/**

 * @file  pqueue.c

 * @brief Implements a priority queue.

 * @note  Copyright (c) 2015 Richard Cochran <richardcochran@gmail.com>

 *

 * This program is free software; you can redistribute it and/or modify

 * it under the terms of the GNU General Public License as published by

 * the Free Software Foundation; either version 2 of the License, or

 * (at your option) any later version.

 *

 * This program is distributed in the hope that it will be useful,

 * but WITHOUT ANY WARRANTY; without even the implied warranty of

 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

 * GNU General Public License for more details.

 *

 * You should have received a copy of the GNU General Public License along

 * with this program; if not, write to the Free Software Foundation, Inc.,

 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1335 USA.

 */

#include <errno.h>

#include <stdlib.h>

#include <string.h>

#include "pqueue.h"

#define parent(x)	(((1 + (x)) >> 1) - 1)

#define left(x)		(((1 + (x)) << 1) - 1)

#define right(x)	(((1 + (x)) << 1))

struct pqueue {

	int len;

	int max;

	int (*cmp)(void *a, void *b);

	void **data;

};

static int pq_greater(struct pqueue *q, int a, int b)

{

	return q->cmp(q->data[a], q->data[b]) > 0 ? 1 : 0;

}

static void heapify(struct pqueue *q, int index)

{

	int i_max = index;

	int left = left(index);

	int right = right(index);

	if (left < q->len) {

		if (pq_greater(q, left, i_max))

			i_max = left;

	}

	if (right < q->len && pq_greater(q, right, i_max)) {

		i_max = right;

	}

	if (i_max != index) {

		void *tmp = q->data[index];

		q->data[index] = q->data[i_max];

		q->data[i_max] = tmp;

		heapify(q, i_max);

	}

}

/* public methods */

struct pqueue *pqueue_create(int max_length,

			     int (*compare)(void *a, void *b))

{

	struct pqueue *q = calloc(1, sizeof(*q));

	if (!q) {

		return NULL;

	}

	q->len = 0;

	q->max = max_length;

	q->cmp = compare;

	q->data = calloc(max_length, sizeof(void *));

	if (!q->data) {

		free(q);

		return NULL;

	}

	return q;

}

void pqueue_destroy(struct pqueue *q)

{

	free(q->data);

	free(q);

}

void *pqueue_extract(struct pqueue *q)

{

	void *data;

	if (!q->len) {

		return NULL;

	}

	data = q->data[0];

	q->data[0] = q->data[q->len - 1];

	q->len--;

	heapify(q, 0);

	return data;

}

int pqueue_insert(struct pqueue *q, void *d)

{

	int index;

	if (q->len >= q->max) {

		void **buf = realloc(q->data, 2 * q->max * sizeof(void *));

		if (buf) {

			q->data = buf;

			q->max *= 2;

		} else {

			return -ENOMEM;

		}

	}

	index = q->len;

	q->len++;

	while (index && (q->cmp(q->data[parent(index)], d) < 0)) {

		q->data[index] = q->data[parent(index)];

		index = parent(index);

	}

	q->data[index] = d;

	return 0;

}

int pqueue_length(struct pqueue *q)

{

	return q->len;

}

void *pqueue_peek(struct pqueue *q)

{

	return q->len ? q->data[0] : (void *) 0;

}