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#define	BASE_NAME	erf
#include "dpml_ux.h"

#if !defined(MAKE_INCLUDE)
#   include STR(BUILD_FILE_NAME)
#endif

/* 
** BASIC DESIGN
** ------------
** 
** The erf/erfc design is based on the following identities:
** 
** 	           2*x     __inf (-x^2)^k
** 	erf(x) = --------  >     --------			(1)
** 	         sqrt(pi) /__0   (k+1)*k!
** 
** 	erfc(x) = 1 - erf(x)					(2)
** 
** 	           exp(-x^2)   __inf     (2k)!
** 	erfc(x) ~  ----------  >     -------------		(3)
** 	           x*sqrt(pi) /__0   k!*(-4*x^2)^k
** 
** 	          exp(-x^2) /  1  1/2 2/2 3/2    \
** 	erfc(x) = --------- | --- --- --- --- ... |		(4)
**	          sqrt(pi)  \ x + x + x + x +    /
** 
** 
** The domain of the two functions is divided into 8 subintervals, symmetrically
** placed around 0.  For each subinterval, the general approach is to perform 
** some primary evaluation and then adjust its sign and add or subtract a
** constant.
** 
** On the first subinterval, from 0 to 1, the primary evaluation is a rational
** approximation to erf(x) of the form x*R(x^2), based on (1).  It should be
** noted here that the upper bound of this interval could be taken as large as
** 2 and still have the terms of R(x^2) be decreasing. However, as the upper
** limit increases past 1, loss of significance when computing 1 - erf(x)
** becomes a problem, so we take the upper limit of the first interval a 1
** because it simplifies the interval determination logic. 
** 
** The second subinterval spans 1 to A, where A is chosen so that if x >= A, the
** correctly rounded value of erf(x) is 1.  On this subinterval, the primary
** evaluation is an approximation to erfc(x), of the form exp(-x*x)*S(x), where
** S(x) is a rational approximation based on (4).
** 
** The third subinterval spans A to B, where B is chosen so that if x >= B, then
** erfc(x) underflows.  On this subinterval, the primary evaluation is an
** approximation to erfc(x) of the form exp(-x*x)*T(1/x^2)/x, where T(1/x^2)
** is a rational approximation based on (3).
** 
** The actual values of A and B are somewhat arbitrary.  For this design we take
** B = 128, since that choice helps simplify the determination of the intervals.
** A is chosen to be 8.75.  The reason for this choice of A is that:
** 
** 	o it meets the requirement that x >= A ==> erf(x) = 1,
** 	o A has very few significant bits, so its fraction can be represented
** 	  in one word
** 	o For this choice of A or larger, the terms in T(1/x^2) decrease
*/ 

