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

namespace HeuristicLab.Common {
  public static class Extensions {
    public static bool IsAlmost(this double x, double y) {
      return Math.Abs(x - y) < 1.0e-12;
    }

    public static T SelectRandom<T>(this IEnumerable<T> xs, Random rand) {
      var n = xs.Count();
      return xs.ElementAt(rand.Next(n));
    }

    public static T SampleProportional<T>(this IEnumerable<T> elements, Random random, IEnumerable<double> weights) {
      double total = weights.Sum();

      var elemEnumerator = elements.GetEnumerator();
      elemEnumerator.MoveNext();
      var weightEnumerator = weights.GetEnumerator();
      weightEnumerator.MoveNext();

      var r = random.NextDouble() * total;
      var agg = weightEnumerator.Current;

      while (agg < r) {
        weightEnumerator.MoveNext();
        elemEnumerator.MoveNext();
        agg += weightEnumerator.Current;
      }
      return elemEnumerator.Current;
    }

    public static double RSq(IEnumerable<double> xs, IEnumerable<double> ys) {
      // two pass implementation, but we don't care
      var meanX = xs.Average();
      var meanY = ys.Average();

      var s = 0.0;
      var ssX = 0.0;
      var ssY = 0.0;
      var xEnum = xs.GetEnumerator();
      var yEnum = ys.GetEnumerator();
      while (xEnum.MoveNext() & yEnum.MoveNext()) {
        var x = xEnum.Current;
        var y = yEnum.Current;
        s += (x - meanX) * (y - meanY);
        ssX += (x - meanX) * (x - meanX);
        ssY += (y - meanY) * (y - meanY);
      }
      if (xEnum.MoveNext() | yEnum.MoveNext()) throw new ArgumentException("lengths are not equal");

      if (s.IsAlmost(0)) return 0;
      if (ssX.IsAlmost(0) || ssY.IsAlmost(0)) return 0;
      return s * s / (ssX * ssY);
    }


  }
}