thodg/libgit2/src/pqueue.c

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• Hash : 1bbacc9f
  Author :
  Date : 2014-02-04T16:46:43
  

  Avoid extra copying in pqueue operations This tweaks the pqueue_up and pqueue_down routines so that they will not do full element swaps but instead carry over the state of the previous loop iteration and only assign elements for which we know the final position. This will avoid a little bit of data assignment which should improve performance in theory. Also got rid of some vector helpers that I'm no longer using.

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• src/pqueue.c

• /*
   * Copyright (C) the libgit2 contributors. All rights reserved.
   *
   * This file is part of libgit2, distributed under the GNU GPL v2 with
   * a Linking Exception. For full terms see the included COPYING file.
   */
  
  #include "pqueue.h"
  #include "util.h"
  
  #define PQUEUE_LCHILD_OF(I) (((I)<<1)+1)
  #define PQUEUE_RCHILD_OF(I) (((I)<<1)+2)
  #define PQUEUE_PARENT_OF(I) (((I)-1)>>1)
  
  int git_pqueue_init(
  	git_pqueue *pq,
  	uint32_t flags,
  	size_t init_size,
  	git_vector_cmp cmp)
  {
  	int error = git_vector_init(pq, init_size, cmp);
  
  	if (!error) {
  		/* mix in our flags */
  		pq->flags |= flags;
  
  		/* if fixed size heap, pretend vector is exactly init_size elements */
  		if ((flags & GIT_PQUEUE_FIXED_SIZE) && init_size > 0)
  			pq->_alloc_size = init_size;
  	}
  
  	return error;
  }
  
  static void pqueue_up(git_pqueue *pq, size_t el)
  {
  	size_t parent_el = PQUEUE_PARENT_OF(el);
  	void *kid = git_vector_get(pq, el);
  
  	while (el > 0) {
  		void *parent = pq->contents[parent_el];
  
  		if (pq->_cmp(parent, kid) <= 0)
  			break;
  
  		pq->contents[el] = parent;
  
  		el = parent_el;
  		parent_el = PQUEUE_PARENT_OF(el);
  	}
  
  	pq->contents[el] = kid;
  }
  
  static void pqueue_down(git_pqueue *pq, size_t el)
  {
  	void *parent = git_vector_get(pq, el), *kid, *rkid;
  
  	while (1) {
  		size_t kid_el = PQUEUE_LCHILD_OF(el);
  
  		if ((kid = git_vector_get(pq, kid_el)) == NULL)
  			break;
  
  		if ((rkid = git_vector_get(pq, kid_el + 1)) != NULL &&
  			pq->_cmp(kid, rkid) > 0) {
  			kid    = rkid;
  			kid_el += 1;
  		}
  
  		if (pq->_cmp(parent, kid) < 0)
  			break;
  
  		pq->contents[el] = kid;
  		el = kid_el;
  	}
  
  	pq->contents[el] = parent;
  }
  
  int git_pqueue_insert(git_pqueue *pq, void *item)
  {
  	int error = 0;
  
  	/* if heap is full, pop the top element if new one should replace it */
  	if ((pq->flags & GIT_PQUEUE_FIXED_SIZE) != 0 &&
  		pq->length >= pq->_alloc_size)
  	{
  		/* skip this item if below min item in heap */
  		if (pq->_cmp(item, git_vector_get(pq, 0)) <= 0)
  			return 0;
  		/* otherwise remove the min item before inserting new */
  		(void)git_pqueue_pop(pq);
  	}
  
  	if (!(error = git_vector_insert(pq, item)))
  		pqueue_up(pq, pq->length - 1);
  
  	return error;
  }
  
  void *git_pqueue_pop(git_pqueue *pq)
  {
  	void *rval = git_pqueue_get(pq, 0);
  
  	if (git_pqueue_size(pq) > 1) {
  		/* move last item to top of heap, shrink, and push item down */
  		pq->contents[0] = git_vector_last(pq);
  		git_vector_pop(pq);
  		pqueue_down(pq, 0);
  	} else {
  		/* all we need to do is shrink the heap in this case */
  		git_vector_pop(pq);
  	}
  
  	return rval;
  }
  

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