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65 lines
1.6 KiB
65 lines
1.6 KiB
// SPDX-License-Identifier: GPL-2.0 |
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/* |
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* rational fractions |
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* |
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* Copyright (C) 2009 emlix GmbH, Oskar Schirmer <oskar@scara.com> |
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* |
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* helper functions when coping with rational numbers |
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*/ |
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#include <linux/rational.h> |
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#include <linux/compiler.h> |
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#include <linux/export.h> |
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/* |
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* calculate best rational approximation for a given fraction |
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* taking into account restricted register size, e.g. to find |
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* appropriate values for a pll with 5 bit denominator and |
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* 8 bit numerator register fields, trying to set up with a |
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* frequency ratio of 3.1415, one would say: |
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* |
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* rational_best_approximation(31415, 10000, |
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* (1 << 8) - 1, (1 << 5) - 1, &n, &d); |
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* |
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* you may look at given_numerator as a fixed point number, |
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* with the fractional part size described in given_denominator. |
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* |
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* for theoretical background, see: |
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* http://en.wikipedia.org/wiki/Continued_fraction |
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*/ |
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void rational_best_approximation( |
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unsigned long given_numerator, unsigned long given_denominator, |
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unsigned long max_numerator, unsigned long max_denominator, |
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unsigned long *best_numerator, unsigned long *best_denominator) |
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{ |
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unsigned long n, d, n0, d0, n1, d1; |
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n = given_numerator; |
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d = given_denominator; |
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n0 = d1 = 0; |
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n1 = d0 = 1; |
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for (;;) { |
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unsigned long t, a; |
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if ((n1 > max_numerator) || (d1 > max_denominator)) { |
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n1 = n0; |
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d1 = d0; |
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break; |
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} |
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if (d == 0) |
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break; |
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t = d; |
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a = n / d; |
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d = n % d; |
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n = t; |
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t = n0 + a * n1; |
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n0 = n1; |
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n1 = t; |
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t = d0 + a * d1; |
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d0 = d1; |
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d1 = t; |
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} |
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*best_numerator = n1; |
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*best_denominator = d1; |
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} |
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EXPORT_SYMBOL(rational_best_approximation);
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