#define _GNU_SOURCE

#include <sys/socket.h>
#include <netdb.h>
#include <string.h>
#include <netinet/in.h>
#include <errno.h>
#include <inttypes.h>

int gethostbyaddr_r(const void* a,
                    socklen_t l,
                    int af,
                    struct hostent* h,
                    char* buf,
                    size_t buflen,
                    struct hostent** res,
                    int* err) {
  union {
    struct sockaddr_in sin;
    struct sockaddr_in6 sin6;
  } sa = {.sin.sin_family = af};
  socklen_t sl = af == AF_INET6 ? sizeof sa.sin6 : sizeof sa.sin;
  int i;

  *res = 0;

  /* Load address argument into sockaddr structure */
  if (af == AF_INET6 && l == 16)
    memcpy(&sa.sin6.sin6_addr, a, 16);
  else if (af == AF_INET && l == 4)
    memcpy(&sa.sin.sin_addr, a, 4);
  else {
    *err = NO_RECOVERY;
    return EINVAL;
  }

  /* Align buffer and check for space for pointers and ip address */
  i = (uintptr_t)buf & sizeof(char*) - 1;
  if (!i)
    i = sizeof(char*);
  if (buflen <= 5 * sizeof(char*) - i + l)
    return ERANGE;
  buf += sizeof(char*) - i;
  buflen -= 5 * sizeof(char*) - i + l;

  h->h_addr_list = (void*)buf;
  buf += 2 * sizeof(char*);
  h->h_aliases = (void*)buf;
  buf += 2 * sizeof(char*);

  h->h_addr_list[0] = buf;
  memcpy(h->h_addr_list[0], a, l);
  buf += l;
  h->h_addr_list[1] = 0;
  h->h_aliases[0] = buf;
  h->h_aliases[1] = 0;

  switch (getnameinfo((void*)&sa, sl, buf, buflen, 0, 0, 0)) {
    case EAI_AGAIN:
      *err = TRY_AGAIN;
      return EAGAIN;
    case EAI_OVERFLOW:
      return ERANGE;
    default:
    case EAI_MEMORY:
    case EAI_SYSTEM:
    case EAI_FAIL:
      *err = NO_RECOVERY;
      return errno;
    case 0:
      break;
  }

  h->h_addrtype = af;
  h->h_name = h->h_aliases[0];
  *res = h;
  return 0;
}