/*
 * 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

#if 0

How do we use this API(?), I hear you say. Well, here is a self-contained
example complete with tedious comments, allocation of buffers and cleanup:

-------------------------------------------------------------------------------
  1 |	void functy_the_function(void)
  2 |	{
  3 |		struct tlv tlv_packet;				/* A packet made of tlv's */
  4 |		struct tlv text;				/* Actually useful data */
  5 |		char *packet;					/* Raw bytes to be pushed via UDP */
  6 |		size_t packet_size;				/* Raw packet size */
  7 |		char message[] = "Get jacked!";			/* Duh! */
  9 |
 10 |		tlv_init(&tlv_packet, REGURAL);			/* Initialise tlv_packet to regular */
 11 |		tlv_init(&text, TEXT);				/* Initialise text packet */
 12 |		tlv_push_data(&text, message, sizeof(message));	/* Push text to appropriate tlv */
 13 |		tlv_push_tlv(&tlv_packet, &text);		/* Push the tlv into tlv packet */
 14 |
 15 |		packet_size = tlv_raw_size(&tlv_packet);	/* Get raw size required to push entire tlv_packet as bytes */
 16 |		packet = (char *)malloc(packet_size);		/* Allocate buffer for outgoing packet */
 17 |		tlv_get_raw(&tlv_packet, packet);		/* Flash tlv_packet to buffer */
 19 |		udp_push(packet, packet_size);			/* Push packet via UDP */
 20 |		udp_flush();					/* Flush to the other side*/
 21 |
 22 |		tlv_destroy(&tlv_packet);			/* Free tlv_packet */
 23 |		free(packet);					/* Free packet itself */
 24 |		return;						/* GTFO and save stack space */
 25 |	}
-------------------------------------------------------------------------------

A few things to note:
	* Calling tlv_destroy() on a tlv of type REGURAL or HEARTBEAT destorys
	  all sub-tlv's too.
	* Tlv's get copies of data, which means the original buffer can be
	  freed immediately.

#endif


#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)

#ifdef __cplusplus
extern "C" {
#endif /* __cplusplus */
/**
 * 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,

/* Data of the following types are other tlv's! */

	/**
	 * Just a TLV container.
	 */
	REGURAL,

	/**
	 * Says "I'm not dead yet.".
	 */
	HEARTBEAT,

	/**
	 * Says "Bring out yer dead!".
	 */
	DISCOVERY
};

/**
 * 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);

/**
 * 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(const 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_data(struct tlv *t, const char *data, size_t size);

/**
 * Pushes a sub-tlv into the packet. 't' can only be REGURAL, HEARTBEAT or
 * DISCOVERY.
 */
int tlv_push_tlv(struct tlv *t, const struct tlv *other);

#ifdef __cplusplus
}
#endif /* __cplusplus */

#endif /* USURPATION_PROTOCOL_H_INCLUDED */