workbench/libs/mathieeesingtrans/ieeesplog10.c

   1 /*
   2     Copyright © 1995-2008, The AROS Development Team. All rights reserved.
   3     $Id$
   4 */
   5
   6 #include "mathieeesingtrans_intern.h"
   7
   8 /*
   9     FUNCTION
  10       Calculate logarithm (base 10) of the given IEEE single precision number
  11
  12     RESULT
  13       IEEE single precision 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       (log is the logarithm with base 10)
  38       y = M * 2^E
  39
  40       <code>
  41       log y = 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_ieeespld.c
  58
  59     HISTORY
  60 */
  61
  62 AROS_LH1(float, IEEESPLog10,
  63     AROS_LHA(float, y, D0),
  64     struct Library *, MathIeeeSingTransBase, 21, MathIeeeSingTrans
  65 )
  66 {
  67     AROS_LIBFUNC_INIT
  68
  69     LONG ld_M, Exponent, Mask = 0x40, i, Sign;
  70
  71     /* check for negative sign */
  72     if ( y < 0)
  73     {
  74         SetSR(Overflow_Bit, Zero_Bit | Negative_Bit | Overflow_Bit);
  75         return 0;
  76     }
  77
  78     /* check for argument == 0 or argument == +infinity */
  79     if (0 == y || IEEESP_Pinfty == y) return y;
  80
  81     /* convert the Exponent of the argument (y) to the ieeesp-format */
  82     Exponent = ((y & IEEESPExponent_Mask) >> 23) - 0x7e ;
  83
  84     if (Exponent < 0 )
  85     {
  86         Exponent =-Exponent;
  87         Sign = IEEESPSign_Mask;
  88     }
  89     else
  90     {
  91         Sign = 0;
  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
 103         Exponent <<= (17 + i);
 104         Exponent &= IEEESPMantisse_Mask;
 105         Exponent |= ((0x85 - i ) << 23);
 106         Exponent |= Sign;
 107     }
 108
 109     ld_M = intern_IEEESPLd((y & IEEESPMantisse_Mask) | 0x3f000000);
 110
 111     /*
 112                       ld M + E
 113         log(fnum1) =  --------
 114                        ld 10
 115     */
 116
 117     return IEEESPMul( IEEESPAdd(ld_M, Exponent), InvLd10);
 118
 119     AROS_LIBFUNC_EXIT
 120 }