workbench/libs/mathtrans/splog10.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 10) 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 0xde5bd8fe.
  33
  34       All other cases:
  35
  36       (ld is the logarithm with base 2)
  37       (log is the logarithm with base 10)
  38       <code>
  39       fnum1 = M * 2^E
  40
  41       log fnum1 = log ( M * 2^E ) =
  42
  43                 = log M + log 2^E =
  44
  45                 = log M + E * log (2) =
  46
  47                   ld M        ld 2
  48                 = ----- + E * ----- =      [ld 2 = 1]
  49                   ld 10       ld 10
  50
  51                   ld M + E
  52                 = --------
  53                   ld 10
  54       </code>
  55
  56       ld 10 can be precalculated, of course.
  57       For calculating ld M see file intern_spld.c
  58
  59     HISTORY
  60 */
  61
  62 AROS_LH1(float, SPLog10,
  63     AROS_LHA(float, fnum1, D0),
  64     struct Library *, MathTransBase, 21, 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     if (0 == fnum1) return 0xde5bd8fe; /* result of the orig. library */
  79
  80     /* convert the Exponent of the argument (fnum1) to the ffp-format */
  81     Exponent = (fnum1 & FFPExponent_Mask) - 0x40;
  82     if (Exponent < 0 )
  83     {
  84         Exponent = -Exponent;
  85         Sign = FFPSign_Mask;
  86     }
  87     else
  88     {
  89         Sign = 0;
  90     }
  91
  92     /* find the number of the highest set bit in the exponent */
  93     if (Exponent != 0)
  94     {
  95         i = 0;
  96         while ( (Mask & Exponent) == 0)
  97         {
  98             i ++;
  99             Mask >>= 1;
 100         }
 101
 102         Exponent <<= (25 + i);
 103         Exponent |= (0x47 - i + Sign);
 104     }
 105
 106     ld_M = intern_SPLd((fnum1 & FFPMantisse_Mask) | 0x40);
 107
 108     /*
 109                       ld M + E
 110         log(fnum1) =  --------
 111                         ld 10
 112     */
 113
 114     return SPMul( SPAdd(ld_M, Exponent), InvLd10);
 115
 116     AROS_LIBFUNC_EXIT
 117 }