sneak/Shared/History.cs

using System;

namespace SemiColinGames {
  // A History is a queue of fixed length N that records the N most recent items Add()ed to it.
  // The mostly-recently-added item is found at index 0; the least-recently-added item is at index
  // N-1. Items older than the History's size are automatically dropped. The underlying
  // implementation is a fixed-size circular array; insertion and access are O(1).
  //
  // Example:
  //   h = new History<int>(3);
  //   h.Add(2); h.Add(3); h.Add(5);
  //   Console.WriteLine("{0} {1} {2}", h[0], h[1], h[2]);  // 5 3 2
  //   h.Add(7);
  //   Console.WriteLine("{0} {1} {2}", h[0], h[1], h[2]);  // 7 5 3
  //   h.Add(11); h.Add(13);
  //   Console.WriteLine("{0} {1} {2}", h[0], h[1], h[2]);  // 13 11 7
  class History<T> {

    // Backing store for the History's items.
    private T[] items;
    // Points at the most-recently-inserted item.
    private int idx = 0;

    public History(int length) {
      items = new T[length];
    }

    public void Add(T item) {
      idx++;
      if (idx >= items.Length) {
        idx -= items.Length;
      }
      items[idx] = item;
    }

    public int Length {
      get => items.Length;
    }

    public T this[int age] {
      get {
        if (age < 0 || age >= items.Length) {
          throw new IndexOutOfRangeException();
        }
        int lookup = idx - age;
        if (lookup < 0) {
          lookup += items.Length;
        }
        return items[lookup];
      }
    }
  }
}