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e_lgammal_r.c

/*
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunPro, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice
 * is preserved.
 * ====================================================
 */

/* Long double expansions are
  Copyright (C) 2001 Stephen L. Moshier <moshier@na-net.ornl.gov>
  and are incorporated herein by permission of the author.  The author 
  reserves the right to distribute this material elsewhere under different
  copying permissions.  These modifications are distributed here under 
  the following terms:

    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Lesser General Public
    License as published by the Free Software Foundation; either
    version 2.1 of the License, or (at your option) any later version.

    This library is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
    Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public
    License along with this library; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307  USA */

/* __ieee754_lgammal_r(x, signgamp)
 * Reentrant version of the logarithm of the Gamma function
 * with user provide pointer for the sign of Gamma(x).
 *
 * Method:
 *   1. Argument Reduction for 0 < x <= 8
 *    Since gamma(1+s)=s*gamma(s), for x in [0,8], we may
 *    reduce x to a number in [1.5,2.5] by
 *          lgamma(1+s) = log(s) + lgamma(s)
 *    for example,
 *          lgamma(7.3) = log(6.3) + lgamma(6.3)
 *                    = log(6.3*5.3) + lgamma(5.3)
 *                    = log(6.3*5.3*4.3*3.3*2.3) + lgamma(2.3)
 *   2. Polynomial approximation of lgamma around its
 *    minimun ymin=1.461632144968362245 to maintain monotonicity.
 *    On [ymin-0.23, ymin+0.27] (i.e., [1.23164,1.73163]), use
 *          Let z = x-ymin;
 *          lgamma(x) = -1.214862905358496078218 + z^2*poly(z)
 *   2. Rational approximation in the primary interval [2,3]
 *    We use the following approximation:
 *          s = x-2.0;
 *          lgamma(x) = 0.5*s + s*P(s)/Q(s)
 *    Our algorithms are based on the following observation
 *
 *                             zeta(2)-1    2    zeta(3)-1    3
 * lgamma(2+s) = s*(1-Euler) + --------- * s  -  --------- * s  + ...
 *                                 2                 3
 *
 *    where Euler = 0.5771... is the Euler constant, which is very
 *    close to 0.5.
 *
 *   3. For x>=8, we have
 *    lgamma(x)~(x-0.5)log(x)-x+0.5*log(2pi)+1/(12x)-1/(360x**3)+....
 *    (better formula:
 *       lgamma(x)~(x-0.5)*(log(x)-1)-.5*(log(2pi)-1) + ...)
 *    Let z = 1/x, then we approximation
 *          f(z) = lgamma(x) - (x-0.5)(log(x)-1)
 *    by
 *                          3       5             11
 *          w = w0 + w1*z + w2*z  + w3*z  + ... + w6*z
 *
 *   4. For negative x, since (G is gamma function)
 *          -x*G(-x)*G(x) = pi/sin(pi*x),
 *    we have
 *          G(x) = pi/(sin(pi*x)*(-x)*G(-x))
 *    since G(-x) is positive, sign(G(x)) = sign(sin(pi*x)) for x<0
 *    Hence, for x<0, signgam = sign(sin(pi*x)) and
 *          lgamma(x) = log(|Gamma(x)|)
 *                  = log(pi/(|x*sin(pi*x)|)) - lgamma(-x);
 *    Note: one should avoid compute pi*(-x) directly in the
 *          computation of sin(pi*(-x)).
 *
 *   5. Special Cases
 *          lgamma(2+s) ~ s*(1-Euler) for tiny s
 *          lgamma(1)=lgamma(2)=0
 *          lgamma(x) ~ -log(x) for tiny x
 *          lgamma(0) = lgamma(inf) = inf
 *          lgamma(-integer) = +-inf
 *
 */

#include "math.h"
#include "math_private.h"

#ifdef __STDC__
static const long double
#else
static long double
#endif
  half = 0.5L,
  one = 1.0L,
  pi = 3.14159265358979323846264L,
  two63 = 9.223372036854775808e18L,

