// SPDX-License-Identifier: GPL-2.0
/*
 * iptool.c - Simple IP subnet calculator
 *
 * Takes an IPv4 address and a prefix length, then calculates:
 *  - IP class
 *  - Network address
 *  - Broadcast address
 *  - Total hosts
 *  - Usable hosts
 *  - Usable IP range
 */

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>

#define PROGRAM_NAME "iptool"

static uint32_t ip_to_u32(const char *ip_str)
{
	int o1, o2, o3, o4;

	if (sscanf(ip_str, "%d.%d.%d.%d", &o1, &o2, &o3, &o4) != 4) {
		fprintf(stderr, "Invalid IP format: %s\n", ip_str);
		exit(1);
	}

	if (o1 < 0 || o1 > 255 || o2 < 0 || o2 > 255 ||
			o3 < 0 || o3 > 255 || o4 < 0 || o4 > 255) {
		fprintf(stderr, "IP octet out of range: %s\n", ip_str);
		exit(1);
	}

	return (o1 << 24) | (o2 << 16) | (o3 << 8) | o4;
}

static uint32_t netmask_from_prefix(int prefix)
{
	return prefix ? 0xFFFFFFFFU << (32 - prefix) : 0;
}

static uint32_t network_addr(uint32_t ip, uint32_t netmask)
{
	return ip & netmask;
}

static uint32_t broadcast_addr(uint32_t ip, uint32_t netmask)
{
	return (ip & netmask) | ~netmask;
}

static uint32_t total_hosts(int prefix)
{
	return 1U << (32 - prefix);
}

static uint32_t usable_hosts(int prefix)
{
	uint32_t th = total_hosts(prefix);

	return (th <= 2) ? 0 : th - 2;
}

static char ip_class(uint32_t ip)
{
	uint8_t first_octet = ip >> 24;

	if (first_octet <= 127)
		return 'A';
	if (first_octet <= 191)
		return 'B';
	if (first_octet <= 223)
		return 'C';
	if (first_octet <= 239)
		return 'D';
	return 'E';
}

static void print_ip(uint32_t ip)
{
	printf("%u.%u.%u.%u",
			(ip >> 24) & 0xFF,
			(ip >> 16) & 0xFF,
			(ip >> 8) & 0xFF,
			ip & 0xFF);
}

int main(int argc, char *argv[])
{
	uint32_t ip, netmask, network, broadcast;
	int prefix;
	uint32_t total, usable;

	if (argc < 3) {
		fprintf(stderr, "Usage: %s <ip> <prefix>\n", PROGRAM_NAME);
		return 1;
	}

	ip = ip_to_u32(argv[1]);
	prefix = atoi(argv[2]);

	if (prefix < 0 || prefix > 32) {
		fprintf(stderr, "Prefix must be 0-32\n");
		return 1;
	}

	netmask   = netmask_from_prefix(prefix);
	network   = network_addr(ip, netmask);
	broadcast = broadcast_addr(ip, netmask);
	total     = total_hosts(prefix);
	usable    = usable_hosts(prefix);

	printf("IP Class: %c\n", ip_class(ip));
	printf("Network: "); print_ip(network); printf("\n");
	printf("Broadcast: "); print_ip(broadcast); printf("\n");
	printf("Total hosts: %u\n", total);
	printf("Usable hosts: %u\n", usable);

	if (usable > 0) {
		printf("Usable IP range: ");
		print_ip(network + 1);
		printf(" - ");
		print_ip(broadcast - 1);
		printf("\n");
	} else {
		printf("No usable IPs in this subnet\n");
	}

	return 0;
}