OreOreLinq.Lottery

OreOreLinq.Lottery.cs

// Created by ERAL
// These codes are licensed under CC0.
// https://creativecommons.org/publicdomain/zero/1.0/

namespace OreOreLinq
{
	using System;
	using System.Collections.Generic;

	public static class Enumerable
	{
		public static TSource Lottery<TSource>(this IEnumerable<TSource> source, Func<TSource,bool> predicate) {
			var result = default(TSource);
			var random = new Random();
			var index = 0;
			foreach (var s in source) {
				if (predicate(s)) {
					++index;
					if (random.Next(index) == 0) {
						result = s;
					}
				}
			}
			if (index == 0) {
				throw new InvalidOperationException();
			}
			return result;
		}
	}
}

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