/* this is a UDP receiver */

#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/inet.h>

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

#define DATA_FRAME 1
#define ACK_FRAME 2
#define PACKETSIZE 10000

struct frame {
	int seq;
	int type;
	int size;
	char buf[PACKETSIZE];
} myframe;

int main(int argc, char *argv[])
{
	int sockfd;
	socklen_t len;
	struct sockaddr_in servaddr;
	struct sockaddr_in cliaddr;
	// int count = 0;
	FILE *fd;
	int last_seq;

	if (argc != 2) {
		printf("Usage : a.out port\n");
		exit(0);
	}

	memset(&servaddr, 0, sizeof(servaddr));
	servaddr.sin_family = AF_INET;
	servaddr.sin_port = htons((int)atoi(argv[1]));
	servaddr.sin_addr.s_addr = htonl(INADDR_ANY);

	if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
		perror(": socket");
		exit(0);
	}

	if ((bind(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr)) < 0)) {
		perror(": bind");
		exit(0);
	}

	if ((fd = fopen("outputfile", "w")) == NULL) {
		printf("can't open outputfile\n");
		exit(0);
	}

	last_seq = -1;
	while (1) {
		memset(&cliaddr, 0, sizeof(servaddr));
		len = sizeof (cliaddr);
		recvfrom(sockfd, &myframe, sizeof(myframe), 0, (struct sockaddr *)&cliaddr, &len);
		if (last_seq < myframe.seq) {
			/* good frame */
		    // note that the sender will not send any framework
		    // with seq no greater than last_seq + 1. So
		    // the condition < is OK here.
			if ((myframe.buf[0] < 'A') || (myframe.buf[0] > 'Z')) exit(0);
			if (myframe.buf[0]=='Z') fprintf(fd, "%c\n", myframe.buf[0]);
			else fprintf(fd, "%c", myframe.buf[0]);
			printf("message %c, seg = %d\n", myframe.buf[0], myframe.seq);
			last_seq = myframe.seq;

			myframe.size = 0;
			myframe.type = ACK_FRAME;
			/* myframe.seq is acked */
			sendto(sockfd, &myframe, sizeof(myframe)-PACKETSIZE+myframe.size, 
					0, (struct sockaddr *)&cliaddr, len);
		} else {
		    // receive some old packets
			/* must resend, otherwise the sender may keep resending a same packet */
			myframe.size = 0;
			myframe.type = ACK_FRAME;
			// we need to update it the last_seq to inform
			// the sender that which packet we have
			// received.
			myframe.seq = last_seq;    
			/* myframe.seq is acked */
			sendto(sockfd, &myframe, sizeof(myframe)-PACKETSIZE+myframe.size, 
					0, (struct sockaddr *)&cliaddr, len);
		}
	}
}