  /* lgam(1+x) = 0.5 x + x a(x)/b(x)
     -0.268402099609375 <= x <= 0
     peak relative error 6.6e-22 */
  a0 = -6.343246574721079391729402781192128239938E2L,
  a1 =  1.856560238672465796768677717168371401378E3L,
  a2 =  2.404733102163746263689288466865843408429E3L,
  a3 =  8.804188795790383497379532868917517596322E2L,
  a4 =  1.135361354097447729740103745999661157426E2L,
  a5 =  3.766956539107615557608581581190400021285E0L,

  b0 =  8.214973713960928795704317259806842490498E3L,
  b1 =  1.026343508841367384879065363925870888012E4L,
  b2 =  4.553337477045763320522762343132210919277E3L,
  b3 =  8.506975785032585797446253359230031874803E2L,
  b4 =  6.042447899703295436820744186992189445813E1L,
  /* b5 =  1.000000000000000000000000000000000000000E0 */


  tc =  1.4616321449683623412626595423257213284682E0L,
  tf = -1.2148629053584961146050602565082954242826E-1,/* double precision */
/* tt = (tail of tf), i.e. tf + tt has extended precision. */
  tt = 3.3649914684731379602768989080467587736363E-18L,
  /* lgam ( 1.4616321449683623412626595423257213284682E0 ) =
-1.2148629053584960809551455717769158215135617312999903886372437313313530E-1 */

  /* lgam (x + tc) = tf + tt + x g(x)/h(x)
     - 0.230003726999612341262659542325721328468 <= x
     <= 0.2699962730003876587373404576742786715318
     peak relative error 2.1e-21 */
  g0 = 3.645529916721223331888305293534095553827E-18L,
  g1 = 5.126654642791082497002594216163574795690E3L,
  g2 = 8.828603575854624811911631336122070070327E3L,
  g3 = 5.464186426932117031234820886525701595203E3L,
  g4 = 1.455427403530884193180776558102868592293E3L,
  g5 = 1.541735456969245924860307497029155838446E2L,
  g6 = 4.335498275274822298341872707453445815118E0L,

  h0 = 1.059584930106085509696730443974495979641E4L,
  h1 =  2.147921653490043010629481226937850618860E4L,
  h2 = 1.643014770044524804175197151958100656728E4L,
  h3 =  5.869021995186925517228323497501767586078E3L,
  h4 =  9.764244777714344488787381271643502742293E2L,
  h5 =  6.442485441570592541741092969581997002349E1L,
  /* h6 = 1.000000000000000000000000000000000000000E0 */


  /* lgam (x+1) = -0.5 x + x u(x)/v(x)
     -0.100006103515625 <= x <= 0.231639862060546875
     peak relative error 1.3e-21 */
  u0 = -8.886217500092090678492242071879342025627E1L,
  u1 =  6.840109978129177639438792958320783599310E2L,
  u2 =  2.042626104514127267855588786511809932433E3L,
  u3 =  1.911723903442667422201651063009856064275E3L,
  u4 =  7.447065275665887457628865263491667767695E2L,
  u5 =  1.132256494121790736268471016493103952637E2L,
  u6 =  4.484398885516614191003094714505960972894E0L,

  v0 =  1.150830924194461522996462401210374632929E3L,
  v1 =  3.399692260848747447377972081399737098610E3L,
  v2 =  3.786631705644460255229513563657226008015E3L,
  v3 =  1.966450123004478374557778781564114347876E3L,
  v4 =  4.741359068914069299837355438370682773122E2L,
  v5 =  4.508989649747184050907206782117647852364E1L,
  /* v6 =  1.000000000000000000000000000000000000000E0 */


  /* lgam (x+2) = .5 x + x s(x)/r(x)
     0 <= x <= 1
     peak relative error 7.2e-22 */
  s0 =  1.454726263410661942989109455292824853344E6L,
  s1 = -3.901428390086348447890408306153378922752E6L,
  s2 = -6.573568698209374121847873064292963089438E6L,
  s3 = -3.319055881485044417245964508099095984643E6L,
  s4 = -7.094891568758439227560184618114707107977E5L,
  s5 = -6.263426646464505837422314539808112478303E4L,
  s6 = -1.684926520999477529949915657519454051529E3L,

  r0 = -1.883978160734303518163008696712983134698E7L,
  r1 = -2.815206082812062064902202753264922306830E7L,
  r2 = -1.600245495251915899081846093343626358398E7L,
  r3 = -4.310526301881305003489257052083370058799E6L,
  r4 = -5.563807682263923279438235987186184968542E5L,
  r5 = -3.027734654434169996032905158145259713083E4L,
  r6 = -4.501995652861105629217250715790764371267E2L,
  /* r6 =  1.000000000000000000000000000000000000000E0 */


/* lgam(x) = ( x - 0.5 ) * log(x) - x + LS2PI + 1/x w(1/x^2)
   x >= 8
   Peak relative error 1.51e-21
   w0 = LS2PI - 0.5 */
  w0 =  4.189385332046727417803e-1L,
  w1 =  8.333333333333331447505E-2L,
  w2 = -2.777777777750349603440E-3L,
  w3 =  7.936507795855070755671E-4L,
  w4 = -5.952345851765688514613E-4L,
  w5 =  8.412723297322498080632E-4L,
  w6 = -1.880801938119376907179E-3L,
  w7 =  4.885026142432270781165E-3L;

