#include <arpa/inet.h>
#include <assert.h>
#include <err.h>
#include <errno.h>
#include <inttypes.h>
#include <linux/if_packet.h>
#include <net/ethernet.h>
#include <net/if.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>

uint32_t crc32_table[256];

void precompute_crc32_table(void) {
	for (size_t index = 0; index < 256; index++) {
		uint32_t value = index;

		for (size_t i = 0; i < 8; i++) {
			uint32_t low_bit = value & 1;
			value = value >> 1;
			if (low_bit) {
				value ^= 0xEDB88320;
			}
		}

		crc32_table[index] = value;
	}
}

uint32_t crc32(unsigned char *buf, size_t len) {
	uint32_t value = 0xffffffff;

	for (size_t index = 0; index < len; index++) {
		value = crc32_table[(value & 0xff) ^ buf[index]] ^ (value >> 8);
	}

	return value ^ 0xffffffff;
}

char hexify(int nybble) {
	assert(0 <= nybble && nybble <= 16);
	return "0123456789abcdef"[nybble];
}

void format_mac(const unsigned char binary_address[6], char formatted[18]) {
	for (size_t i = 0; i < 6; i++) {
		unsigned char byte = binary_address[i];
		formatted[3*i] = hexify(byte >> 4);
		formatted[3*i + 1] = hexify(byte & 0xf);
		formatted[3*i + 2] = ':';
	}
	formatted[17] = '\0';
}

int main(int argc, char **argv) {
	if (argc != 2) {
		fprintf(stderr, "Usage: %s interface\n", argv[0]);
		exit(1);
	}

	// Set up CRC32
	precompute_crc32_table();

	// Create a packet socket
	errno = 0;
	int packet_socket = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
	if (packet_socket == -1) {
		err(1, "socket");
	}

	// Find the index of the network interface
	struct ifreq ifr;
	strncpy(ifr.ifr_name, argv[1], IFNAMSIZ);
	errno = 0;
	if (ioctl(packet_socket, SIOCGIFINDEX, &ifr) == -1) {
		err(1, "ioctl");
	}

	// Bind to the network interface
	struct sockaddr_ll sll;
	sll.sll_family = AF_PACKET;
	sll.sll_protocol = htons(ETH_P_ALL);
	sll.sll_ifindex = ifr.ifr_ifindex;
	errno = 0;
	if (bind(packet_socket, (const struct sockaddr*)&sll, sizeof(sll)) == -1) {
		err(1, "bind");
	}

	for(;;) {
		unsigned char frame[1522]; // 1522 is the largest a 802.3 frame can be

		if (fflush(stdout) == -1) {
			err(1, "fflush");
		}

		errno = 0;
		ssize_t frame_length = recv(packet_socket, frame, sizeof(frame), MSG_TRUNC);
		if (frame_length == -1) {
			err(1, "recv");
		}
		size_t frame_data_length = frame_length <= 1522 ? frame_length : 1522;

		if (frame_length < 64) {
			errno = 0;
			if (printf("Runt frame (%zdB)\n", frame_length) == -1) {
				err(1, "printf");
			}
			continue;
		}

		// Compute the CRC32 of the frame
		uint32_t crc32_residue = crc32(frame, frame_data_length);

		if (crc32_residue != 0xc704dd7b) {
			errno = 0;
			if (printf("CRC failed (%04"PRIx32")\n", crc32_residue) == -1) {
				err(1, "printf");
			}
			continue;
		}

		// Extract the MACs
		// 012345 012345
		// dest   source
		char destination_mac[18], source_mac[18];
		format_mac(&frame[0], destination_mac);
		format_mac(&frame[6], source_mac);

		// Extract EtherType / length field
		// 012345 012345 01
		// dest   source ^^
		// It is stored in the network byte order, that is, high byte 1st
		uint16_t ethertype = (frame[12] << 8) | frame[13];

		const char *ethertype_meaning = NULL;
		if (ethertype <= 1500) {
			ethertype_meaning = "packet length";
		} else if (ethertype < 0x0600) {
			ethertype_meaning = "undefined";
		} else if (ethertype == 0x0800) {
			ethertype_meaning = "IPv4";
		} else if (ethertype == 0x0806) {
			ethertype_meaning = "ARP";
		} else if (ethertype == 0x86dd) {
			ethertype_meaning = "IPv6";
		}

		errno = 0;
		if(printf("%s -> %s ethertype: %04" PRIx16, source_mac, destination_mac, ethertype) == -1) {
			err(1, "printf");
		}

		errno = 0;
		if (ethertype_meaning == NULL) {
			if (printf(", length: %zd\n", frame_length) == -1) {
				err(1, "printf");
			}
		} else {
			if (printf(" (%s), length: %zd\n", ethertype_meaning, frame_length) == -1) {
				err(1, "printf");
			}
		}
	}

	return 0;
}