#define HI_WORD_OF_8_PT_75		0x8c00000000000000ull

/* 
** 
** IMPLEMENTATION STRATEGY
** -----------------------
** 
** Based on the above definitions and equation (2) we can construct table 1
** which shows how erf(x) and erfc(x) are computed based on which interval
** they lie in.  In the table we refer to the primary evaluations in the first,
** second and third subintervals as ERF(x), MID(x) and ERFC(x) respectively.
** 
** 
** 	Sub-Interval	Index	  erf(x)	  erfc(x)
** 	------------	-----	-----------	------------
**	(-Inf,  -128]     7	-1              2
**	(-128, -8.75]     6	-1              2
**	[-8.75,   -1)     5	-1 + MID(|x|)   2 - MID(|x|)
**	(-1,       0)     4	 0 - ERF(|x|)   1 + ERF(|x|)
**	( 0,       1)     0	 0 + ERF(|x|)   1 - ERF(|x|)
**	[ 1,    8.75]     1	 1 - MID(|x|)	0 + MID(x)
** 	( 8.75,  128]     2	 1         	0 + ERFC(|x|)
** 	( 128,  +Inf]     3	 1         	underflow
** 
** 			    Table 1
** 			    -------
** 
** Ignoring for the time being, that underflows may need to be signaled, the
** evaluation scheme for each subinterval, for both functions, is of the form:
** 
** 			   c + t*F(x) 			(5)
** where
** 
** 		o t is +/-1
** 		o c is -1, 0, 1 or 2
** 		o F(x) is ERF(x), MID(x), ERFC(x), UNDERFLOW(x) or 0
** 
** Based on the above, we implement erf and erfc as calls into a common
** evaluation routine, C_UX_ERF, that determines the interval the argument
** lies in and then dispatches to the appropriate evaluation code.
** 
** 
** MAPPING INTERVALS TO EVALUATIONS
** --------------------------------
** 
** The mapping from interval to evaluation function can be done via a switch
** statement on the interval.  The cases for ERFC(x) and UNDERFLOW need to check
** for whether an erf(x) or erfc(x) evaluation is being performed.
** 
** The selection of the constants, c can be accomplished by encoding the 
** appropriate values of c for erfc(x) in a "bit string" that can be indexed
** by the interval number.  Actually, rather then encoding the constants
** themselves we encode the index into an unpacked constant table.  Letting
** the index for c = -1, 0, 1 and 2 be c + 1 (i.e the indices 0, 1, 2 and
** 3 correspond to the constants -1, 0, 1 and 2), we can create two integers,
** defined by:
** 
** 		 1   1   1
** 		 4   2   0   8   6   4   2   0: bit position
** 	      +---+---+---+---+---+---+---+---+
** 	erfc: | 3 | 3 | 3 | 2 | 1 | 1 | 1 | 2 |
** 	      +---+---+---+---+---+---+---+---+
** 	      +---+---+---+---+---+---+---+---+
** 	erf:  | 0 | 0 | 0 | 1 | 2 | 2 | 2 | 1 |
** 	      +---+---+---+---+---+---+---+---+
** 
** that map indices of the constants to the intervals. Note that given one of
** the above integers, we can determine if an erf or erfc evaluation is being
** performed by looking at the low bit.
*/ 

#define MAP_BIT_WIDTH			0x2
#define MAP_MASK			MAKE_MASK(MAP_BIT_WIDTH, 0)
#define MAP_IT(a,b,c,s)		 			\
		((a << (7*MAP_BIT_WIDTH)) |		\
		 (a << (6*MAP_BIT_WIDTH)) |		\
		 (a << (5*MAP_BIT_WIDTH)) |		\
		 (b << (4*MAP_BIT_WIDTH)) |		\
		 (c << (3*MAP_BIT_WIDTH)) |		\
		 (c << (2*MAP_BIT_WIDTH)) |		\
		 (c << (1*MAP_BIT_WIDTH)) |		\
		 (b << (0*MAP_BIT_WIDTH)) |		\
		 s)

#define ERFC_INTERVAL_TO_CONSTANT_MAP 	MAP_IT(3, 2, 1, UX_SIGN_BIT)
#define ERF_INTERVAL_TO_CONSTANT_MAP 	MAP_IT(0, 1, 2, 0)

#define IS_ERF_EVALUATION(i)	(i & 1)
#define IS_ERFC_EVALUATION(i)	((i & 1) == 0)

#define INTERVAL(i)	i

/*
** CALCULATING MID(x)
** -------------------
**
** The rational expression that needs to be evaluated for mid(x) is particularly
** ill behaved from the point of view of the general unpacked rational
** evaluation routine.  So ill behaved in fact, that the general routine can
** not be used for the evaluation.  The problem is that over the range
** [1, 8.75), the evaluation cannot be formulated in such a way that the
** terms decrease in magnitude and at the same time have the argument be less
** that 1 is absolute value.  Since this is the evaluation in the math library
** that has these characteristics, the special evaluation code for this case
** is included here.
**
** The solution to the problem is to use a special (less efficient) packed format for
** the evaluation.  See dpml_ux_ops.c for at description of the format.
*/


