workbench/libs/mathieeesingtrans/ieeesplog.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 e) 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       (ln is the logarithm with base e)
  38       y = M * 2^E
  39
  40       <code>
  41       ln y = 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_ieeespld.c
  58
  59     HISTORY
  60 */
  61
  62 AROS_LH1(float, IEEESPLog,
  63     AROS_LHA(float, y, D0),
  64     struct Library *, MathIeeeSingTransBase, 14, 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
  94     /* find the number of the highest set bit in the exponent */
  95     if (Exponent != 0)
  96     {
  97         i = 0;
  98         while ( (Mask & Exponent) == 0)
  99         {
 100             i ++;
 101             Mask >>= 1;
 102         }
 103
 104         Exponent <<= (17 + i);
 105         Exponent &= IEEESPMantisse_Mask;
 106         Exponent |= ((0x85 - i ) << 23);
 107         Exponent |= Sign;
 108     }
 109
 110     ld_M = intern_IEEESPLd((y & IEEESPMantisse_Mask) | 0x3f000000);
 111
 112     /*
 113                       ld M + E
 114         ln(fnum1) =  --------
 115                        ld e
 116     */
 117
 118     return IEEESPMul( IEEESPAdd(ld_M, Exponent), InvLde);
 119
 120     AROS_LIBFUNC_EXIT
 121 }