#include<sys/socket.h>
#include<sys/types.h>
#include<netinet/ip.h>
#include<arpa/inet.h>
#include<stdio.h>
#include<unistd.h>
#include<string.h>

int main(){
   struct sockaddr_in sad;
   sad.sin_port = htons(5141);
   sad.sin_addr.s_addr = INADDR_ANY;
   sad.sin_family = AF_INET;

   int skt = socket(AF_INET, SOCK_DGRAM, 0);
   bind(skt, (struct sockaddr *)(&sad), sizeof(struct sockaddr_in));
	// No listen.  We're using UDP
	char buffer[20];
	long int a, b;
	struct sockaddr_in cad;
	socklen_t cad_size = sizeof(struct sockaddr_in);
	for(;;){
		recvfrom(skt, buffer, 20, 0, (struct sockaddr*)&cad, &cad_size);
		a = *(long int*)(buffer+4);
		b = *(long int*)(buffer+12);
		buffer[4] = 0;
		printf("Message from %s was %s, arguments %ld and %ld\n", inet_ntoa(cad.sin_addr), buffer, a, b);
		if(!strncmp(buffer, "DIVI", 4))
			a = a / b;
		else if (!strncmp(buffer, "MODU", 4))
			a = a / b;
		else {
			strcpy(buffer, "LAZY");
			a = 1;
			memcpy(buffer+4, &a, 8);
			sendto(skt, buffer, 12, 0, (struct sockaddr*)&cad, cad_size);
			printf("Sending LAZY\n");
			continue;
		}	
		strcpy(buffer, "RESL");
		memcpy(buffer+4, &a, 8);
		/* This next line assumes that the client has not sent another request
		 * If they did, and a request or an answer got lost, they could have the wrong answer!
		 * Problem scenario:
		 * 	Client sends request D
		 * 	Request D gets lost
		 * 	Client sends request E
		 * 	We get request E and reply
		 * 	The client assumes our answer to E is the answer to E
		 * 	The client assumes we never answered E, and has the wrong answer to D
		 *
		 * One way to fix it:  Include the original question with the answer!
		 * 	DNS does this
		 * 	Will make the message bigger
		 * Another way:  Include a token (a request number) 
		 * 	This works, but we'll have to track the token
		 * What if somebody steals the token!
		 * 	Then they can pretend to be us
		 */
		sendto(skt, buffer, 12, 0, (struct sockaddr*)&cad, cad_size);
		printf("Sent RESL\n");
	}
}