interval.h

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/*
 * Copyright 2011,2012,2013 Didier Barvaux
 *
 * This 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 library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
 */

/**
 * @file interval.h
 * @brief Compute the interpretation interval for LSB and W-LSB encoding
 * @author Didier Barvaux <didier.barvaux@toulouse.viveris.com>
 */

#ifndef ROHC_COMMON_INTERVAL_H
#define ROHC_COMMON_INTERVAL_H

#include "dllexport.h"

#include <stdlib.h>
#include <stdint.h>


/**
 * @brief the different values of the shift parameter of the LSB algorithm
 *
 * The shift parameter is also named 'p' in some RFCs.
 *
 * Some values are the real values to use directly. Some others are code
 * that means that the real value to use shall be computed from the number
 * of least significant bits that are transmitted.
 */
typedef enum
{
        ROHC_LSB_SHIFT_IP_ID  =  0,  /**< real value for IP-ID */
        ROHC_LSB_SHIFT_RTP_TS =  2,  /**< need to compute real value for RTP TS */
        ROHC_LSB_SHIFT_RTP_SN =  3,  /**< need to compute real value for RTP SN */
        ROHC_LSB_SHIFT_ESP_SN =  3,  /**< need to compute real value for ESP SN */
        ROHC_LSB_SHIFT_SN     = -1,  /**< real value for non-RTP SN */
        ROHC_LSB_SHIFT_VAR    =  1,  /**< real value is variable */
} rohc_lsb_shift_t;


/**
 * @brief An interval of 16-bit values
 *
 * Lower and upper bound values are always included in the interval.
 *
 * The upper bound may be greater that the lower bound of the interval if the
 * interval straddles the interval boundaries.
 *
 * Example of interval that does not straddle field boundaries:
 *   [1, 3]
 *
 * Example of interval that straddles field boundaries (16-bit field):
 *   [65530, 4]
 */
struct rohc_interval16
{
        uint16_t min;  /**< The lower bound of the interval */
        uint16_t max;  /**< The upper bound of the interval */
};


/**
 * @brief An interval of 32-bit values
 *
 * Lower and upper bound values are always included in the interval.
 *
 * The upper bound may be greater that the lower bound of the interval if the
 * interval straddles the interval boundaries.
 *
 * Example of interval that does not straddle field boundaries:
 *   [1, 3]
 *
 * Example of interval that straddles field boundaries (32-bit field):
 *   [65530, 4]
 */
struct rohc_interval32
{
        uint32_t min;  /**< The lower bound of the interval */
        uint32_t max;  /**< The upper bound of the interval */
};


/*
 * Public function prototypes:
 */

static inline int32_t rohc_interval_compute_p(const size_t k,
                                              const rohc_lsb_shift_t p)
        __attribute__((warn_unused_result, const));

struct rohc_interval16 ROHC_EXPORT rohc_f_16bits(const uint16_t v_ref,
                                                 const size_t k,
                                                 const rohc_lsb_shift_t p)
        __attribute__((warn_unused_result, const));

struct rohc_interval32 ROHC_EXPORT rohc_f_32bits(const uint32_t v_ref,
                                                 const size_t k,
                                                 const rohc_lsb_shift_t p)
        __attribute__((warn_unused_result, const));


/**
 * @brief Compute the shift parameter p for the f function
 *
 * @param k  The number of least significant bits of the value that are
 *           transmitted
 * @param p  The shift parameter (may be negative)
 * @return   The computed shift parameter p
 */
static inline int32_t rohc_interval_compute_p(const size_t k,
                                              const rohc_lsb_shift_t p)
{
        int32_t computed_p;

        /* determine the real p value to use */
        switch(p)
        {
                case ROHC_LSB_SHIFT_RTP_TS: /* special computation for RTP TS encoding */
                {
                        if(k <= 2)
                        {
                                computed_p = 0;
                        }
                        else
                        {
                                computed_p = (1 << (k - 2)) - 1;
                        }
                }
                break;

                /* special computation for RTP and ESP SN encoding */
                case ROHC_LSB_SHIFT_RTP_SN: /* = ROHC_LSB_SHIFT_ESP_SN */
                {
                        if(k <= 4)
                        {
                                computed_p = 1;
                        }
                        else
                        {
                                computed_p = (1 << (k - 5)) - 1;
                        }
                }
                break;

                default: /* otherwise: use the p value given as parameter */
                {
                        computed_p = p;
                }
        }

        return computed_p;
}

#endif