package org.apache.lucene.util;

/**
 * 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.
 */

/**
 * This class was inspired by CGLIB, but provides a better
 * QuickSort algorithm without additional InsertionSort
 * at the end.
 * To use, subclass and override the four abstract methods
 * which compare and modify your data.
 * Allows custom swap so that two arrays can be sorted
 * at the same time.
 * @lucene.internal
 */
public abstract class SorterTemplate {

  private static final int MERGESORT_THRESHOLD = 12;
  private static final int QUICKSORT_THRESHOLD = 7;

  /** Implement this method, that swaps slots {@code i} and {@code j} in your data */
  protected abstract void swap(int i, int j);
  
  /** Compares slots {@code i} and {@code j} of you data.
   * Should be implemented like <code><em>valueOf(i)</em>.compareTo(<em>valueOf(j)</em>)</code> */
  protected abstract int compare(int i, int j);

  /** Implement this method, that stores the value of slot {@code i} as pivot value */
  protected abstract void setPivot(int i);
  
  /** Implements the compare function for the previously stored pivot value.
   * Should be implemented like <code>pivot.compareTo(<em>valueOf(j)</em>)</code> */
  protected abstract int comparePivot(int j);
  
  /** Sorts via stable in-place InsertionSort algorithm
   *(ideal for small collections which are mostly presorted). */
  public final void insertionSort(int lo, int hi) {
    for (int i = lo + 1 ; i <= hi; i++) {
      for (int j = i; j > lo; j--) {
        if (compare(j - 1, j) > 0) {
          swap(j - 1, j);
        } else {
          break;
        }
      }
    }
  }

  /** Sorts via in-place, but unstable, QuickSort algorithm.
   * For small collections falls back to {@link #insertionSort(int,int)}. */
  public final void quickSort(final int lo, final int hi) {
    if (hi <= lo) return;
    // from Integer's Javadocs: ceil(log2(x)) = 32 - numberOfLeadingZeros(x - 1)
    quickSort(lo, hi, (Integer.SIZE - Integer.numberOfLeadingZeros(hi - lo)) << 1);
  }
  
  private void quickSort(int lo, int hi, int maxDepth) {
    // fall back to insertion when array has short length
    final int diff = hi - lo;
    if (diff <= QUICKSORT_THRESHOLD) {
      insertionSort(lo, hi);
      return;
    }
    
    // fall back to merge sort when recursion depth gets too big
    if (--maxDepth == 0) {
      mergeSort(lo, hi);
      return;
    }
    
    final int mid = lo + (diff >>> 1);
    
    if (compare(lo, mid) > 0) {
      swap(lo, mid);
    }

    if (compare(mid, hi) > 0) {
      swap(mid, hi);
      if (compare(lo, mid) > 0) {
        swap(lo, mid);
      }
    }
    
    int left = lo + 1;
    int right = hi - 1;

    setPivot(mid);
    for (;;) {
      while (comparePivot(right) < 0)
        --right;

      while (left < right && comparePivot(left) >= 0)
        ++left;

      if (left < right) {
        swap(left, right);
        --right;
      } else {
        break;
      }
    }

    quickSort(lo, left, maxDepth);
    quickSort(left + 1, hi, maxDepth);
  }
  
  /** Sorts via stable in-place MergeSort algorithm
   * For small collections falls back to {@link #insertionSort(int,int)}. */
  public final void mergeSort(int lo, int hi) {
    final int diff = hi - lo;
    if (diff <= MERGESORT_THRESHOLD) {
      insertionSort(lo, hi);
      return;
    }
    
    final int mid = lo + (diff >>> 1);
    
    mergeSort(lo, mid);
    mergeSort(mid, hi);
    merge(lo, mid, hi, mid - lo, hi - mid);
  }

  private void merge(int lo, int pivot, int hi, int len1, int len2) {
    if (len1 == 0 || len2 == 0) {
      return;
    }
    if (len1 + len2 == 2) {
      if (compare(pivot, lo) < 0) {
          swap(pivot, lo);
      }
      return;
    }
    int first_cut, second_cut;
    int len11, len22;
    if (len1 > len2) {
      len11 = len1 >>> 1;
      first_cut = lo + len11;
      second_cut = lower(pivot, hi, first_cut);
      len22 = second_cut - pivot;
    } else {
      len22 = len2 >>> 1;
      second_cut = pivot + len22;
      first_cut = upper(lo, pivot, second_cut);
      len11 = first_cut - lo;
    }
    rotate(first_cut, pivot, second_cut);
    final int new_mid = first_cut + len22;
    merge(lo, first_cut, new_mid, len11, len22);
    merge(new_mid, second_cut, hi, len1 - len11, len2 - len22);
  }

  private void rotate(int lo, int mid, int hi) {
    int lot = lo;
    int hit = mid - 1;
    while (lot < hit) {
      swap(lot++, hit--);
    }
    lot = mid; hit = hi - 1;
    while (lot < hit) {
      swap(lot++, hit--);
    }
    lot = lo; hit = hi - 1;
    while (lot < hit) {
      swap(lot++, hit--);
    }
  }

  private int lower(int lo, int hi, int val) {
    int len = hi - lo;
    while (len > 0) {
      final int half = len >>> 1,
        mid = lo + half;
      if (compare(mid, val) < 0) {
        lo = mid + 1;
        len = len - half -1;
      } else {
        len = half;
      }
    }
    return lo;
  }

  private int upper(int lo, int hi, int val) {
    int len = hi - lo;
    while (len > 0) {
      final int half = len >>> 1,
        mid = lo + half;
      if (compare(val, mid) < 0) {
        len = half;
      } else {
        lo = mid + 1;
        len = len - half -1;
      }
    }
    return lo;
  }

}