workbench/libs/mathffp/spfloor.c

   1 /*
   2     Copyright © 1995-2004, The AROS Development Team. All rights reserved.
   3     $Id$
   4 */
   5
   6 #include "mathffp_intern.h"
   7
   8 /*****************************************************************************
   9
  10     NAME */
  11
  12         AROS_LH1(float, SPFloor,
  13
  14 /*  SYNOPSIS */
  15         AROS_LHA(float, y, D0),
  16
  17 /*  LOCATION */
  18         struct LibHeader *, MathBase, 15, Mathffp)
  19
  20 /*  FUNCTION
  21         Calculate the largest integer ffp-number
  22         less than or equal to fnum
  23
  24     INPUTS
  25
  26     RESULT
  27         FFP number
  28
  29         Flags:
  30           zero     : result is zero
  31           negative : result is negative
  32           overflow : 0 (???)
  33
  34     EXAMPLE
  35        floor(10.5) = 10
  36        floor(0.5)  = 0
  37        floor(-0.5) = -1
  38        floor(-10.5)= -11
  39
  40     BUGS
  41
  42     INTERNALS
  43         ALGORITHM:
  44         The integer part of a ffp number are the left "exponent"-bits
  45         of the mantisse!
  46         Therefore:
  47         Test the exponent for <= 0. This has to be done separately!
  48         If the sign is negative then return -1 otherwise return 0.
  49
  50         Generate a mask of exponent(y) (where y is the given ffp-number)
  51         bits starting with bit 31.
  52         If y < 0 then test whether it is already an integer. If not
  53         then y = y - 1 and generate that mask again. Use the
  54         mask on the mantisse.
  55
  56 *****************************************************************************/
  57 {
  58     AROS_LIBFUNC_INIT
  59
  60     LONG Mask = 0x80000000;
  61
  62     if (((char)y & FFPExponent_Mask)  <= 0x40)
  63     {
  64         if ((char)y < 0)
  65         {
  66             SetSR(Negative_Bit, Zero_Bit | Negative_Bit | Overflow_Bit);
  67             return 0x800000C1; /* -1 */
  68         }
  69         else
  70         {
  71             SetSR(Zero_Bit, Zero_Bit | Negative_Bit | Overflow_Bit);
  72             return 0;
  73         }
  74     }
  75
  76     /* |fnum| >= 1 */
  77     Mask >>= ( ((char) y & FFPExponent_Mask) - 0x41);
  78     Mask |= FFPSign_Mask | FFPExponent_Mask;
  79
  80     /* fnum is negative */
  81     if ((char) y < 0)
  82     {
  83         /* is there anything behind the dot? */
  84         if (0 != (y & (~Mask)) )
  85         {
  86             Mask = 0x80000000;
  87             y    = SPAdd(y, 0x800000c1); /* y = y -1; */
  88             Mask >>= ((char) y & FFPExponent_Mask) - 0x41;
  89             Mask |= FFPSign_Mask | FFPExponent_Mask;
  90         }
  91     }
  92
  93     if((char) y < 0)
  94     {
  95         SetSR(Negative_Bit, Zero_Bit | Negative_Bit | Overflow_Bit);
  96     }
  97
  98     return y & Mask;
  99
 100     AROS_LIBFUNC_EXIT
 101 }