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
  10     NAME */
  11
  12         AROS_LH1(float, IEEESPLog,
  13
  14 /*  SYNOPSIS */
  15         AROS_LHA(float, y, D0),
  16
  17 /*  LOCATION */
  18         struct Library *, MathIeeeSingTransBase, 14, MathIeeeSingTrans)
  19
  20 /*  FUNCTION
  21         Calculate logarithm (base e) of the given IEEE single precision number
  22
  23     INPUTS
  24
  25     RESULT
  26         IEEE single precision number
  27
  28         flags:
  29         zero     : result is zero
  30         negative : result is negative
  31         overflow : argument was negative
  32
  33     BUGS
  34
  35     INTERNALS
  36         ALGORITHM:
  37
  38         If the Argument is negative set overflow-flag and return 0.
  39         If the Argument is 0 return 0xffffffff.
  40
  41         All other cases:
  42
  43         (ld is the logarithm with base 2)
  44         (ln is the logarithm with base e)
  45         y = M * 2^E
  46
  47         ln y = ln ( M * 2^E ) =
  48
  49            = ln M + ln 2^E =
  50
  51            = ln M + E * ln (2) =
  52
  53              ld M        ld 2
  54            = ----- + E * ----- =      [ld 2 = 1]
  55              ld e        ld e
  56
  57              ld M + E
  58            = --------
  59              ld e
  60
  61         ld e can be precalculated, of course.
  62         For calculating ld M see file intern_ieeespld.c
  63
  64 *****************************************************************************/
  65 {
  66     AROS_LIBFUNC_INIT
  67
  68     LONG ld_M, Exponent, Mask = 0x40, i, Sign;
  69
  70     /* check for negative sign */
  71     if ( y < 0)
  72     {
  73         SetSR(Overflow_Bit, Zero_Bit | Negative_Bit | Overflow_Bit);
  74         return 0;
  75     }
  76
  77     /* check for argument == 0 or argument == +infinity */
  78     if (0 == y || IEEESP_Pinfty == y) return y;
  79
  80     /* convert the Exponent of the argument (y) to the ieeesp-format */
  81     Exponent = ((y & IEEESPExponent_Mask) >> 23) - 0x7e ;
  82
  83     if (Exponent < 0 )
  84     {
  85         Exponent =-Exponent;
  86         Sign = IEEESPSign_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
 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         ln(fnum1) =  --------
 114                        ld e
 115     */
 116
 117     return IEEESPMul( IEEESPAdd(ld_M, Exponent), InvLde);
 118
 119     AROS_LIBFUNC_EXIT
 120 }