/*
* The olsr.org Optimized Link-State Routing daemon(olsrd)
* Copyright (c) 2004, Andreas Tonnesen(andreto@olsr.org)
* 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.
*
*/
#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
*/
uint8_t
reltime_to_me(const olsr_reltime interval)
{
uint8_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;
*
* VTIME_SCALE_FACTOR = 1/16
*
* => unscaled_interval = interval(ms) / 1000 * 16
* = interval(ms) / 125 * 2
*/
const unsigned int unscaled_interval = interval / 125 * 2;
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
*
* VTIME_SCALE_FACTOR = 1/16
*
* => a = (16 * interval(ms) / 1000 * 16 / (1 << b)) - 16
* = (interval(ms) * 256 / 1000 / (1 << b)) - 16
* = (interval(ms) * 32 / 125 / (1<**= 5
* = (interval(ms) - (125 << (b-1))) / 125 / (1 << (b-5))
* = (interval(ms) - (125 << (b-1))) / (125 << (b-5))
*
* 2. case: b <= 5
* = (interval(ms) - (125 << (b-1))) / 125 * (1 << (5-b))
* = (interval(ms) - (125 << (b-1))) * (1 << (5-b)) / 125
*/
if (b >= 5) {
a = (interval - (125 << (b - 1))) / (125 << (b - 5));
} else {
a = (interval - (125 << (b - 1))) * (1 << (5 - b)) / 125;
}
b += a >> 4;
a &= 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
*
* VTIME_SCALE_FACTOR = 1/16
*
* => value(ms) = ((16 + a) << b) / 256 * 1000
*
* 1. case: b >= 8
* = ((16 + a) << (b-8)) * 1000
*
* 2. case: b <= 8
* = ((16 + a) * 1000) >> (8-b)
*/
olsr_reltime
me_to_reltime(const uint8_t me)
{
const uint8_t a = me >> 4;
const uint8_t b = me & 0x0F;
if (b >= 8) {
return ((16 + a) << (b - 8)) * 1000;
}
return ((16 + a) * 1000) >> (8 - b);
}
/*
* Local Variables:
* c-basic-offset: 2
* indent-tabs-mode: nil
* End:
*/
**