#include <errno.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define NUM_ELEMENTS 8

typedef struct creds creds_t;

typedef struct creds {
  void (*welcome)(void);
  uint64_t buffer[NUM_ELEMENTS];
} creds_t;
static creds_t *creds;

/* internal prototypes */
static void insert(uint64_t value, int64_t index);

static void print(void);

static void s3cr3t(void);

/* entry point */
int main(int argc, char **argv) {
  if (argc < 3) {
    printf("usage: %s <hex value> <index>\n", argv[0]);
    return -1;
  }

  char *endptr;
  errno = 0;
  uint64_t value = (uint64_t)strtoull(argv[1], &endptr, 16);
  if (errno || *endptr) {
    fprintf(stderr, "Conversion error, non-convertible part: %s\n", endptr);
    exit(EXIT_FAILURE);
  }

  errno = 0;
  uint64_t index = (uint64_t)strtoull(argv[2], &endptr, 10);
  if (errno || *endptr) {
    fprintf(stderr, "Conversion error, non-convertible part: %s\n", endptr);
    exit(EXIT_FAILURE);
  }

  if (index < 0) {
    fprintf(stderr, "Negative index\n");
    exit(EXIT_FAILURE);
  }

  creds = (creds_t *)calloc(1, sizeof(creds_t));
  if (!creds) {
    perror("calloc");
    return -1;
  }

  creds->welcome = print;
  insert(value, index);
  creds->welcome();
  free(creds);
  return 0;
}

/* internal function definitions */
static void insert(uint64_t value, int64_t index) {
  if (index >= NUM_ELEMENTS) {
    fprintf(stderr, "Invalid index!\n");
    exit(EXIT_FAILURE);
  }

  creds->buffer[index] = value;
}

static void print(void) {
  for (int i = 0; i < NUM_ELEMENTS; ++i) {
    printf("Index %d: %#lx\n", i, creds->buffer[i]);
  }
}

static void s3cr3t(void) { printf("You've gathered secret material!\n"); }