package com.github.davidmoten.rx2;
   
   public final class Statistics {
   
       private final long count;
       private final double sumX;
       private final double sumX2;
   
       private Statistics(long count, double sumX, double sumX2) {
          this.count = count;
          this.sumX = sumX;
          this.sumX2 = sumX2;
      }
  
      public static Statistics create() {
          return new Statistics(0, 0, 0);
      }
  
      public Statistics add(Number number) {
          double x = number.doubleValue();
          return new Statistics(count + 1, sumX + x, sumX2 + x * x);
      }
  
      public long count() {
          return count;
      }
  
      public double sum() {
          return sumX;
      }
  
      public double sumSquares() {
          return sumX2;
      }
  
      public double mean() {
          return sumX / count;
      }
  
      public double sd() {
          double m = mean();
          return Math.sqrt(sumX2 / count - m * m);
      }
  
      @Override
      public String toString() {
          StringBuilder builder = new StringBuilder();
          builder.append("Statistics [count=");
          builder.append(count);
          builder.append(", sum=");
          builder.append(sum());
          builder.append(", sumSquares=");
          builder.append(sumSquares());
          builder.append(", mean=");
          builder.append(mean());
          builder.append(", sd=");
          builder.append(sd());
          builder.append("]");
          return builder.toString();
      }
  }