/*
** C_UX_ERF is the common erf/erfc evaluation routine
*/

#if !defined(C_UX_ERF)
#    define C_UX_ERF	__INTERNAL_NAME(C_ux_erf__)
#endif

static void
C_UX_ERF(
  _X_FLOAT   * packed_argument,
  U_WORD       interval_to_constant_map,
  _X_FLOAT   * packed_result
  OPT_EXCEPTION_INFO_DECLARATION )
    {  
    WORD fp_class, index;
    WORD const * class_to_action_map;
    UX_SIGN_TYPE  sign;
    UX_EXPONENT_TYPE exponent;
    UX_FLOAT unpacked_argument, tmp[3], *eval_result;

    fp_class = UNPACK(
        packed_argument,
        &unpacked_argument,
        IS_ERF_EVALUATION(interval_to_constant_map) ?
            ERF_CLASS_TO_ACTION_MAP : ERFC_CLASS_TO_ACTION_MAP,
        packed_result
        OPT_EXCEPTION_INFO_ARGUMENT);

    if (0 > fp_class)
        return;

    /* Determine interval */

    exponent = G_UX_EXPONENT(&unpacked_argument);
    if (exponent < 4)
        index = (exponent <= 0) ? 0 : 1;
    else if (exponent > 4)
        index = (exponent < 8) ? 2 : 3;
    else 
        index = (G_UX_MSD(&unpacked_argument) < HI_WORD_OF_8_PT_75) ? 1 : 2;

    index += G_UX_SIGN(&unpacked_argument) ? 4 : 0;
    P_UX_SIGN(&unpacked_argument, 0);

    /*
    ** Branch to appropriate action code.
    */

    sign = UX_SIGN_BIT & interval_to_constant_map;
    eval_result = & tmp[0];
    switch (index)
        {
	case INTERVAL(4):
            sign ^= UX_SIGN_BIT;
            /* Fall through */

	case INTERVAL(0):
            EVALUATE_RATIONAL(
                &unpacked_argument,
                ERF_COEF_ARRAY,
                ERF_COEF_ARRAY_DEGREE,
                NUMERATOR_FLAGS(SQUARE_TERM | POST_MULTIPLY)
                  | DENOMINATOR_FLAGS(SQUARE_TERM),
                eval_result);
	    break;

	case INTERVAL(1):
            sign ^= UX_SIGN_BIT;
            /* Fall through */

	case INTERVAL(5):
             EVALUATE_PACKED_POLY( &unpacked_argument,
                 MID_NUM_COEF_ARRAY_DEGREE, MID_NUM_COEF_ARRAY,
                 MID_NUM_SCALE_MASK, MID_NUM_SCALE_BIAS, &tmp[1]);
             EVALUATE_PACKED_POLY( &unpacked_argument,
                 MID_DEN_COEF_ARRAY_DEGREE, MID_DEN_COEF_ARRAY,
                 MID_DEN_SCALE_MASK, MID_DEN_SCALE_BIAS, &tmp[2]);
             DIVIDE(&tmp[1], &tmp[2], FULL_PRECISION, eval_result);
             goto multiply_by_exp_m_x_sqr;
	     break;

	case INTERVAL(2):
             if (IS_ERF_EVALUATION(interval_to_constant_map))
                 goto default_label;

             /* Compute z*T(z^2) for z = 8/x */

             sign = 0;
             DIVIDE( NOT_USED, &unpacked_argument, FULL_PRECISION, &tmp[2]);
             EVALUATE_RATIONAL(
                &tmp[2],
                ERFC_COEF_ARRAY,
                ERFC_COEF_ARRAY_DEGREE,
                NUMERATOR_FLAGS(SQUARE_TERM | POST_MULTIPLY)
                  | DENOMINATOR_FLAGS(SQUARE_TERM) | P_SCALE(3),
                eval_result);
 
             /* Fall through */

        multiply_by_exp_m_x_sqr:

