If.cs

namespace SuperLinq;

public static partial class SuperEnumerable
{
	/// <summary>
	///	    Returns an enumerable sequence if the evaluation result of the given condition is <see langword="true"/>,
	///     otherwise returns an empty sequence.
	/// </summary>
	/// <typeparam name="TResult">
	///	    Result sequence element type.
	/// </typeparam>
	/// <param name="condition">
	///	    Condition to evaluate.
	/// </param>
	/// <param name="thenSource">
	///	    Sequence to return in case the condition evaluates true.
	/// </param>
	/// <returns>
	///	    The given input sequence if the condition evaluates <see langword="true"/>; otherwise, an empty sequence.
	/// </returns>
	/// <exception cref="ArgumentNullException">
	///	    <paramref name="condition"/> or <paramref name="thenSource"/> is <see langword="null"/>.
	/// </exception>
	/// <remarks>
	/// <para>
	///	    <paramref name="condition"/> is not evaluated until enumeration. If the value is <see langword="true"/>,
	///     then <paramref name="thenSource"/> is enumerated. Otherwise, the sequence will be empty.
	/// </para>
	/// </remarks>
	public static IEnumerable<TResult> If<TResult>(Func<bool> condition, IEnumerable<TResult> thenSource)
	{
		return If(condition, thenSource, []);
	}

	/// <summary>
	///	    Returns an enumerable sequence based on the evaluation result of the given condition.
	/// </summary>
	/// <typeparam name="TResult">
	///	    Result sequence element type.
	/// </typeparam>
	/// <param name="condition">
	///	    Condition to evaluate.
	/// </param>
	/// <param name="thenSource">
	///	    Sequence to return in case the condition evaluates <see langword="true"/>.
	/// </param>
	/// <param name="elseSource">
	///	    Sequence to return in case the condition evaluates <see langword="false"/>.
	/// </param>
	/// <returns>
	///	    If the condition evaluates <see langword="true"/>, then the sequence represented by <paramref
	///     name="thenSource"/>; otherwise the sequence represented by <paramref name="elseSource"/>.
	/// </returns>
	/// <exception cref="ArgumentNullException">
	///	    <paramref name="condition"/>, <paramref name="thenSource"/>, or <paramref name="elseSource"/> is <see
	///     langword="null"/>.
	/// </exception>
	/// <remarks>
	/// <para>
	///	    <paramref name="condition"/> is not evaluated until enumeration. If the value is <see langword="true"/>,
	///     then <paramref name="thenSource"/> is enumerated. Otherwise, <paramref name="elseSource"/> will be
	///     enumerated.
	/// </para>
	/// </remarks>
	public static IEnumerable<TResult> If<TResult>(
		Func<bool> condition,
		IEnumerable<TResult> thenSource,
		IEnumerable<TResult> elseSource)
	{
		ArgumentNullException.ThrowIfNull(condition);
		ArgumentNullException.ThrowIfNull(thenSource);
		ArgumentNullException.ThrowIfNull(elseSource);

		return Case(
			condition,
			new Dictionary<bool, IEnumerable<TResult>>
			{
				[true] = thenSource,
				[false] = elseSource,
			});
	}
}