| Oskar Schirmer | 8759ef3 | 2009-06-11 14:51:15 +0100 | [diff] [blame] | 1 | /* |
| 2 | * rational fractions |
| 3 | * |
| 4 | * Copyright (C) 2009 emlix GmbH, Oskar Schirmer <os@emlix.com> |
| 5 | * |
| 6 | * helper functions when coping with rational numbers |
| 7 | */ |
| 8 | |
| 9 | #include <linux/rational.h> |
| 10 | |
| 11 | /* |
| 12 | * calculate best rational approximation for a given fraction |
| 13 | * taking into account restricted register size, e.g. to find |
| 14 | * appropriate values for a pll with 5 bit denominator and |
| 15 | * 8 bit numerator register fields, trying to set up with a |
| 16 | * frequency ratio of 3.1415, one would say: |
| 17 | * |
| 18 | * rational_best_approximation(31415, 10000, |
| 19 | * (1 << 8) - 1, (1 << 5) - 1, &n, &d); |
| 20 | * |
| 21 | * you may look at given_numerator as a fixed point number, |
| 22 | * with the fractional part size described in given_denominator. |
| 23 | * |
| 24 | * for theoretical background, see: |
| 25 | * http://en.wikipedia.org/wiki/Continued_fraction |
| 26 | */ |
| 27 | |
| 28 | void rational_best_approximation( |
| 29 | unsigned long given_numerator, unsigned long given_denominator, |
| 30 | unsigned long max_numerator, unsigned long max_denominator, |
| 31 | unsigned long *best_numerator, unsigned long *best_denominator) |
| 32 | { |
| 33 | unsigned long n, d, n0, d0, n1, d1; |
| 34 | n = given_numerator; |
| 35 | d = given_denominator; |
| 36 | n0 = d1 = 0; |
| 37 | n1 = d0 = 1; |
| 38 | for (;;) { |
| 39 | unsigned long t, a; |
| 40 | if ((n1 > max_numerator) || (d1 > max_denominator)) { |
| 41 | n1 = n0; |
| 42 | d1 = d0; |
| 43 | break; |
| 44 | } |
| 45 | if (d == 0) |
| 46 | break; |
| 47 | t = d; |
| 48 | a = n / d; |
| 49 | d = n % d; |
| 50 | n = t; |
| 51 | t = n0 + a * n1; |
| 52 | n0 = n1; |
| 53 | n1 = t; |
| 54 | t = d0 + a * d1; |
| 55 | d0 = d1; |
| 56 | d1 = t; |
| 57 | } |
| 58 | *best_numerator = n1; |
| 59 | *best_denominator = d1; |
| 60 | } |
| 61 | |
| 62 | EXPORT_SYMBOL(rational_best_approximation); |