Read a MIDI SDS data packet

read_packet.c

static int
read_packet(midi_t midi,
            unsigned char *buf,
            unsigned int channel_num,
            unsigned int modded_packet_num,
            err_t err) {
    const int checksum_byte = 125;
    const time_t timeout_sec = 1;
    int raw_bytes_read, bytes_read;
    char strbuf[500]; strbuf[0] = '\0';
    unsigned char c;
    unsigned char checksum;

 start:

    raw_bytes_read = bytes_read = 0;

    printf("Reading Packet %d\n", modded_packet_num);

    time_t start_time, now;
    start_time = time(NULL);

    while (bytes_read < SDS_PACKET_LENGTH) {
        now = time(NULL);

        if (now - start_time > timeout_sec) {
            printf("Done!\n");
            exit(0);
        }

        if (! midi_read(midi, &c)) {
            err_set2(err, "Could not read midi.");
            return 0;
	} else {
            raw_bytes_read++;
        }

	switch(bytes_read) {
        case 0:
            if (c == 0xf0) {
                buf[bytes_read++] = c;
            }
            break;
        case 1:
            if (c == 0x7e) {
                checksum = c;
		buf[bytes_read++] = c;
            }
            break;
        case 2:
            if (c == channel_num) {
		checksum ^= c;
                buf[bytes_read++] = c;
            }
            break;
        case 3:
            if (c == 0x02) {
                checksum ^= c;
                buf[bytes_read++] = c;
            }
            break;
        case 4:
            if (c == modded_packet_num) {
	        checksum ^= c;
                buf[bytes_read++] = c;
            }
            break;
        case 125:
            buf[bytes_read++] = c;
            break;
        case 126:
            if (c == 0xf7) {
                buf[bytes_read++] = c;

                checksum &= 0x7f;
                if (checksum != buf[checksum_byte]) {
		    fprintf(stderr, "Calculated checksum = %02X. Actual checksum = %02X.\n",
                            checksum, buf[checksum_byte]);

                    if (send_response(midi, channel_num, modded_packet_num, RESPONSE_NAK, err)) {
                        // attempt to re-read sample                                                                                                                                                           
		        printf("Sent NAK...\n");
                        goto start;
                    }
                }
            }
            break;
        default:
            checksum ^= c;
            buf[bytes_read++] = c;
            break;
        }
    }

    sds_serialize_packet(strbuf, buf, bytes_read);
    printf("Total bytes read = %d\n", bytes_read);
    printf("Received Packet %s\n", strbuf);

    return bytes_read;
}