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# STC [cpque](../include/stc/cpque.h): Priority Queue
A priority queue is a container adaptor that provides constant time lookup of the largest (by default) element, at the expense of logarithmic insertion and extraction.
A user-provided ***i_cmp*** may be defined to set the ordering, e.g. using ***-c_default_cmp*** would cause the smallest element to appear as the top() value.
Note that **cpque** does not support `i_valraw` and `i_valto`, so only cloning via `i_valfrom` is available.
See the c++ class [std::priority_queue](https://en.cppreference.com/w/cpp/container/priority_queue) for a functional reference.
## Header file and declaration
```c
#define i_val // value: REQUIRED
#define i_cmp // three-way compare two i_val* : REQUIRED IF i_val/i_valraw is a non-integral type
#define i_valdrop // destroy value func - defaults to empty destruct
#define i_valraw // convertion type
#define i_valfrom // convertion func i_valraw => i_val - defaults to plain copy
#define i_valto // convertion func i_val* => i_valraw.
#define i_tag // defaults to i_val
#define i_type // container type name
#include <stc/cpque.h>
```
`X` should be replaced by the value of `i_tag` in all of the following documentation.
## Methods
```c
cpque_X cpque_X_init(void); // create empty pri-queue.
cpque_X cpque_X_with_capacity(size_t cap);
cpque_X cpque_X_with_size(size_t size, i_val null);
cpque_X cpque_X_clone(cpque_X pq);
void cpque_X_clear(cpque_X* self);
bool cpque_X_reserve(cpque_X* self, size_t n);
void cpque_X_shrink_to_fit(cpque_X* self);
void cpque_X_copy(cpque_X* self, cpque_X other);
void cpque_X_drop(cpque_X* self); // destructor
size_t cpque_X_size(cpque_X pq);
bool cpque_X_empty(cpque_X pq);
i_val* cpque_X_top(const cpque_X* self);
void cpque_X_make_heap(cpque_X* self); // heapify the vector.
void cpque_X_push(cpque_X* self, i_val value);
void cpque_X_emplace(cpque_X* self, i_valraw raw); // converts from raw
void cpque_X_pop(cpque_X* self);
void cpque_X_erase_at(cpque_X* self, size_t idx);
i_val cpque_X_value_clone(i_val value);
```
## Types
| Type name | Type definition | Used to represent... |
|:-------------------|:--------------------------------------|:------------------------|
| `cpque_X` | `struct {cpque_X_value* data; ...}` | The cpque type |
| `cpque_X_value` | `i_val` | The cpque element type |
## Example
```c
#include <stc/crandom.h>
#include <stdio.h>
#define i_val int64_t
#define i_cmp -c_default_cmp // min-heap
#define i_tag i
#include <stc/cpque.h>
int main()
{
size_t N = 10000000;
stc64_t rng = stc64_init(1234);
stc64_uniform_t dist = stc64_uniform_init(0, N * 10);
// Declare heap, with defered drop()
c_auto (cpque_i, heap)
{
// Push ten million random numbers to priority queue, plus some negative ones.
c_forrange (N)
cpque_i_push(&heap, stc64_uniform(&rng, &dist));
c_apply(v, cpque_i_push(&heap, v), int, {-231, -32, -873, -4, -343});
// Extract and display the fifty smallest.
c_forrange (50) {
printf("%" PRIdMAX " ", *cpque_i_top(&heap));
cpque_i_pop(&heap);
}
}
}
```
Output:
```
-873 -343 -231 -32 -4 3 5 6 18 23 31 54 68 87 99 105 107 125 128 147 150 155 167 178 181 188 213 216 272 284 287 302 306 311 313 326 329 331 344 348 363 367 374 385 396 399 401 407 412 477
```
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