#include <arpa/inet.h>
#include <ctype.h>
#include <errno.h>
#include <string.h>
#include <sys/socket.h>

static int hexval(unsigned c) {
  if (c - '0' < 10)
    return c - '0';
  c |= 32;
  if (c - 'a' < 6)
    return c - 'a' + 10;
  return -1;
}

int inet_pton(int af, const char* restrict s, void* restrict a0) {
  uint16_t ip[8];
  unsigned char* a = a0;
  int i, j, v, d, brk = -1, need_v4 = 0;

  if (af == AF_INET) {
    for (i = 0; i < 4; i++) {
      for (v = j = 0; j < 3 && isdigit(s[j]); j++)
        v = 10 * v + s[j] - '0';
      if (j == 0 || (j > 1 && s[0] == '0') || v > 255)
        return 0;
      a[i] = v;
      if (s[j] == 0 && i == 3)
        return 1;
      if (s[j] != '.')
        return 0;
      s += j + 1;
    }
    return 0;
  } else if (af != AF_INET6) {
    errno = EAFNOSUPPORT;
    return -1;
  }

  if (*s == ':' && *++s != ':')
    return 0;

  for (i = 0;; i++) {
    if (s[0] == ':' && brk < 0) {
      brk = i;
      ip[i & 7] = 0;
      if (!*++s)
        break;
      if (i == 7)
        return 0;
      continue;
    }
    for (v = j = 0; j < 4 && (d = hexval(s[j])) >= 0; j++)
      v = 16 * v + d;
    if (j == 0)
      return 0;
    ip[i & 7] = v;
    if (!s[j] && (brk >= 0 || i == 7))
      break;
    if (i == 7)
      return 0;
    if (s[j] != ':') {
      if (s[j] != '.' || (i < 6 && brk < 0))
        return 0;
      need_v4 = 1;
      i++;
      break;
    }
    s += j + 1;
  }
  if (brk >= 0) {
    memmove(ip + brk + 7 - i, ip + brk, 2 * (i + 1 - brk));
    for (j = 0; j < 7 - i; j++)
      ip[brk + j] = 0;
  }
  for (j = 0; j < 8; j++) {
    *a++ = ip[j] >> 8;
    *a++ = ip[j];
  }
  if (need_v4 && inet_pton(AF_INET, (void*)s, a - 4) <= 0)
    return 0;
  return 1;
}