#include <fcntl.h>
#include <semaphore.h>
#include <stdatomic.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <unistd.h>

struct data {
  atomic_bool initialized;
  struct {
    sem_t sem;
    char text[256];
  } item[5];
} *data;

int main(int argc, char **argv) {
  if (argc < 2 || argc > 3) {
    printf("Usage:\n"
           "  %s <num>             prints <text> from item <num>\n"
           "  %s <num> <text>      puts <text> to item <num>\n"
           "  %s <num1> <num2>     copies to item <num1> the text from item <num2>\n",
           argv[0], argv[0], argv[0]);
    return 1;
  }

  int fd = shm_open("/fiveItems", O_RDWR | O_CREAT | O_EXCL, 0666);
  char createSucceeded = (fd != -1);
  if (fd == -1) fd = shm_open("/fiveItems", O_RDWR, 0666);
  if (fd == -1) { perror("shm_open"); exit(1); }

  ftruncate(fd, sizeof(struct data));
  data = mmap(NULL, sizeof(struct data), PROT_WRITE | PROT_READ, MAP_SHARED, fd, 0);

  if (createSucceeded) {
    for (int i = 0; i < 5; ++i)
      sem_init(&data->item[i].sem, 1, 1);
    data->initialized = 1;
  } else
    while (!data->initialized);

  int argOne = atoi(argv[1]);
  if (argOne >= 5 || argOne < 0) return 1;

  if (argc == 2) {
    // print an item
    sem_wait(&data->item[argOne].sem);
    printf("%s\n", data->item[argOne].text);
    sem_post(&data->item[argOne].sem);
    return 0;
  }

  char *e;
  int argTwo = strtol(argv[2], &e, 10);
  if (*e == 0 && argTwo < 5 && argTwo >= 0 && argOne != argTwo) {
    // copy item to item
    sem_wait(&data->item[argOne].sem);
    sem_wait(&data->item[argTwo].sem);
    memcpy(data->item[argOne].text, data->item[argTwo].text, sizeof(data->item[argOne].text));
    sem_post(&data->item[argTwo].sem);
    sem_post(&data->item[argOne].sem);
  } else {
    // assign new value
    sem_wait(&data->item[argOne].sem);
    memset(data->item[argOne].text, 0, sizeof(data->item[argOne].text));
    strncpy(data->item[argOne].text, argv[2], sizeof(data->item[argOne].text) - 1);
    sem_post(&data->item[argOne].sem);
  }

  return 0;
}