using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace HeuristicLab.Algorithms.Bandits {
  /* see: Streeter and Smith: A simple distribution-free approach to the max k-armed bandit problem, Proceedings  of the 12th
International Conference, CP 2006, Nantes, France, September 25-29, 2006. pp 560-574 */

  public class ChernoffIntervalEstimationPolicy : IPolicy {
    private readonly double delta;

    public ChernoffIntervalEstimationPolicy(double delta = 0.01) {
      this.delta = delta;
    }
    public int SelectAction(Random random, IEnumerable<IPolicyActionInfo> actionInfos) {
      Debug.Assert(actionInfos.Any());
      // select best
      var myActionInfos = actionInfos.OfType<DefaultPolicyActionInfo>().ToArray(); // TODO: performance
      int k = myActionInfos.Length;
      int totalTries = myActionInfos.Where(a => !a.Disabled).Sum(a => a.Tries);
      int bestAction = -1;
      double bestQ = double.NegativeInfinity;
      for (int a = 0; a < myActionInfos.Length; a++) {
        if (myActionInfos[a].Disabled) continue;
        if (myActionInfos[a].Tries == 0) return a;

        var sumReward = myActionInfos[a].SumReward;
        var tries = myActionInfos[a].Tries;

        var avgReward = sumReward / tries;

        // page 5 of "A simple distribution-free appraoch to the max k-armed bandit problem"
        // var alpha = Math.Log(2 * totalTries * k / delta);
        double alpha = Math.Log(2) + Math.Log(totalTries) + Math.Log(k) - Math.Log(delta); // total tries is max tries in the original paper
        var q = avgReward + (alpha + Math.Sqrt(2 * tries * avgReward * alpha + alpha * alpha)) / tries;
        if (q > bestQ) {
          bestQ = q;
          bestAction = a;
        }
      }
      Debug.Assert(bestAction >= 0);
      return bestAction;
    }

    public IPolicyActionInfo CreateActionInfo() {
      return new DefaultPolicyActionInfo();
    }

    public override string ToString() {
      return string.Format("ChernoffIntervalEstimationPolicy({0:F2})", delta);
    }
  }
}