/*
 * Copyright (C) 2009-2012 Free Software Foundation, Inc.
 *
 * Author: Jonathan Bastien-Filiatrault
 *
 * This file is part of GNUTLS.
 *
 * The GNUTLS library 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; either version 2.1 of
 * the License, or (at your option) any later version.
 *
 * This library 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, see <http://www.gnu.org/licenses/>
 *
 */

#include "gnutls_mbuffers.h"
#include "gnutls_errors.h"

/* Here be mbuffers */

/* A note on terminology:
 *
 * Variables named bufel designate a single buffer segment (mbuffer_st
 * type). This type is textually referred to as a "segment" or a
 * "buffer element".
 *
 * Variables named buf desigate a chain of buffer segments
 * (mbuffer_head_st type).  This type is textually referred to as a
 * "buffer head" or simply as "buffer".
 *
 * Design objectives:
 *
 * - Make existing code easier to understand.
 * - Make common operations more efficient by avoiding unnecessary
 *    copying.
 * - Provide a common datatype with a well-known interface to move
 *    data around and through the multiple protocol layers.
 * - Enable a future implementation of DTLS, which needs the concept
 *    of record boundaries.
 */


/* Initialize a buffer head.
 *
 * Cost: O(1)
 */
void _mbuffer_head_init(mbuffer_head_st * buf)
{
	buf->head = NULL;
	buf->tail = NULL;

	buf->length = 0;
	buf->byte_length = 0;
}

/* Deallocate all buffer segments and reset the buffer head.
 *
 * Cost: O(n)
 * n: Number of segments currently in the buffer.
 */
void _mbuffer_head_clear(mbuffer_head_st * buf)
{
	mbuffer_st *bufel, *next;

	for (bufel = buf->head; bufel != NULL; bufel = next) {
		next = bufel->next;
		gnutls_free(bufel);
	}

	_mbuffer_head_init(buf);
}

/* Append a segment to the end of this buffer.
 *
 * Cost: O(1)
 */
void _mbuffer_enqueue(mbuffer_head_st * buf, mbuffer_st * bufel)
{
	bufel->next = NULL;
	bufel->prev = buf->tail;

	buf->length++;
	buf->byte_length += bufel->msg.size - bufel->mark;

	if (buf->tail != NULL)
		buf->tail->next = bufel;
	else
		buf->head = bufel;
	buf->tail = bufel;
}

/* Remove a segment from the buffer.
 *
 * Cost: O(1)
 *
 * Returns the buffer following it.
 */
mbuffer_st *_mbuffer_dequeue(mbuffer_head_st * buf, mbuffer_st * bufel)
{
	mbuffer_st *ret = bufel->next;

	if (buf->tail == bufel)	/* if last */
		buf->tail = bufel->prev;

	if (buf->head == bufel)	/* if first */
		buf->head = bufel->next;

	if (bufel->prev)
		bufel->prev->next = bufel->next;

	if (bufel->next)
		bufel->next->prev = NULL;

	buf->length--;
	buf->byte_length -= bufel->msg.size - bufel->mark;

	bufel->next = bufel->prev = NULL;

	return ret;
}

/* Get a reference to the first segment of the buffer and
 * remove it from the list.
 *
 * Used to start iteration.
 *
 * Cost: O(1)
 */
mbuffer_st *_mbuffer_head_pop_first(mbuffer_head_st * buf)
{
	mbuffer_st *bufel = buf->head;

	if (buf->head == NULL)
		return NULL;

	_mbuffer_dequeue(buf, bufel);

	return bufel;
}

/* Get a reference to the first segment of the buffer and its data.
 *
 * Used to start iteration or to peek at the data.
 *
 * Cost: O(1)
 */
mbuffer_st *_mbuffer_head_get_first(mbuffer_head_st * buf,
				    gnutls_datum_t * msg)
{
	mbuffer_st *bufel = buf->head;

	if (msg) {
		if (bufel) {
			msg->data = bufel->msg.data + bufel->mark;
			msg->size = bufel->msg.size - bufel->mark;
		} else {
			msg->data = NULL;
			msg->size = 0;
		}
	}
	return bufel;
}

/* Get a reference to the next segment of the buffer and its data.
 *
 * Used to iterate over the buffer segments.
 *
 * Cost: O(1)
 */
mbuffer_st *_mbuffer_head_get_next(mbuffer_st * cur, gnutls_datum_t * msg)
{
	mbuffer_st *bufel = cur->next;

	if (msg) {
		if (bufel) {
			msg->data = bufel->msg.data + bufel->mark;
			msg->size = bufel->msg.size - bufel->mark;
		} else {
			msg->data = NULL;
			msg->size = 0;
		}
	}
	return bufel;
}

