workbench/libs/mathtrans/splog.c

   1 /*
   2     Copyright © 1995-2008, The AROS Development Team. All rights reserved.
   3     $Id$
   4 */
   5
   6 #include "mathtrans_intern.h"
   7
   8 /*
   9     FUNCTION
  10       Calculate logarithm (base e) of the given ffp number
  11
  12     RESULT
  13       ffp-number
  14
  15       flags:
  16         zero     : result is zero
  17         negative : result is negative
  18         overflow : argument was negative
  19
  20     NOTES
  21
  22     EXAMPLE
  23
  24     BUGS
  25
  26     SEE ALSO
  27
  28     INTERNALS
  29       ALGORITHM:
  30
  31       If the Argument is negative set overflow-flag and return 0.
  32       If the Argument is 0 return 0xffffffff.
  33
  34       All other cases:
  35
  36       (ld is the logarithm with base 2)
  37       (ln is the logarithm with base e)
  38      <code>
  39       fnum1 = M * 2^E
  40
  41       ln fnum1 = ln ( M * 2^E ) =
  42
  43                = ln M + ln 2^E =
  44
  45                = ln M + E * ln (2) =
  46
  47                  ld M        ld 2
  48                = ----- + E * ----- =      [ld 2 = 1]
  49                  ld e        ld e
  50
  51                  ld M + E
  52                = --------
  53                  ld e
  54      </code>
  55
  56       ld e can be precalculated, of course.
  57       For calculating ld M see file intern_spld.c
  58
  59     HISTORY
  60 */
  61
  62 AROS_LH1(float, SPLog,
  63     AROS_LHA(float, fnum1, D0),
  64     struct Library *, MathTransBase, 14, MathTrans
  65 )
  66 {
  67     AROS_LIBFUNC_INIT
  68
  69     LONG ld_M, Exponent, Mask = 0x40, i, Sign;
  70
  71     /* check for negative sign */
  72     if ((char) fnum1 < 0)
  73     {
  74         SetSR(Overflow_Bit, Zero_Bit | Negative_Bit | Overflow_Bit);
  75         return 0;
  76     }
  77
  78     /* check for argument == 0 -> return (-infinity) */
  79     if (fnum1 == 0) return FFP_Ninfty;
  80
  81     /* convert the Exponent of the argument (fnum1) to the ffp-format */
  82     Exponent = (fnum1 & FFPExponent_Mask) - 0x40;
  83     if (Exponent < 0 )
  84     {
  85         Exponent =-Exponent;
  86         Sign = FFPSign_Mask;
  87     }
  88     else
  89     {
  90         Sign = 0;
  91     }
  92
  93     /* find the number of the highest set bit in the exponent */
  94     if (Exponent != 0)
  95     {
  96         i = 0;
  97         while ( (Mask & Exponent) == 0)
  98         {
  99             i ++;
 100             Mask >>= 1;
 101         }
 102         Exponent <<= (25 + i);
 103         Exponent |= (0x47 - i + Sign);
 104     }
 105
 106     ld_M = intern_SPLd((fnum1 & FFPMantisse_Mask) | 0x40);
 107
 108     /*               ld M + E
 109     ** ln(fnum1) =  --------
 110     **                 ld e
 111     */
 112
 113     return SPMul( SPAdd(ld_M, Exponent), InvLde);
 114
 115     AROS_LIBFUNC_EXIT
 116 }