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

typedef union {
  struct {
    float x, y;
  };
  struct {
    unsigned int next;
  };
} item_t;

typedef struct {
  item_t* items;
  size_t size;
  size_t used;
  size_t num_inserts;
  size_t num_deletes;
  size_t num_allocations;
} bulk_data_t;

void delete_item(bulk_data_t* bd, size_t index) {
  if (index >= bd->size) return;
  bd->items[index].next = bd->items[0].next;
  bd->items[0].next = index;
  bd->num_deletes++;
}

size_t alloc_item(bulk_data_t* bd) {
  bd->num_inserts++;
  size_t slot = bd->items[0].next;
  bd->items[0].next = bd->items[slot].next;
  if (slot) return slot;
  if (bd->used < bd->size) {
    slot = bd->used++;
    return slot;
  }
  bd->size += 5;
  bd->items = reallocarray(bd->items, bd->size, sizeof(item_t));
  bd->num_allocations++;
  return bd->used++;
}

int main() {
  srand(time(0));

  bulk_data_t bd = { NULL, 1, 1, 1, 0, 0 };
  bd.items = reallocarray(bd.items, bd.size, sizeof(item_t));
  bd.items[0].next = 1;
  for (int i = 0; i < 1000; i++) {
    if (rand() % 2 == 0) {
      alloc_item(&bd);
    } else {
      delete_item(&bd, rand() % bd.size);
    }
  }
  printf(
    "%zu - %zu - %zu - %zu - %zu\n",
    bd.size,
    bd.used,
    bd.num_allocations,
    bd.num_inserts,
    bd.num_deletes
  );
  return 0;
}