/* Remove the first segment from the buffer.
 *
 * Used to dequeue data from the buffer. Not yet exposed in the
 * internal interface since it is not yet needed outside of this unit.
 *
 * Cost: O(1)
 */
static inline void remove_front(mbuffer_head_st * buf)
{
	mbuffer_st *bufel = buf->head;

	if (!bufel)
		return;

	_mbuffer_dequeue(buf, bufel);
	gnutls_free(bufel);
}

/* Remove a specified number of bytes from the start of the buffer.
 *
 * Useful for uses that treat the buffer as a simple array of bytes.
 *
 * If more than one mbuffer_st have been removed it
 * returns 1, 0 otherwise and an error code on error.
 *
 * Cost: O(n)
 * n: Number of segments needed to remove the specified amount of data.
 */
int _mbuffer_head_remove_bytes(mbuffer_head_st * buf, size_t bytes)
{
	size_t left = bytes;
	mbuffer_st *bufel, *next;
	int ret = 0;

	if (bytes > buf->byte_length) {
		gnutls_assert();
		return GNUTLS_E_INVALID_REQUEST;
	}

	for (bufel = buf->head; bufel != NULL && left > 0; bufel = next) {
		next = bufel->next;

		if (left >= (bufel->msg.size - bufel->mark)) {
			left -= (bufel->msg.size - bufel->mark);
			remove_front(buf);
			ret = 1;
		} else {
			bufel->mark += left;
			buf->byte_length -= left;
			left = 0;
		}
	}
	return ret;
}

/* Allocate a buffer segment. The segment is not initially "owned" by
 * any buffer.
 *
 * maximum_size: Amount of data that this segment can contain.
 * size: Amount of useful data that is contained in this
 *  buffer. Generally 0, but this is a shortcut when a fixed amount of
 *  data will immediately be added to this segment.
 *
 * Returns the segment or NULL on error.
 *
 * Cost: O(1)
 */
mbuffer_st *_mbuffer_alloc(size_t payload_size, size_t maximum_size)
{
	mbuffer_st *st;

	st = gnutls_malloc(maximum_size + sizeof(mbuffer_st));
	if (st == NULL) {
		gnutls_assert();
		return NULL;
	}

	/* set the structure to zero */
	memset(st, 0, sizeof(*st));

	/* payload points after the mbuffer_st structure */
	st->msg.data = (uint8_t *) st + sizeof(mbuffer_st);
	st->msg.size = payload_size;
	st->maximum_size = maximum_size;

	return st;
}

/* Copy data into a segment. The segment must not be part of a buffer
 * head when using this function.
 *
 * Bounds checking is performed by this function.
 *
 * Returns 0 on success or an error code otherwise.
 *
 * Cost: O(n)
 * n: number of bytes to copy
 */
int
_mbuffer_append_data(mbuffer_st * bufel, void *newdata,
		     size_t newdata_size)
{
	if (bufel->msg.size + newdata_size <= bufel->maximum_size) {
		memcpy(&bufel->msg.data[bufel->msg.size], newdata,
		       newdata_size);
		bufel->msg.size += newdata_size;
	} else {
		gnutls_assert();
		return GNUTLS_E_INVALID_REQUEST;
	}

	return 0;
}

/* Takes a buffer in multiple chunks and puts all the data in a single
 * contiguous segment.
 *
 * Returns 0 on success or an error code otherwise.
 *
 * Cost: O(n)
 * n: number of segments initially in the buffer
 */
int _mbuffer_linearize(mbuffer_head_st * buf)
{
	mbuffer_st *bufel, *cur;
	gnutls_datum_t msg;
	size_t pos = 0;

	if (buf->length <= 1)
		/* Nothing to do */
		return 0;

	bufel = _mbuffer_alloc(buf->byte_length, buf->byte_length);
	if (!bufel) {
		gnutls_assert();
		return GNUTLS_E_MEMORY_ERROR;
	}

	for (cur = _mbuffer_head_get_first(buf, &msg);
	     msg.data != NULL; cur = _mbuffer_head_get_next(cur, &msg)) {
		memcpy(&bufel->msg.data[pos], msg.data, msg.size);
		pos += msg.size;
	}

	_mbuffer_head_clear(buf);
	_mbuffer_enqueue(buf, bufel);

	return 0;
}