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# STC [cpque](../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 argument `<`or `>` must be supplied to set the ordering, e.g. using `>` would cause the smallest element to appear as the top().
See the c++ class [std::priority_queue](https://en.cppreference.com/w/cpp/container/priority_queue) for a functional reference.
## Declaration
```c
using_cpque(X, ctype, direction)
```
The macro `using_cpque()` must be instantiated in the global scope. **cpque** uses normally **cvec_X**
or **cdeq_X** as underlying implementation, specified as `ctype`. The *direction* must be given as
`<` or `>`, specifying *max-heap* or *min-heap* for the priority queue.
Note that the function *`ctype`_value_compare(x, y)* defined by the underlying vector type is used to
compare values (priorities). `X` is a type tag name and will affect the names of all cpque types and methods.
Declaring `using_cpque(i, cvec_i, >)`, `X` should be replaced by `i` in the following documentation.
## Header file
All cpque definitions and prototypes are available by including a single header file.
```c
#include "stc/cpque.h"
```
## Methods
```c
cpque_X cpque_X_init(void);
cpque_X cpque_X_clone(cpque_X pq);
void cpque_X_make_heap(cpque_X* self);
void cpque_X_del(cpque_X* self); // destructor
size_t cpque_X_size(cpque_X pq);
bool cpque_X_empty(cpque_X pq);
const
cpque_X_value_t* cpque_X_top(const cpque_X* self);
void cpque_X_push(cpque_X* self, cpque_X_value_t value);
void cpque_X_emplace(cpque_X* self, cpque_X_rawvalue_t raw);
void cpque_X_emplace_n(cpque_X *self, const cpque_X_rawvalue_t arr[], size_t size);
void cpque_X_pop(cpque_X* self);
void cpque_X_erase_at(cpque_X* self, size_t idx);
cpque_X_value_t cpque_X_value_clone(cpque_X_value_t val);
```
## Types
| Type name | Type definition | Used to represent... |
|:---------------------|:--------------------------------------|:------------------------|
| `cpque_X` | `struct {cpque_X_value_t* data; ...}` | The cpque type |
| `cpque_X_value_t` | Depends on underlying container type | The cpque element type |
| `cpque_X_rawvalue_t` | " | cpque raw value type |
## Example
```c
#include <stdio.h>
#include "stc/cpque.h"
#include "stc/crandom.h"
using_cvec(i, int64_t);
using_cpque(i, cvec_i, >); // adaptor type, '>' = min-heap
int main()
{
size_t N = 10000000;
stc64_t rng = stc64_init(1234);
stc64_uniform_t dist = stc64_uniform_init(0, N * 10);
cpque_i heap = cpque_i_init();
// Push ten million random numbers to priority queue, plus some negative ones.
c_forrange (N)
cpque_i_push(&heap, stc64_uniform(&rng, &dist));
c_emplace_items(&heap, cpque_i, {-231, -32, -873, -4, -343});
// Extract and display the fifty smallest.
c_forrange (50) {
printf("%zd ", *cpque_i_top(&heap));
cpque_i_pop(&heap);
}
cpque_i_del(&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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