include/heap.h

/*====================================================================*
 -  Copyright (C) 2001 Leptonica.  All rights reserved.
 -  This software is distributed in the hope that it will be
 -  useful, but with NO WARRANTY OF ANY KIND.
 -  No author or distributor accepts responsibility to anyone for the
 -  consequences of using this software, or for whether it serves any
 -  particular purpose or works at all, unless he or she says so in
 -  writing.  Everyone is granted permission to copy, modify and
 -  redistribute this source code, for commercial or non-commercial
 -  purposes, with the following restrictions: (1) the origin of this
 -  source code must not be misrepresented; (2) modified versions must
 -  be plainly marked as such; and (3) this notice may not be removed
 -  or altered from any source or modified source distribution.
 *====================================================================*/

#ifndef  LEPTONICA_HEAP_H
#define  LEPTONICA_HEAP_H

/*
 *  heap.h
 *
 *      Expandable priority queue configured as a heap for arbitrary void* data
 *
 *      The L_Heap is used to implement a priority queue.  The elements
 *      in the heap are ordered in either increasing or decreasing key value.
 *      The key is a float field 'keyval' that is required to be
 *      contained in the elements of the queue.
 * 
 *      The heap is a simple binary tree with the following constraints:
 *         - the key of each node is >= the keys of the two children
 *         - the tree is complete, meaning that each level (1, 2, 4, ...)
 *           is filled and the last level is filled from left to right
 *
 *      The tree structure is implicit in the queue array, with the
 *      array elements numbered as a breadth-first search of the tree
 *      from left to right.  It is thus guaranteed that the largest
 *      (or smallest) key belongs to the first element in the array.
 *
 *      Heap sort is used to sort the array.  Once an array has been
 *      sorted as a heap, it is convenient to use it as a priority queue,
 *      because the min (or max) elements are always at the root of
 *      the tree (element 0), and once removed, the heap can be
 *      resorted in not more than log[n] steps, where n is the number
 *      of elements on the heap.  Likewise, if an arbitrary element is
 *      added to the end of the array A, the sorted heap can be restored
 *      in not more than log[n] steps.
 *
 *      A L_Heap differs from a L_Queue in that the elements in the former
 *      are sorted by a key.  Internally, the array is maintained
 *      as a queue, with a pointer to the end of the array.  The
 *      head of the array always remains at array[0].  The array is
 *      maintained (sorted) as a heap.  When an item is removed from
 *      the head, the last item takes its place (thus reducing the
 *      array length by 1), and this is followed by array element
 *      swaps to restore the heap property.   When an item is added,
 *      it goes at the end of the array, and is swapped up to restore
 *      the heap.  If the ptr array is full, adding another item causes
 *      the ptr array size to double.
 *      
 *      For further implementation details, see heap.c.
 */

struct L_Heap
{
    l_int32      nalloc;      /* size of allocated ptr array                 */
    l_int32      n;           /* number of elements stored in the heap       */
    void       **array;       /* ptr array                                   */
    l_int32      direction;   /* L_SORT_INCREASING or L_SORT_DECREASING      */
};
typedef struct L_Heap  L_HEAP;


#endif  /* LEPTONICA_HEAP_H */