/*
 * 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.beam.sdk.util;

import static com.google.common.base.Preconditions.checkArgument;

import java.util.Arrays;
import org.apache.beam.sdk.transforms.Combine;

/**
 * Keep track of the moving minimum/maximum/sum of sampled long values. The minimum/maximum/sum
 * is over at most the user-specified last {@code samplePeriodMs}, and is updated every
 * {@code sampleUpdateMs}.
 */
public class MovingFunction {
  /**
   * How frequently to update the moving function, in ms.
   */
  private final long sampleUpdateMs;

  /**
   * How many buckets are considered 'significant'?
   */
  private final int numSignificantBuckets;

  /**
   * How many samples are considered 'significant'?
   */
  private final int numSignificantSamples;

  /**
   * Function for combining sample values.
   */
  private final Combine.BinaryCombineLongFn function;

  /**
   * Minimum/maximum/sum of all values per bucket.
   */
  private final long[] buckets;

  /**
   * How many samples have been added to each bucket.
   */
  private final int[] numSamples;

  /**
   * Time of start of current bucket.
   */
  private long currentMsSinceEpoch;

  /**
   * Index of bucket corresponding to above timestamp, or -1 if no entries.
   */
  private int currentIndex;

  public MovingFunction(long samplePeriodMs, long sampleUpdateMs,
                        int numSignificantBuckets, int numSignificantSamples,
                        Combine.BinaryCombineLongFn function) {
    this.sampleUpdateMs = sampleUpdateMs;
    this.numSignificantBuckets = numSignificantBuckets;
    this.numSignificantSamples = numSignificantSamples;
    this.function = function;
    int n = (int) (samplePeriodMs / sampleUpdateMs);
    buckets = new long[n];
    Arrays.fill(buckets, function.identity());
    numSamples = new int[n];
    Arrays.fill(numSamples, 0);
    currentMsSinceEpoch = -1;
    currentIndex = -1;
  }

  /**
   * Flush stale values.
   */
  private void flush(long nowMsSinceEpoch) {
    checkArgument(nowMsSinceEpoch >= 0, "Only positive timestamps supported");
    if (currentIndex < 0) {
      currentMsSinceEpoch = nowMsSinceEpoch - (nowMsSinceEpoch % sampleUpdateMs);
      currentIndex = 0;
    }
    checkArgument(nowMsSinceEpoch >= currentMsSinceEpoch, "Attempting to move backwards");
    int newBuckets =
        Math.min((int) ((nowMsSinceEpoch - currentMsSinceEpoch) / sampleUpdateMs),
                 buckets.length);
    while (newBuckets > 0) {
      currentIndex = (currentIndex + 1) % buckets.length;
      buckets[currentIndex] = function.identity();
      numSamples[currentIndex] = 0;
      newBuckets--;
      currentMsSinceEpoch += sampleUpdateMs;
    }
  }

  /**
   * Add {@code value} at {@code nowMsSinceEpoch}.
   */
  public void add(long nowMsSinceEpoch, long value) {
    flush(nowMsSinceEpoch);
    buckets[currentIndex] = function.apply(buckets[currentIndex], value);
    numSamples[currentIndex]++;
  }

  /**
   * Return the minimum/maximum/sum of all retained values within samplePeriodMs
   * of {@code nowMsSinceEpoch}.
   */
  public long get(long nowMsSinceEpoch) {
    flush(nowMsSinceEpoch);
    long result = function.identity();
    for (long bucket : buckets) {
      result = function.apply(result, bucket);
    }
    return result;
  }

  /**
   * Is the current result 'significant'? Ie is it drawn from enough buckets
   * or from enough samples?
   */
  public boolean isSignificant() {
    int totalSamples = 0;
    int activeBuckets = 0;
    for (int i = 0; i < buckets.length; i++) {
      totalSamples += numSamples[i];
      if (numSamples[i] > 0) {
        activeBuckets++;
      }
    }
    return activeBuckets >= numSignificantBuckets || totalSamples >= numSignificantSamples;
  }
}
