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phoenix |
/***********************************************************************
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** File: scalb.c
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**
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** Contains: C source code for implementations of floating-point
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** scalb functions defined in header <fp.h>. In
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** particular, this file contains implementations of
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** functions scalb and scalbl for double and long double
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** formats on PowerPC platforms.
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**
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** Written by: Jon Okada, SANEitation Engineer, ext. 4-4838
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**
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** Copyright: © 1992 by Apple Computer, Inc., all rights reserved
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**
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** Change History ( most recent first ):
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**
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** 28 May 97 ali made an speed improvement for large n,
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** removed scalbl.
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** 12 Dec 92 JPO First created.
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**
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***********************************************************************/
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typedef union
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{
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struct {
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#if defined(__BIG_ENDIAN__)
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unsigned long int hi;
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unsigned long int lo;
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#else
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unsigned long int lo;
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unsigned long int hi;
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#endif
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} words;
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double dbl;
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} DblInHex;
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static const double twoTo1023 = 8.988465674311579539e307; // 0x1p1023
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static const double twoToM1022 = 2.225073858507201383e-308; // 0x1p-1022
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/***********************************************************************
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double scalb( double x, long int n ) returns its argument x scaled
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by the factor 2^m. NaNs, signed zeros, and infinities are propagated
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by this function regardless of the value of n.
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Exceptions: OVERFLOW/INEXACT or UNDERFLOW inexact may occur;
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INVALID for signaling NaN inputs ( quiet NaN returned ).
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Calls: none.
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***********************************************************************/
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double scalb ( double x, int n )
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{
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DblInHex xInHex;
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xInHex.words.lo = 0UL; // init. low half of xInHex
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if ( n > 1023 )
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{ // large positive scaling
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if ( n > 2097 ) // huge scaling
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return ( ( x * twoTo1023 ) * twoTo1023 ) * twoTo1023;
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while ( n > 1023 )
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{ // scale reduction loop
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x *= twoTo1023; // scale x by 2^1023
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n -= 1023; // reduce n by 1023
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}
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}
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else if ( n < -1022 )
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{ // large negative scaling
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if ( n < -2098 ) // huge negative scaling
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return ( ( x * twoToM1022 ) * twoToM1022 ) * twoToM1022;
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while ( n < -1022 )
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{ // scale reduction loop
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x *= twoToM1022; // scale x by 2^( -1022 )
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n += 1022; // incr n by 1022
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}
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}
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/*******************************************************************************
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* -1022 <= n <= 1023; convert n to double scale factor. *
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*******************************************************************************/
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xInHex.words.hi = ( ( unsigned long ) ( n + 1023 ) ) << 20;
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return ( x * xInHex.dbl );
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}
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