#include <stdlib.h>
#include <stdio.h>

typedef struct {
  size_t count;
  size_t capacity;
} Header;

#define ARR_INIT_CAPACITY 256

#define arr_len(arr) ((Header *)(arr) - 1)->count

#define arr_push(arr, x)                                                       \
  do {                                                                         \
    if ((arr) == NULL) {                                                       \
      size_t st = sizeof(*(arr)) * ARR_INIT_CAPACITY + sizeof(Header);         \
      Header *header = malloc(st);                                             \
      header->count = 0;                                                       \
      header->capacity = ARR_INIT_CAPACITY;                                    \
      (arr) = (void *)(header + 1);                                            \
    }                                                                          \
    Header *header = (Header *)(arr) - 1;                                      \
    if (header->count >= header->capacity) {                                   \
      header->capacity *= 2;                                                   \
      header =                                                                 \
          realloc(header, sizeof(*(arr)) * header->capacity + sizeof(Header)); \
      (arr) = (void *)(header + 1);                                            \
    }                                                                          \
    (arr)[header->count++] = (x);                                              \
  } while (0)

#define arr_pop(arr)                                                           \
  do {                                                                         \
    if ((arr) == NULL) {                                                       \
      size_t st = sizeof(*(arr)) * ARR_INIT_CAPACITY + sizeof(Header);         \
      Header *header = malloc(st);                                             \
      header->count = 0;                                                       \
      header->capacity = ARR_INIT_CAPACITY;                                    \
      (arr) = (void *)(header + 1);                                            \
    }                                                                          \
    Header *header = (Header *)(arr) - 1;                                      \
    if (header->count > 0)                                                     \
      header->count--;                                                         \
	} while(0)

#define arr_free(arr) free((Header*)(arr)-1)

int main(){
  int *numbers = NULL;
  // [header][numbers]
  //         ^
  float *data = NULL;
  arr_push(numbers, 23);
  arr_push(numbers, 456);
  arr_push(numbers, 3128);
  arr_push(numbers, 79);
  arr_push(numbers, 69);
  printf("\nnumbers (nb_elements: %zu)\n", arr_len(numbers));
  for (int i = 0; i < arr_len(numbers); i++) {
	printf("%d\n", numbers[i]);
  }


  arr_push(data, 67);
  arr_push(data, 54);
  arr_push(data, 34.5);
  printf("\ndata (nb_elements: %zu)\n", arr_len(data));
  for (int i = 0; i < arr_len(data); i++) {
	printf("%f\n", data[i]);
  }
  arr_free(numbers);
  arr_free(data);
  return 0;
}