A way to repeatedly randomly choose from an initially provided set of possibilities.

RandomGrabBag.cs

// Copyright Rupert Key, 2016
// Published under BSD license (commercial-friendly use as you wish but recognize owner wrote it)

using System.Collections.Generic;
// uses UnityEngine for Random.Range()

namespace UGS.Random {

	/// <summary>
	/// A way to repeatedly randomly choose from an initially provided set of possibilities.
	/// Acts similarly to a deck of cards.
	/// In much the same way that taking one card from a deck removes the possibility that that
	/// card will be chosen again until the deck is reset (all cards returned and deck shuffled).
	/// Use by:
	///  1. constructing with all the possibilities (<see cref="RandomGrabBag{T}"/>),
	///  2. if <see cref="hasChoicesLeft"/>
	///  3. call <see cref="chooseOne"/>
	///  4. <see cref="reset"/> to re-start with all possibilities.
	/// Note: Each reset generates some garbage.
	/// </summary>
	/// <typeparam name="T"></typeparam>
	public class RandomGrabBag<T> {

		private readonly T[] choices;

		private readonly List<int> choiceIndicies;

		public int numEntriesTotal { get { return choices.Length; } }

		/// <summary>
		/// Construct with these possible choices.
		/// </summary>
		/// <param name="choices"></param>
		public RandomGrabBag(T[] choices) {
			this.choices =  choices;
			choiceIndicies = new List<int>();
			for (int i = choices.Length - 1; i >= 0; i--) {
				choiceIndicies.Add(i);
			}
		}

		/// <summary>
		/// Reset to choose from all choices again.
		/// </summary>
		public void reset() {
			choiceIndicies.Clear();
			for (int i = choices.Length - 1; i >= 0; i--) {
				choiceIndicies.Add(i);
			}
		}

		/// <summary>
		/// Returns whether there are choices remaining.
		/// </summary>
		/// <returns>true if there are values left; else false.</returns>
		public bool hasChoicesLeft() {
			return 0 < choiceIndicies.Count;
		}

		/// <summary>
		/// Get one randomly chosen value (like picking a card from the deck).
		/// </summary>
		/// <returns>The chosen card.</returns>
		public T chooseOne() {
			if (0 >= choiceIndicies.Count) {
				throw new System.IndexOutOfRangeException("No choices left");
			}

			var chosenIndexIndex = UnityEngine.Random.Range(0, choiceIndicies.Count - 1);
			var chosenIndex = choiceIndicies[chosenIndexIndex];
			choiceIndicies.RemoveAt(chosenIndexIndex);
			return choices[chosenIndex];
		}

	}

}