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pq.c
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pq.c
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/* pq.c - priority queue */
#include <stdlib.h>
#include "pq.h"
pq
make_pq(unsigned int size)
{
pq q;
q = malloc(sizeof(struct pq) + size * sizeof(job));
if (!q) return NULL;
q->size = size;
q->used = 0;
return q;
}
static void
swap(pq q, unsigned int a, unsigned int b)
{
job j;
j = q->heap[a];
q->heap[a] = q->heap[b];
q->heap[b] = j;
}
#define PARENT(i) (((i-1))>>1)
#define CHILD_LEFT(i) (((i)<<1)+1)
#define CHILD_RIGHT(i) (((i)<<1)+2)
static int
cmp(pq q, unsigned int a, unsigned int b)
{
return job_cmp(q->heap[a], q->heap[b]);
}
static void
bubble_up(pq q, unsigned int k)
{
int p;
if (k == 0) return;
p = PARENT(k);
if (cmp(q, p, k) <= 0) return;
swap(q, k, p);
bubble_up(q, p);
}
static void
bubble_down(pq q, unsigned int k)
{
int l, r, s;
l = CHILD_LEFT(k);
r = CHILD_RIGHT(k);
s = k;
if (l < q->used && cmp(q, l, k) < 0) s = l;
if (r < q->used && cmp(q, r, s) < 0) s = r;
if (s == k) return; /* already satisfies the heap property */
swap(q, k, s);
bubble_down(q, s);
}
/* assumes there is at least one item in the queue */
static void
delete_min(pq q)
{
q->heap[0] = q->heap[--q->used];
if (q->used) bubble_down(q, 0);
}
int
pq_give(pq q, job j)
{
int k;
if (q->used >= q->size) return 0;
k = q->used++;
q->heap[k] = j;
bubble_up(q, k);
return 1;
}
job
pq_take(pq q)
{
job j;
if (q->used == 0) return NULL;
j = q->heap[0];
delete_min(q);
return j;
}