Trim a CIDR string to make it compatible with inet_net_pton

cidr_trim.cc

#ifdef __FreeBSD__
#include <sys/socket.h>
#endif
#include <arpa/inet.h>
#include <netinet/in.h>

#include <algorithm>  // find, find_if_not, reverse_copy, for_each
#include <functional> // bind1st, std::equal_to
#include <string>     // string
#include <iostream>   // cout

// Trim a CIDR string so (excess) zero groups are trimmed out
// inet_net_pton is very picky about how many address groups are specified, so
// this method trims any '::' which is adjacent to the '/' char in a CIDR.
// 2001:db8::/32 is not understood by inet_net_pton, since with 32 bits it's
// supposed to be 2001:db8/32, this method fixes that, unfortunately it also
// trimmes zero groups that might be needed such as 2001:db8::/48.
std::string TrimCidr(const std::string &input) {
  // Find the network sep '/' starting from the rear where it's closest
  auto net_it = std::find(input.crbegin(), input.crend(), '/');
  if (net_it != input.crend()) {

    // Advance the iterator to skip past the '/' itself
    std::advance(net_it, 1);

    // If this is not a ':' char, then early return (for speed)
    if (*net_it != ':') {
      return input;
    }

    // Find location of first non ':' char
    auto trim_it = std::find_if_not(net_it,
        input.crend(), std::bind1st(std::equal_to<char>(),':'));

    // If the address contains anything else than ':' then trim excess ':'
    if (trim_it != input.crend()) {
      std::string output;
      output.reserve(input.size());
      std::reverse_copy(trim_it, input.crend(), std::back_inserter(output));
      std::reverse_copy(input.crbegin(), net_it, std::back_inserter(output));
      return output;
    }
  }
  return input;
}

int main() {

  auto tests = {
    "0.0.0.0/0",
    "::0/0",
    "2001:db8::/32",  // This will fail without being trimmed
    "2001:db8::/48",  // This fails when trimmed
    "2001:db8::ff00:42:8329",
    "2a03:2880:2130:cf05:face:b00c:0:1",
    "192.168.0.16/28",
    "192.168/16",
    "1.2.3.4",
    "0::0/0",  // Fails but that's okay since the correct form is ::/0
    "::ffff:1.2.3.0/120",
    "2001:1448:281::/48",
    "::/0",
  };

  std::for_each(tests.begin(), tests.end(), [](std::string t) {
    auto cidr = TrimCidr(t);
    std::cout << t << " -> " << cidr << " - ";
    int bits;
    in_addr ip4;
    in6_addr ip6;
    bits = inet_net_pton(AF_INET, cidr.c_str(), &ip4, sizeof(ip4));
    if ((bits = inet_net_pton(AF_INET, cidr.c_str(), &ip4, sizeof(ip4))) != -1) {
      std::cout << "IPv4 CIDR with " << bits << " bits" << std::endl;
    } else if ((bits = inet_net_pton(AF_INET6, cidr.c_str(), &ip6, sizeof(ip6))) != -1) {
      std::cout << "IPv6 CIDR with " << bits << " bits (trimmed)" << std::endl;
    } else if ((bits = inet_net_pton(AF_INET6, t.c_str(), &ip6, sizeof(ip6))) != -1) {
      std::cout << "IPv6 CIDR with " << bits << " bits (original)" << std::endl;
    } else {
      std::cout << "Unknown CIDR " << std::endl;
    }
  });

}