             /*
             ** In order to avoid excessive errors in the final result, we
             ** compute exp(-x^2) as
             **
             **		exp(-x^2) = exp(-(hi + lo))
             **		          = exp(-hi)*exp(-lo)
             **		          ~ exp(-hi)*(1 - lo)
             **		          = exp(-hi) - lo*exp(-hi)
             */

             EXTENDED_MULTIPLY(&unpacked_argument, &unpacked_argument, &tmp[1],
                 &tmp[2]);
             P_UX_SIGN( &tmp[1], UX_SIGN_BIT);
             UX_EXP( &tmp[1], &tmp[1]);
             MULTIPLY(&tmp[2], &tmp[1], &tmp[2]);
             ADDSUB(&tmp[1], &tmp[2], SUB | NO_NORMALIZATION, &tmp[1]);

             MULTIPLY(&tmp[1], eval_result, eval_result);
	     break;

	case INTERVAL(3):
             if (IS_ERFC_EVALUATION(interval_to_constant_map))
                 { /* Dummy up underflow result and "zero" index */
                 UX_SET_SIGN_EXP_MSD(&tmp[0], 0, UX_UNDERFLOW_EXPONENT, UX_MSB);
	         break;
                 }
             /* Fall through */

        default:
        default_label:
             eval_result = UX_ZERO;
	     break;
        }

    /* Adjust sign of the evaluation and add in constant */

    P_UX_SIGN(&tmp[0], sign);
    index = (interval_to_constant_map >> (MAP_BIT_WIDTH*index)) & MAP_MASK;
    WORD_TO_UX(index - 1, &tmp[1]);
    ADDSUB(eval_result, &tmp[1], ADD | NO_NORMALIZATION, &tmp[0]);

    PACK(
        &tmp[0],
        packed_result,
        ERFC_UNDERFLOW,
        NOT_USED
        OPT_EXCEPTION_INFO_ARGUMENT);
    }


/*
** The following two entry points implement erfl and erfcl by calling the
** C_UX_ERF routine with the appropriate parameters
*/

#undef  F_ENTRY_NAME
#define F_ENTRY_NAME	F_ERF_NAME

X_X_PROTO(F_ENTRY_NAME, packed_result, packed_argument)
    {
    EXCEPTION_INFO_DECL
    DECLARE_X_FLOAT(packed_result)

    INIT_EXCEPTION_INFO;
    C_UX_ERF(
        PASS_ARG_X_FLOAT(packed_argument),
        ERF_INTERVAL_TO_CONSTANT_MAP,
        PASS_RET_X_FLOAT(packed_result)
        OPT_EXCEPTION_INFO);

    RETURN_X_FLOAT(packed_result);

    }

#undef  F_ENTRY_NAME
#define F_ENTRY_NAME	F_ERFC_NAME

X_X_PROTO(F_ENTRY_NAME, packed_result, packed_argument)
    {
    EXCEPTION_INFO_DECL
    DECLARE_X_FLOAT(packed_result)

    INIT_EXCEPTION_INFO;
    C_UX_ERF(
        PASS_ARG_X_FLOAT(packed_argument),
        ERFC_INTERVAL_TO_CONSTANT_MAP,
        PASS_RET_X_FLOAT(packed_result)
        OPT_EXCEPTION_INFO);

    RETURN_X_FLOAT(packed_result);

    }



#if defined(MAKE_INCLUDE)

    @divert -append divertText

    precision = ceil(UX_PRECISION/8) + 4;

#   undef TABLE_NAME

    START_TABLE;

