www.pudn.com > system.rar > atan.c

/****************************************************************************/ 
/*  atan   v2.54                                                            */ 
/*  Copyright (c) 1995-2004 Texas Instruments Incorporated                  */ 
/****************************************************************************/ 
#include  
#include  
 
/****************************************************************************/ 
/*  ATAN() - Arctangent							    */ 
/*									    */ 
/*  Based on the algorithm from "Software Manual for the Elementary         */ 
/*  Functions", Cody and Waite, Prentice Hall 1980, chapter 11.             */ 
/*									    */ 
/*  if x > 1, x = 1 / x 						    */ 
/*  if x > 2 - sqrt(3), x = (x * sqrt(3) - 1) / (sqrt(3) + x)		    */ 
/*  g = x * x								    */ 
/*  R = polynomial expression						    */ 
/*									    */ 
/*  result = (t * (x + x * R) + an) * s					    */ 
/****************************************************************************/ 
double atan(double x) 
{ 
    double g, p, q; 
    int    sign = x < 0.0;       /* REMEMBER SIGN (1 == negative) */ 
    int    t = 0;                /* PARTIAL RESULT SIGN FLAG      */ 
    int    n = 0; 
    double result;  
 
    static const _DATA_ACCESS double a[4] = {0.0, 0.52359877559829887308,  
		     	     		        1.57079632679489661923, 
		                     		1.04719755119659774615}; 
 
    if ((x = fabs(x)) > 1.0) 
    { 
	x = 1.0 / x;	 
	n = 2;	 
 
	/******************************************************************/ 
	/* the partial result needs to be negated                         */ 
	/******************************************************************/ 
	t = 1;     
    } 
 
    /**********************************************************************/ 
    /* for x > (2 - sqrt(3)  )                                            */ 
    /**********************************************************************/ 
    if (x > TWO_SQRT3)			 
    { 
	/******************************************************************/ 
	/* x = (x * sqrt(3) -1)	/ (sqroot(3) + x)                         */ 
	/******************************************************************/ 
	x = (x * SQRTTHREE - 1.0) / (SQRTTHREE + x); 
	++n; 			 
    } 
 
    /*********************************************************************/ 
    /* determine polynomial expression                                   */ 
    /*********************************************************************/ 
    g = x * x;	 
 
#if BITS<=24 
    p = (ATP1 * g + ATP0) * g; 
    q = g + ATQ0; 
#elif BITS>=25 && BITS<=32 
    p = (ATP1 * g + ATP0) * g; 
    q = (g + ATQ1) * g + ATQ0; 
#elif BITS>=33 && BITS<=50 
    p = ((ATP2 * g + ATP1) * g + ATP0) * g; 
    q = ((g + ATQ2) * g + ATQ1) * g + ATQ0; 
#else 
    p = (((ATP3 * g + ATP2) * g + ATP1) * g + ATP0) * g; 
    q = (((g + ATQ3) * g + ATQ2) * g + ATQ1) * g + ATQ0; 
#endif 
 
    /**********************************************************************/ 
    /* calculate the result multiplied by the correct sign                */ 
    /**********************************************************************/ 
    result = ((p / q) * x + x);  
    result = (t ? -result : result) + a[n]; 
    return sign ? -result : result;  
}