/*
 * Usurpataion --- client-server protocol interface.
 *
 * Copyright (C) 2019 Ramūnas Mažeikis
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; version 2.1
 * of the License.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA
 */

#ifndef USURPATION_PROTOCOL_H_INCLUDED
#define USURPATION_PROTOCOL_H_INCLUDED

#include <errno.h>

#define E_TLV_OVERFLOW		(1 << 0)
#define E_UNKNOWN_TYPE		(1 << 1)
#define END_OF_PACKET		(1 << 2)

#define TLV_SZ_MAX_RAW		(MTU - 64)

/**
 * Regular packets contain tlv's defined by tlv_type.
 *
 * Hearbeat packet tell daemon that device is still alive and listening.
 *
 * Discovery packets are used for what they say.
 */
enum packet_type {
	REGURAL,
	HEARTBEAT,
	DISCOVERY
};

/**
 * Message sequence number since beggining of sesssion.
 *
 * Mainly used for identifying lost messages.
 */
typedef unsigned int msg_idx_t;

enum tlv_type {
	/**
	 * NULL-terminated string. To be put in a queue to display on the
	 * screen.
	 */
	TEXT,

	/** Fixed point. 1 decimal digit of precision. */
	FPI1,

	/** Literally time_t */
	TIMESTAMP,

	/** Represents a request for lost message. Data is unsigned integer
	 * that uniquely identifies the message.
	 */
	REQUEST,

	/**
	 * Response to request. Begins with unsigned integer that represents
	 * which message is being repeated and the actual null-terminated
	 * message after that.
	 */
	REPLY,

	/**
	 * UUID that represents a particular device.
	 */
	UUID
};

/**
 * Packet data itself is a special type of tlv. A packet is either regular,
 * hearbeat or discovery.
 *
 * May be used to send data.
 */
struct tlv_packet {
	enum packet_type type;
	size_t size;
	size_t offset;
	char *data;	/* Bytes representing tlv's */
};

/**
 * Literally type-length-value
 * */
struct tlv {
	enum tlv_type type;
	size_t length;
	void *data;
};

/**
 * Keeps state of the parsing process.
 *
 * Related functions return one tlv at a time.
 */
struct tlv_parser {
	char *data;
	size_t offset;
	size_t size;
};

/**
 * Fills tlv structure to represent the next tlv in the packet.
 *
 * Returns END_OF_PACKET if all tlv's were read of E_TLV_OVERFLOW, if the last
 * tlv, according to its declared size should not fit in a packet.
 */
int tlv_get(struct tlv_parser *parser, struct tlv *ret);

/**
 * Appends data to the next packet to be sent. Type of data is determined by
 * enum tlv_type.
 *
 * In case of overflow return E_TLV_OVERFLOW.
 *
 * On next call after retreiving last packet returns END_OF_PACKET.
 *
 * Overflow can be detected after forming tlv header. This means that the
 * packet may have changes.
 * */
int tlv_push_data(struct tlv_packet *packet, enum tlv_type type, char *data);

/**
 * Resets offset to 0 and set entire buffer to 0.
 */
void tlv_clear_data(struct tlv_packet *packet);

/**
 * Initialises tlv to sane values.
 */
void tlv_init(struct tlv *t, enum tlv_type type);

/**
 * Frees data held in the tlv structure. The structure itself shall be freed as
 * needed by calling code.
 */
void tlv_destroy(struct tlv *t);

/**
 * Tells amount of bytes needed to push tlv to a buffer
 */
size_t tlv_raw_size(struct tlv *t);

/**
 * Pushes tlv to buffer as contiguous data. Check tlv size with tlv_raw_size
 * beforehand. If you don't do that and overflow --- tough tiddy.
 */
int tlv_get_raw(struct tlv *t, char *buf);

/**
 * Pushes data to tlv. Returns E_TLV_OVERFLOW if pushing data would cause the
 * final size to be greater than TLV_SZ_MAX_RAW. In case of such error the data is left
 * untouched.
 */
int tlv_push(struct tlv *t, const char *data, size_t size);

#endif /* USURPATION_PROTOCOL_H_INCLUDED */