mantissa.c

wifi 无线网络路由协议OLSR linux下C代码

/*
 * The olsr.org Optimized Link-State Routing daemon(olsrd)
 * Copyright (c) 2004, Andreas T鴑nesen([e-mail])
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without 
 * modification, are permitted provided that the following conditions 
 * are met:
 *
 * * Redistributions of source code must retain the above copyright 
 *   notice, this list of conditions and the following disclaimer.
 * * Redistributions in binary form must reproduce the above copyright 
 *   notice, this list of conditions and the following disclaimer in 
 *   the documentation and/or other materials provided with the 
 *   distribution.
 * * Neither the name of olsr.org, olsrd nor the names of its 
 *   contributors may be used to endorse or promote products derived 
 *   from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS 
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE 
 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, 
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, 
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; 
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER 
 * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN 
 * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 
 * POSSIBILITY OF SUCH DAMAGE.
 *
 * Visit http://www.olsr.org for more information.
 *
 * If you find this software useful feel free to make a donation
 * to the project. For more information see the website or contact
 * the copyright holders.
 *
 * $Id: mantissa.c,v 1.9 2007/08/28 20:45:17 bernd67 Exp $
 */

#include "mantissa.h"

/**
 *Function that converts a double to a mantissa/exponent
 *product as described in RFC3626:
 *
 * value = C*(1+a/16)*2^b [in seconds]
 *
 *  where a is the integer represented by the four highest bits of the
 *  field and b the integer represented by the four lowest bits of the
 *  field.
 *
 *@param interval the time interval to process
 *
 *@return a 8-bit mantissa/exponent product
 */

olsr_u8_t
double_to_me(const double interval)
{
    olsr_u8_t a, b;

    /* It is sufficent to compare the integer part since we test on >=.
     * So we have now only a floating point division and the rest of the loop
     * are only integer operations.
     */
    const unsigned int unscaled_interval = interval / VTIME_SCALE_FACTOR;
    b = 0;
    while (unscaled_interval >= (1U << b)) {
        b++;
    }

    if(b == 0) {
        a = 1;
        b = 0;
    } else {
        b--;
        if (b > 15) {
            a = 15;
            b = 15;
        } else {
            /* And again some maths simplification from the former version:
             *    a = 16 * ((double)interval / (VTIME_SCALE_FACTOR * (double)(1 << b)) - 1)
             * Since interval is already double:
             *    a = 16 * (interval / (VTIME_SCALE_FACTOR * (double)(1 << b)) - 1)
             * first, we can get rid of parentheses and change the * to a /
             *    a = 16 * (interval / VTIME_SCALE_FACTOR / (double)(1 << b) - 1)
             * then we make an integer addition from the floating point addition
             *    a = (int)(16.0 * interval / VTIME_SCALE_FACTOR / (double)(1 << b)) - 16
             * and we loose an unnecessary cast
             *    a = (int)(16.0 * interval / VTIME_SCALE_FACTOR / (1 << b)) - 16
             */
            a = (int)(16.0 * interval / VTIME_SCALE_FACTOR / (1 << b)) - 16;
            b += a >> 4;
            a &= 0x0f;
        }
    }
    //printf("Generated mantissa/exponent(%d/%d): %d from %f\n", a, b, (olsr_u8_t) (a*16+b), interval);  //printf("Resolves back to: %f\n", me_to_double(((a<<4)|(b&0x0F))));
    return (a << 4) | (b & 0x0F);
}

/**
 * Function for converting a mantissa/exponent 8bit value back
 * to double as described in RFC3626:
 *
 * value = C*(1+a/16)*2^b [in seconds]
 *
 *  where a is the integer represented by the four highest bits of the
 *  field and b the integer represented by the four lowest bits of the
 *  field.
 *
 * me is the 8 bit mantissa/exponent value
 *
 * To avoid expensive floating maths, we transform the equation:
 *     value = C * (1 + a / 16) * 2^b
 * first, we make an int addition from the floating point addition:
 *     value = C * ((16 + a) / 16) * 2^b
 * then we get rid of a pair of parentheses
 *     value = C * (16 + a) / 16 * 2^b
 * and now we make an int multiplication from the floating point one
 *     value = C * (16 + a) * 2^b / 16
 * so that we can make a shift from the multiplication
 *     value = C * ((16 + a) << b) / 16
 * and sionce C and 16 are constants
 *     value = ((16 + a) << b) * C / 16
 */
double me_to_double(const olsr_u8_t me)
{
    const olsr_u8_t a = me >> 4;
    const olsr_u8_t b = me & 0x0F;
    return ((16 + a) << b) * VTIME_SCALE_FACTOR / 16.0;
}