/*---------------------------------------------------------------------------+
   |  poly_2xm1.c                                                              |
   |                                                                           |
   | Function to compute 2^x-1 by a polynomial approximation.                  |
   |                                                                           |
   | Copyright (C) 1992,1993,1994,1997                                         |
   |                  W. Metzenthen, 22 Parker St, Ormond, Vic 3163, Australia |
   |                  E-mail   billm@suburbia.net                              |
   |                                                                           |
   |                                                                           |
   +---------------------------------------------------------------------------*/
  
  #include "exception.h"
  #include "reg_constant.h"
  #include "fpu_emu.h"
  #include "fpu_system.h"
  #include "control_w.h"
  #include "poly.h"

  #define	HIPOWER	11

  static const unsigned long long lterms[HIPOWER] = {
  	0x0000000000000000LL,	/* This term done separately as 12 bytes */
  	0xf5fdeffc162c7543LL,
  	0x1c6b08d704a0bfa6LL,
  	0x0276556df749cc21LL,
  	0x002bb0ffcf14f6b8LL,
  	0x0002861225ef751cLL,
  	0x00001ffcbfcd5422LL,
  	0x00000162c005d5f1LL,
  	0x0000000da96ccb1bLL,
  	0x0000000078d1b897LL,
  	0x000000000422b029LL

  };
  
  static const Xsig hiterm = MK_XSIG(0xb17217f7, 0xd1cf79ab, 0xc8a39194);
  
  /* Four slices: 0.0 : 0.25 : 0.50 : 0.75 : 1.0,
     These numbers are 2^(1/4), 2^(1/2), and 2^(3/4)
   */
  static const Xsig shiftterm0 = MK_XSIG(0, 0, 0);
  static const Xsig shiftterm1 = MK_XSIG(0x9837f051, 0x8db8a96f, 0x46ad2318);
  static const Xsig shiftterm2 = MK_XSIG(0xb504f333, 0xf9de6484, 0x597d89b3);
  static const Xsig shiftterm3 = MK_XSIG(0xd744fcca, 0xd69d6af4, 0x39a68bb9);
  
  static const Xsig *shiftterm[] = { &shiftterm0, &shiftterm1,

  	&shiftterm2, &shiftterm3
  };

  
  /*--- poly_2xm1() -----------------------------------------------------------+
   | Requires st(0) which is TAG_Valid and < 1.                                |
   +---------------------------------------------------------------------------*/

  int poly_2xm1(u_char sign, FPU_REG *arg, FPU_REG *result)

  {

  	long int exponent, shift;
  	unsigned long long Xll;
  	Xsig accumulator, Denom, argSignif;
  	u_char tag;

  	exponent = exponent16(arg);

  
  #ifdef PARANOID

  	if (exponent >= 0) {	/* Don't want a |number| >= 1.0 */
  		/* Number negative, too large, or not Valid. */
  		EXCEPTION(EX_INTERNAL | 0x127);
  		return 1;
  	}

  #endif /* PARANOID */

  	argSignif.lsw = 0;
  	XSIG_LL(argSignif) = Xll = significand(arg);
  
  	if (exponent == -1) {
  		shift = (argSignif.msw & 0x40000000) ? 3 : 2;
  		/* subtract 0.5 or 0.75 */
  		exponent -= 2;
  		XSIG_LL(argSignif) <<= 2;
  		Xll <<= 2;
  	} else if (exponent == -2) {
  		shift = 1;
  		/* subtract 0.25 */
  		exponent--;
  		XSIG_LL(argSignif) <<= 1;
  		Xll <<= 1;
  	} else
  		shift = 0;
  
  	if (exponent < -2) {
  		/* Shift the argument right by the required places. */
  		if (FPU_shrx(&Xll, -2 - exponent) >= 0x80000000U)
  			Xll++;	/* round up */
  	}
  
  	accumulator.lsw = accumulator.midw = accumulator.msw = 0;
  	polynomial_Xsig(&accumulator, &Xll, lterms, HIPOWER - 1);
  	mul_Xsig_Xsig(&accumulator, &argSignif);
  	shr_Xsig(&accumulator, 3);
  
  	mul_Xsig_Xsig(&argSignif, &hiterm);	/* The leading term */
  	add_two_Xsig(&accumulator, &argSignif, &exponent);
  
  	if (shift) {
  		/* The argument is large, use the identity:
  		   f(x+a) = f(a) * (f(x) + 1) - 1;
  		 */
  		shr_Xsig(&accumulator, -exponent);
  		accumulator.msw |= 0x80000000;	/* add 1.0 */
  		mul_Xsig_Xsig(&accumulator, shiftterm[shift]);
  		accumulator.msw &= 0x3fffffff;	/* subtract 1.0 */
  		exponent = 1;
  	}
  
  	if (sign != SIGN_POS) {
  		/* The argument is negative, use the identity:
  		   f(-x) = -f(x) / (1 + f(x))
  		 */
  		Denom.lsw = accumulator.lsw;
  		XSIG_LL(Denom) = XSIG_LL(accumulator);
  		if (exponent < 0)
  			shr_Xsig(&Denom, -exponent);
  		else if (exponent > 0) {
  			/* exponent must be 1 here */
  			XSIG_LL(Denom) <<= 1;
  			if (Denom.lsw & 0x80000000)
  				XSIG_LL(Denom) |= 1;
  			(Denom.lsw) <<= 1;
  		}
  		Denom.msw |= 0x80000000;	/* add 1.0 */
  		div_Xsig(&accumulator, &Denom, &accumulator);

  	}

  	/* Convert to 64 bit signed-compatible */
  	exponent += round_Xsig(&accumulator);

  	result = &st(0);
  	significand(result) = XSIG_LL(accumulator);
  	setexponent16(result, exponent);

  	tag = FPU_round(result, 1, 0, FULL_PRECISION, sign);

  	setsign(result, sign);
  	FPU_settag0(tag);

  	return 0;

  
  }