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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 IEnumerator<T> GetEnumerator()
{
yield return first;
foreach (var v in rest) yield return v;
}
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 T : class 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);
}
}