    TABLE_COMMENT("erf class-to-action-mapping");
    PRINT_CLASS_TO_ACTION_TBL_DEF( "ERF_CLASS_TO_ACTION_MAP\t");
    PRINT_64_TBL_ITEM( CLASS_TO_ACTION_DISP(1) +
	      CLASS_TO_ACTION( F_C_SIG_NAN,    RETURN_QUIET_NAN, 0) +
              CLASS_TO_ACTION( F_C_QUIET_NAN,  RETURN_VALUE,     0) +
              CLASS_TO_ACTION( F_C_POS_INF,    RETURN_VALUE,     1) +
              CLASS_TO_ACTION( F_C_NEG_INF,    RETURN_NEGATIVE,  1) +
              CLASS_TO_ACTION( F_C_POS_NORM,   RETURN_UNPACKED,  0) +
              CLASS_TO_ACTION( F_C_NEG_NORM,   RETURN_UNPACKED,  0) +
              CLASS_TO_ACTION( F_C_POS_DENORM, RETURN_UNPACKED,  0) +
              CLASS_TO_ACTION( F_C_NEG_DENORM, RETURN_UNPACKED,  0) +
              CLASS_TO_ACTION( F_C_POS_ZERO,   RETURN_VALUE,     0) +
              CLASS_TO_ACTION( F_C_NEG_ZERO ,  RETURN_VALUE,     0) );

        PRINT_U_TBL_ITEM( /* data 1 */ ONE );


    TABLE_COMMENT("erfc class-to-action-mapping");
    PRINT_CLASS_TO_ACTION_TBL_DEF( "ERFC_CLASS_TO_ACTION_MAP");
    PRINT_64_TBL_ITEM( CLASS_TO_ACTION_DISP(1) +
              CLASS_TO_ACTION( F_C_SIG_NAN,    RETURN_QUIET_NAN, 0) +
              CLASS_TO_ACTION( F_C_QUIET_NAN,  RETURN_VALUE,     0) +
              CLASS_TO_ACTION( F_C_POS_INF,    RETURN_VALUE,     1) +
              CLASS_TO_ACTION( F_C_NEG_INF,    RETURN_VALUE,     3) +
              CLASS_TO_ACTION( F_C_POS_NORM,   RETURN_UNPACKED,  0) +
              CLASS_TO_ACTION( F_C_NEG_NORM,   RETURN_UNPACKED,  0) +
              CLASS_TO_ACTION( F_C_POS_DENORM, RETURN_VALUE,     2) +
              CLASS_TO_ACTION( F_C_NEG_DENORM, RETURN_VALUE,     2) +
              CLASS_TO_ACTION( F_C_POS_ZERO,   RETURN_VALUE,     2) +
              CLASS_TO_ACTION( F_C_NEG_ZERO ,  RETURN_VALUE,     2) );

        PRINT_U_TBL_ITEM( /* data 1 */ ZERO );
        PRINT_U_TBL_ITEM( /* data 2 */  ONE );
        PRINT_U_TBL_ITEM( /* data 3 */  TWO );

    TABLE_COMMENT("unpacked 0 constant");
    PRINT_UX_TBL_ADEF_ITEM( "UX_ZERO\t\t\t", 0);

    /*
    ** The remaining mphoc computes the coefficients for the various rational
    ** evaluations.  The erf/erfc approximations are rather difficult to
    ** compute and consequently the Remes algorithm requires a long time to
    ** converge.  In order to speed up the process for the normal case, we
    ** compute rational approximation of specific degrees, rather than using
    ** the REMES_FIND_RATIONAL option.
    */

#   if UX_PRECISION != 128
#        error "Rational coefficient degrees may be invalid for this precision"
#   endif
 
    /*
    ** Generate coefficients for erf(x) evaluation on [0,1)
    */

    zero_value = 2/sqrt(pi);
    function __erf(x)
        {
        if (x == 0)
            return zero_value;
        else
            return erf(x)/x;
        }

    save_precision = precision;
    precision = ceil(UX_PRECISION/8) + 8;
    max_arg = 1;

    num_degree = 10;
    den_degree = 10;
    TABLE_COMMENT("Fixed point coefficients for erf(x) evaluation");
    remes(REMES_STATIC + REMES_RELATIVE_WEIGHT + REMES_SQUARE_ARG,
        0, max_arg, __erf, num_degree, den_degree, &ux_rational_coefs);

    precision = save_precision;

