src/add-ons/accelerants/ati/mach64_util.cpp

   1 /*
   2         Haiku ATI video driver adapted from the X.org ATI driver.
   3
   4         Copyright 1997 through 2004 by Marc Aurele La France (TSI @ UQV), tsi@xfree86.org
   5
   6         Copyright 2009 Haiku, Inc.  All rights reserved.
   7         Distributed under the terms of the MIT license.
   8
   9         Authors:
  10         Gerald Zajac 2009
  11 */
  12
  13 #include "accelerant.h"
  14 #include "mach64.h"
  15
  16
  17 #define MAX_INT ((int)((unsigned int)(-1) >> 2))
  18
  19
  20
  21 void
  22 Mach64_ReduceRatio(int *numerator, int *denominator)
  23 {
  24         // Reduce a fraction by factoring out the largest common divider of the
  25         // fraction's numerator and denominator.
  26
  27         int multiplier = *numerator;
  28         int divider = *denominator;
  29         int remainder;
  30
  31         while ((remainder = multiplier % divider)) {
  32                 multiplier = divider;
  33                 divider = remainder;
  34         }
  35
  36         *numerator /= divider;
  37         *denominator /= divider;
  38 }
  39
  40
  41
  42 int
  43 Mach64_Divide(int numerator, int denominator, int shift, const int roundingKind)
  44 {
  45         // Using integer arithmetic and avoiding overflows, this function finds the
  46         // rounded integer that best approximates
  47         //
  48         //              numerator      shift
  49         //              ----------- * 2
  50         //              denominator
  51         //
  52         // using the specified rounding (floor (<0), nearest (=0) or ceiling (>0)).
  53
  54         Mach64_ReduceRatio(&numerator, &denominator);
  55
  56         // Deal with left shifts but try to keep the denominator even.
  57         if (denominator & 1) {
  58                 if (denominator <= MAX_INT) {
  59                         denominator <<= 1;
  60                         shift++;
  61                 }
  62         } else {
  63                 while ((shift > 0) && !(denominator & 3)) {
  64                         denominator >>= 1;
  65                         shift--;
  66                 }
  67         }
  68
  69         // Deal with right shifts.
  70         while (shift < 0) {
  71                 if ((numerator & 1) && (denominator <= MAX_INT))
  72                         denominator <<= 1;
  73                 else
  74                         numerator >>= 1;
  75
  76                 shift++;
  77         }
  78
  79         int rounding = 0;                                       // Default to floor
  80
  81         if (!roundingKind)                                      // nearest
  82                 rounding = denominator >> 1;
  83         else if (roundingKind > 0)                      // ceiling
  84                 rounding = denominator - 1;
  85
  86         return ((numerator / denominator) << shift) +
  87                    ((((numerator % denominator) << shift) + rounding) / denominator);
  88 }