/*
 * 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.arrow.algorithm.search;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;

import org.apache.arrow.algorithm.sort.VectorValueComparator;
import org.apache.arrow.vector.ValueVector;
import org.apache.arrow.vector.compare.Range;
import org.apache.arrow.vector.compare.RangeEqualsVisitor;

/**
 * Search for a value in the vector by multiple threads.
 * This is often used in scenarios where the vector is large or
 * low response time is required.
 * @param <V> the vector type.
 */
public class ParallelSearcher<V extends ValueVector> {

  /**
   * The target vector to search.
   */
  private final V vector;

  /**
   * The thread pool.
   */
  private final ExecutorService threadPool;

  /**
   * The number of threads to use.
   */
  private final int numThreads;

  /**
   * The position of the key in the target vector, if any.
   */
  private int keyPosition = -1;

  /**
   * Constructs a parallel searcher.
   * @param vector the vector to search.
   * @param threadPool the thread pool to use.
   * @param numThreads the number of threads to use.
   */
  public ParallelSearcher(V vector, ExecutorService threadPool, int numThreads) {
    this.vector = vector;
    this.threadPool = threadPool;
    this.numThreads = numThreads;
  }

  /**
   * Search for the key in the target vector.
   * @param keyVector the vector containing the search key.
   * @param keyIndex the index of the search key in the key vector.
   * @return the position of a matched value in the target vector,
   *     or -1 if none is found. Please note that if there are multiple
   *     matches of the key in the target vector, this method makes no
   *     guarantees about which instance is returned.
   *     For an alternative search implementation that always finds the first match of the key,
   *     see {@link VectorSearcher#linearSearch(ValueVector, VectorValueComparator, ValueVector, int)}.
   * @throws ExecutionException if an exception occurs in a thread.
   * @throws InterruptedException if a thread is interrupted.
   */
  public int search(V keyVector, int keyIndex) throws ExecutionException, InterruptedException {
    keyPosition = -1;
    final int valueCount = vector.getValueCount();
    final CompletableFuture<Boolean>[] futures = new CompletableFuture[numThreads];
    for (int i = 0; i < futures.length; i++) {
      futures[i] = new CompletableFuture<>();
    }

    for (int i = 0; i < numThreads; i++) {
      final int tid = i;
      threadPool.submit(() -> {
        // convert to long to avoid overflow
        int start = (int) (((long) valueCount) * tid / numThreads);
        int end = (int) ((long) valueCount) * (tid + 1) / numThreads;

        if (start >= end) {
          // no data assigned to this task.
          futures[tid].complete(false);
          return;
        }

        RangeEqualsVisitor visitor = new RangeEqualsVisitor(vector, keyVector, null);
        Range range = new Range(0, 0, 1);
        for (int pos = start; pos < end; pos++) {
          if (keyPosition != -1) {
            // the key has been found by another task
            futures[tid].complete(false);
            return;
          }
          range.setLeftStart(pos).setRightStart(keyIndex);
          if (visitor.rangeEquals(range)) {
            keyPosition = pos;
            futures[tid].complete(true);
            return;
          }
        }

        // no match value is found.
        futures[tid].complete(false);
      });
    }

    CompletableFuture.allOf(futures).get();
    return keyPosition;
  }
}