    PRINT_FIXED_128_TBL_ADEF("ERF_COEF_ARRAY\t\t");
    degree = print_ux_rational_coefs(num_degree, den_degree, 0);
    PRINT_WORD_DEF("ERF_COEF_ARRAY_DEGREE\t", degree);


    /*
    ** Generate coefficients for erfc(x) evaluation on [8.75, 128)
    */

    zero_value = 1/sqrt(pi);

    function __erfc(z)
        {
        auto x;

        if (z == 0)
            return zero_value;

        x = 8/z;
        return exp(x*x)*x*erfc(x);
        }


    save_precision = precision;
    precision = ceil(UX_PRECISION/8) + 8;
    min_arg = 0;
    max_arg = 8/8.75;

    num_degree = 10;
    den_degree = 10;
    TABLE_COMMENT("Fixed point coefficients for erfc(x) evaluation");
    remes(REMES_STATIC + REMES_RELATIVE_WEIGHT + REMES_SQUARE_ARG,
        min_arg, max_arg, __erfc, num_degree, den_degree, &ux_rational_coefs);

    precision = save_precision;

    PRINT_FIXED_128_TBL_ADEF("ERFC_COEF_ARRAY\t\t");
    degree = print_ux_rational_coefs(num_degree, den_degree, -3);
    PRINT_WORD_DEF("ERFC_COEF_ARRAY_DEGREE\t", degree);

    /*
    ** Generate coefficients for mid(x) evaluation on [1,8.75).
    */

    function __mid(x) { return exp(x*x)*erfc(x); }

    save_precision = precision;
    precision = ceil(UX_PRECISION/8) + 8;
    min_arg = 1;
    max_arg = 8.75;

    num_degree = 16;
    den_degree = 17;
    remes(REMES_STATIC + REMES_RELATIVE_WEIGHT + REMES_LINEAR_ARG +
        REMES_INIT_LEFT_CHEBY, min_arg, max_arg, __mid, num_degree, den_degree,
        &ux_rational_coefs);

    precision = save_precision;


    /*
    ** Now convert numerator and denominator to "packed" form and print them out
    */

    procedure cvt_and_print_packed(degree, base_index)
        {
        find_exponent_width_and_bias(degree, base_index);
        cvt_to_packed(degree, base_index, packed_exponent_width,
          packed_exponent_bias);
        print_packed(degree, base_index);
        }

    TABLE_COMMENT("Packed coefficients for mid numerator evaluation");
    PRINT_FIXED_128_TBL_ADEF("MID_NUM_COEF_ARRAY\t");
    PRINT_WORD_DEF("MID_NUM_COEF_ARRAY_DEGREE", num_degree);
    cvt_and_print_packed(num_degree, 0);
    PRINT_WORD_DEF("MID_NUM_SCALE_BIAS\t", packed_exponent_bias);
    PRINT_WORD_DEF("MID_NUM_SCALE_MASK\t", (1 << packed_exponent_width) - 1);

    TABLE_COMMENT("Packed coefficients for mid denominator evaluation");
    PRINT_FIXED_128_TBL_ADEF("MID_DEN_COEF_ARRAY\t");
    PRINT_WORD_DEF("MID_DEN_COEF_ARRAY_DEGREE", den_degree);
    cvt_and_print_packed(den_degree, num_degree + 1);
    PRINT_WORD_DEF("MID_DEN_SCALE_BIAS\t", packed_exponent_bias);
    PRINT_WORD_DEF("MID_DEN_SCALE_MASK\t", (1 << packed_exponent_width) - 1);

    END_TABLE;

    @end_divert
    @eval my $tableText;						\
          my $outText    = MphocEval( GetStream( "divertText" ) );	\
          my $defineText = Egrep( "#define", $outText, \$tableText );	\
             $outText    = "$tableText\n\n$defineText";			\
          my $headerText = GetHeaderText( STR(BUILD_FILE_NAME),         \
                           "Definitions and constants erf and erfc",	\
                              __FILE__ );				\
             print "$headerText\n\n$outText\n";

#endif

