/*
* The olsr.org Optimized Link-State Routing daemon(olsrd)
* Copyright (c) 2004, Andreas Tønnesen(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
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*
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*
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* 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;
}