"/></returns>
public static IEnumerable<T> Append<T>(this IEnumerable<T> seq, T app)
=> new AppendEnumerable<T>(seq, app);
private sealed class AppendEnumerable<T> : IEnumerable<T>
{
private readonly IEnumerable<T> rest;
private readonly T last;
public AppendEnumerable(IEnumerable<T> rest, T last)
{
this.rest = rest;
this.last = last;
}
public AppendEnumerator GetEnumerator() => new AppendEnumerator(this);
public struct AppendEnumerator : IEnumerator<T>
{
private readonly IEnumerator<T> restEnum;
private readonly AppendEnumerable<T> enumerable;
private int state;
internal AppendEnumerator(AppendEnumerable<T> enumerable)
{
this.enumerable = enumerable;
restEnum = enumerable.rest.GetEnumerator();
state = 0;
Current = default;
}
public T Current { get; private set; }
object IEnumerator.Current => Current;
public void Dispose() => restEnum.Dispose();
public bool MoveNext()
{
switch (state)
{
case 0:
if (!restEnum.MoveNext())
{
state = 1;
goto case 1;
}
else
Current = restEnum.Current;
return true;
case 1:
Current = enumerable.last;
state++;
return true;
case 2:
default:
return false;
}
}
public void Reset()
{
restEnum.Reset();
state = 0;
}
}
IEnumerator<T> IEnumerable<T>.GetEnumerator() => GetEnumerator();
IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}
/// <summary>
/// LINQ extension method that filters <see langword="null"/> elements out of an enumeration.
/// </summary>
/// <typeparam name="T">the type of the enumeration</typeparam>
/// <param name="self">the enumeration to filter</param>
/// <returns>a filtered enumerable</returns>
public static IEnumerable<T> NonNull<T>(this IEnumerable<T> self) where T : class
=> self.Where(o => o != null);
/// <summary>
/// LINQ extension method that filters <see langword="null"/> elements out of an enumeration based on a converter.
/// </summary>
/// <typeparam name="T">the type of the enumeration</typeparam>
/// <typeparam name="U">the type to compare to null</typeparam>
/// <param name="self">the enumeration to filter</param>
/// <param name="pred">the predicate to select for filtering</param>
/// <returns>a filtered enumerable</returns>
public static IEnumerable<T> NonNull<T, U>(this IEnumerable<T> self, Func<T, U> pred) where U : class
=> self.Where(o => pred(o) != null);
/// <summary>
/// LINQ extension method that filters <see langword="null"/> elements from an enumeration of nullable types.
/// </summary>
/// <typeparam name="T">the underlying type of the nullable enumeration</typeparam>
/// <param name="self">the enumeration to filter</param>
/// <returns>a filtered enumerable</returns>
public static IEnumerable<T> NonNull<T>(this IEnumerable<T?> self) where T : struct
=> self.Where(o => o != null).Select(o => o.Value);
}
}