lib/rational.c

   1 /*
   2  * rational fractions
   3  *
   4  * Copyright (C) 2009 emlix GmbH, Oskar Schirmer <oskar@scara.com>
   5  *
   6  * helper functions when coping with rational numbers
   7  */
   8
   9 #include <linux/rational.h>
  10 #include <linux/compiler.h>
  11 #include <linux/export.h>
  12
  13 /*
  14  * calculate best rational approximation for a given fraction
  15  * taking into account restricted register size, e.g. to find
  16  * appropriate values for a pll with 5 bit denominator and
  17  * 8 bit numerator register fields, trying to set up with a
  18  * frequency ratio of 3.1415, one would say:
  19  *
  20  * rational_best_approximation(31415, 10000,
  21  *              (1 << 8) - 1, (1 << 5) - 1, &n, &d);
  22  *
  23  * you may look at given_numerator as a fixed point number,
  24  * with the fractional part size described in given_denominator.
  25  *
  26  * for theoretical background, see:
  27  * http://en.wikipedia.org/wiki/Continued_fraction
  28  */
  29
  30 void rational_best_approximation(
  31         unsigned long given_numerator, unsigned long given_denominator,
  32         unsigned long max_numerator, unsigned long max_denominator,
  33         unsigned long *best_numerator, unsigned long *best_denominator)
  34 {
  35         unsigned long n, d, n0, d0, n1, d1;
  36         n = given_numerator;
  37         d = given_denominator;
  38         n0 = d1 = 0;
  39         n1 = d0 = 1;
  40         for (;;) {
  41                 unsigned long t, a;
  42                 if ((n1 > max_numerator) || (d1 > max_denominator)) {
  43                         n1 = n0;
  44                         d1 = d0;
  45                         break;
  46                 }
  47                 if (d == 0)
  48                         break;
  49                 t = d;
  50                 a = n / d;
  51                 d = n % d;
  52                 n = t;
  53                 t = n0 + a * n1;
  54                 n0 = n1;
  55                 n1 = t;
  56                 t = d0 + a * d1;
  57                 d0 = d1;
  58                 d1 = t;
  59         }
  60         *best_numerator = n1;
  61         *best_denominator = d1;
  62 }
  63
  64 EXPORT_SYMBOL(rational_best_approximation);