// Two functions below: one returns T[] and one returns IEnumerable<T> using yield.

// Has been tested for uniform distribution and has excellent performance.
// Should NOT be used as a shuffling algorithm by requesting all items to be picked, as this will return the set of values unchanged.

// Requires System.Random object:
// private static readonly Random Random = new Random();

/// <summary>
/// Randomly picks '<paramref name="count"/>' number of elements from the set of <paramref name="values"/>.
/// </summary>
/// <typeparam name="T">The type of values.</typeparam>
/// <param name="values">The values to pick from.</param>
/// <param name="count">The number of values to pick.</param>
/// <exception cref="ArgumentNullException">Thrown if <paramref name="values"/> is null.</exception>
/// <exception cref="ArgumentOutOfRangeException">Thrown if <paramref name="count"/> is negative.</exception>
/// <exception cref="ArgumentException">Throw if <paramref name="count"/> is greater than the number of values.</exception>
public static T[] RandomlyPick<T>(this IEnumerable<T> values, int count)
{
  if (values == null) { throw new ArgumentNullException(nameof(values)); }
  if (count < 0) { throw new ArgumentOutOfRangeException(nameof(count), "Count cannot be negative."); }

  T[] allValues = values as T[] ?? values.ToArray();
  if (count == allValues.Length) { return allValues; }
  if (count > allValues.Length) { throw new ArgumentException("Count is greater than number of values.", nameof(count)); }

  T[] returnValues = new T[count];

  for (int i = 0; i < count; i++)
  {
    int index = Random.Next(i, allValues.Length);
    T temp = allValues[i];
    returnValues[i] = allValues[i] = allValues[index];
    allValues[index] = temp;
  }

  return returnValues;
}

/// <summary>
/// Randomly picks '<paramref name="count"/>' number of elements from the set of <paramref name="values"/>.
/// </summary>
/// <typeparam name="T">The type of values.</typeparam>
/// <param name="values">The values to pick from.</param>
/// <param name="count">The number of values to pick.</param>
/// <exception cref="ArgumentNullException">Thrown if <paramref name="values"/> is null.</exception>
/// <exception cref="ArgumentOutOfRangeException">Thrown if <paramref name="count"/> is negative.</exception>
/// <exception cref="ArgumentException">Throw if <paramref name="count"/> is greater than the number of values.</exception>
public static IEnumerable<T> RandomlyPick<T>(this IEnumerable<T> values, int count)
{
  if (values == null) { throw new ArgumentNullException(nameof(values)); }
  if (count < 0) { throw new ArgumentOutOfRangeException(nameof(count), "Count cannot be negative."); }

  T[] allValues = values as T[] ?? values.ToArray();
  if (count == allValues.Length)
  {
    foreach (T value in allValues) { yield return value; }
    yield break;
  }
  if (count > allValues.Length) { throw new ArgumentException("Count is greater than number of values.", nameof(count)); }

  for (int i = 0; i < count; i++)
  {
    int index = Random.Next(i, allValues.Length);
    T temp = allValues[i];
    yield return allValues[i] = allValues[index];
    allValues[index] = temp;
  }
}