/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You under the Apache License, Version 2.0
    * (the "License"); you may not use this file except in compliance with
    * the License.  You may obtain a copy of the License at
    *
    *      http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.commons.statistics.descriptive;
  
  import java.math.BigInteger;
  
  /**
   * Returns the sum of the squares of the available values. Uses the following definition:
   *
   * <p>\[ \sum_{i=1}^n x_i^2 \]
   *
   * <p>where \( n \) is the number of samples.
   *
   * <ul>
   *   <li>The result is zero if no values are observed.
   * </ul>
   *
   * <p>The implementation uses an exact integer sum to compute the sum of squared values.
   * The exact sum is returned using {@link #getAsBigInteger()}. Methods that return {@code int} or
   * {@code long} primitives will raise an exception if the result overflows.
   *
   * <p>Note that the implementation does not use {@code BigInteger} arithmetic; for
   * performance the sum is computed using primitives to create an unsigned 192-bit integer.
   * Support is provided for at least 2<sup>63</sup> observations.
   *
   * <p>This class is designed to work with (though does not require)
   * {@linkplain java.util.stream streams}.
   *
   * <p><strong>This implementation is not thread safe.</strong>
   * If multiple threads access an instance of this class concurrently,
   * and at least one of the threads invokes the {@link java.util.function.IntConsumer#accept(int) accept} or
   * {@link StatisticAccumulator#combine(StatisticResult) combine} method, it must be synchronized externally.
   *
   * <p>However, it is safe to use {@link java.util.function.IntConsumer#accept(int) accept}
   * and {@link StatisticAccumulator#combine(StatisticResult) combine}
   * as {@code accumulator} and {@code combiner} functions of
   * {@link java.util.stream.Collector Collector} on a parallel stream,
   * because the parallel implementation of {@link java.util.stream.Stream#collect Stream.collect()}
   * provides the necessary partitioning, isolation, and merging of results for
   * safe and efficient parallel execution.
   *
   * @since 1.1
   */
  public final class LongSumOfSquares implements LongStatistic, StatisticAccumulator<LongSumOfSquares> {
  
      /** Sum of the squared values. */
      private final UInt192 sumSq;
  
      /**
       * Create an instance.
       */
      private LongSumOfSquares() {
          this(UInt192.create());
      }
  
      /**
       * Create an instance.
       *
       * @param sumSq Sum of the squared values.
       */
      private LongSumOfSquares(UInt192 sumSq) {
          this.sumSq = sumSq;
      }
  
      /**
       * Creates an instance.
       *
       * <p>The initial result is zero.
       *
       * @return {@code LongSumOfSquares} instance.
       */
      public static LongSumOfSquares create() {
          return new LongSumOfSquares();
      }
  
      /**
       * Returns an instance populated using the input {@code values}.
       *
       * <p>When the input is an empty array, the result is zero.
       *
       * @param values Values.
       * @return {@code LongSumOfSquares} instance.
       */
      public static LongSumOfSquares of(long... values) {
          final UInt192 ss = UInt192.create();
         for (final long x : values) {
             ss.addSquare(x);
         }
         return new LongSumOfSquares(ss);
     }
 
     /**
      * Returns an instance populated using the specified range of {@code values}.
      *
      * <p>When the range is empty, the result is zero.
      *
      * @param values Values.
      * @param from Inclusive start of the range.
      * @param to Exclusive end of the range.
      * @return {@code LongSumOfSquares} instance.
      * @throws IndexOutOfBoundsException if the sub-range is out of bounds
      * @since 1.2
      */
     public static LongSumOfSquares ofRange(long[] values, int from, int to) {
         Statistics.checkFromToIndex(from, to, values.length);
         return createFromRange(values, from, to);
     }
 
     /**
      * Create an instance using the specified range of {@code values}.
      *
      * <p>Warning: No range checks are performed.
      *
      * @param values Values.
      * @param from Inclusive start of the range.
      * @param to Exclusive end of the range.
      * @return {@code LongSumOfSquares} instance.
      */
     static LongSumOfSquares createFromRange(long[] values, int from, int to) {
         final UInt192 ss = UInt192.create();
         for (int i = from; i < to; i++) {
             ss.addSquare(values[i]);
         }
         return new LongSumOfSquares(ss);
     }
 
     /**
      * Gets the sum of squares.
      *
      * <p>This is package private for use in {@link IntStatistics}.
      *
      * @return the sum of squares
      */
     UInt192 getSumOfSquares() {
         return sumSq;
     }
 
     /**
      * Updates the state of the statistic to reflect the addition of {@code value}.
      *
      * @param value Value.
      */
     @Override
     public void accept(long value) {
         sumSq.addSquare(value);
     }
 
     /**
      * Gets the sum of squares of all input values.
      *
      * <p>When no values have been added, the result is zero.
      *
      * <p>Warning: This will raise an {@link ArithmeticException}
      * if the result is not within the range {@code [0, 2^31)}.
      *
      * @return sum of all values.
      * @throws ArithmeticException if the {@code result} overflows an {@code int}
      * @see #getAsBigInteger()
      */
     @Override
     public int getAsInt() {
         return sumSq.toIntExact();
     }
 
     /**
      * Gets the sum of squares of all input values.
      *
      * <p>When no values have been added, the result is zero.
      *
      * <p>Warning: This will raise an {@link ArithmeticException}
      * if the result is not within the range {@code [0, 2^63)}.
      *
      * @return sum of all values.
      * @throws ArithmeticException if the {@code result} overflows a {@code long}
      * @see #getAsBigInteger()
      */
     @Override
     public long getAsLong() {
         return sumSq.toLongExact();
     }
 
     /**
      * Gets the sum of squares of all input values.
      *
      * <p>When no values have been added, the result is zero.
      *
      * <p>Note that this conversion can lose information about the precision of the
      * {@code BigInteger} value.
      *
      * @return sum of squares of all values.
      * @see #getAsBigInteger()
      */
     @Override
     public double getAsDouble() {
         return sumSq.toDouble();
     }
 
     /**
      * Gets the sum of squares of all input values.
      *
      * <p>When no values have been added, the result is zero.
      *
      * @return sum of squares of all values.
      */
     @Override
     public BigInteger getAsBigInteger() {
         return sumSq.toBigInteger();
     }
 
     @Override
     public LongSumOfSquares combine(LongSumOfSquares other) {
         sumSq.add(other.sumSq);
         return this;
     }
 }