#ifdef __STDC__
static const long double zero = 0.0L;
#else
static long double zero = 0.0L;
#endif

#ifdef __STDC__
static long double
sin_pi (long double x)
#else
static long double
sin_pi (x)
     long double x;
#endif
{
  long double y, z;
  int n, ix;
  u_int32_t se, i0, i1;

  GET_LDOUBLE_WORDS (se, i0, i1, x);
  ix = se & 0x7fff;
  ix = (ix << 16) | (i0 >> 16);
  if (ix < 0x3ffd8000) /* 0.25 */
    return __sinl (pi * x);
  y = -x;               /* x is assume negative */

  /*
   * argument reduction, make sure inexact flag not raised if input
   * is an integer
   */
  z = __floorl (y);
  if (z != y)
    {                   /* inexact anyway */
      y  *= 0.5;
      y = 2.0*(y - __floorl(y));          /* y = |x| mod 2.0 */
      n = (int) (y*4.0);
    }
  else
    {
      if (ix >= 0x403f8000)  /* 2^64 */
      {
        y = zero; n = 0;                 /* y must be even */
      }
      else
      {
      if (ix < 0x403e8000)  /* 2^63 */
        z = y + two63;  /* exact */
      GET_LDOUBLE_WORDS (se, i0, i1, z);
      n = i1 & 1;
      y  = n;
      n <<= 2;
      }
    }

  switch (n)
    {
    case 0:
      y = __sinl (pi * y);
      break;
    case 1:
    case 2:
      y = __cosl (pi * (half - y));
      break;
    case 3:
    case 4:
      y = __sinl (pi * (one - y));
      break;
    case 5:
    case 6:
      y = -__cosl (pi * (y - 1.5));
      break;
    default:
      y = __sinl (pi * (y - 2.0));
      break;
    }
  return -y;
}


#ifdef __STDC__
long double
__ieee754_lgammal_r (long double x, int *signgamp)
#else
long double
__ieee754_lgammal_r (x, signgamp)
     long double x;
     int *signgamp;
#endif
{
  long double t, y, z, nadj, p, p1, p2, q, r, w;
  int i, ix;
  u_int32_t se, i0, i1;

  *signgamp = 1;
  GET_LDOUBLE_WORDS (se, i0, i1, x);
  ix = se & 0x7fff;

  if ((ix | i0 | i1) == 0)
    return one / fabsl (x);

  ix = (ix << 16) | (i0 >> 16);

  /* purge off +-inf, NaN, +-0, and negative arguments */
  if (ix >= 0x7fff0000)
    return x * x;

  if (ix < 0x3fc08000) /* 2^-63 */
    {                   /* |x|<2**-63, return -log(|x|) */
      if (se & 0x8000)
      {
        *signgamp = -1;
        return -__ieee754_logl (-x);
      }
      else
      return -__ieee754_logl (x);
    }
  if (se & 0x8000)
    {
      t = sin_pi (x);
      if (t == zero)
      return one / fabsl (t); /* -integer */
      nadj = __ieee754_logl (pi / fabsl (t * x));
      if (t < zero)
      *signgamp = -1;
      x = -x;
    }

  /* purge off 1 and 2 */
  if ((((ix - 0x3fff8000) | i0 | i1) == 0)
      || (((ix - 0x40008000) | i0 | i1) == 0))
    r = 0;
  else if (ix < 0x40008000) /* 2.0 */
    {
      /* x < 2.0 */
      if (ix <= 0x3ffee666) /* 8.99993896484375e-1 */
      {
        /* lgamma(x) = lgamma(x+1) - log(x) */
        r = -__ieee754_logl (x);
        if (ix >= 0x3ffebb4a) /* 7.31597900390625e-1 */
          {
            y = x - one;
            i = 0;
          }
        else if (ix >= 0x3ffced33)/* 2.31639862060546875e-1 */
          {
            y = x - (tc - one);
            i = 1;
          }
        else
          {
            /* x < 0.23 */
            y = x;
            i = 2;
          }
      }
      else
      {
        r = zero;
        if (ix >= 0x3fffdda6) /* 1.73162841796875 */
          {
            /* [1.7316,2] */
            y = x - 2.0;
            i = 0;
          }
        else if (ix >= 0x3fff9da6)/* 1.23162841796875 */
          {
            /* [1.23,1.73] */
            y = x - tc;
            i = 1;
          }
        else
          {
            /* [0.9, 1.23] */
            y = x - one;
            i = 2;
          }
      }
      switch (i)
      {
      case 0:
        p1 = a0 + y * (a1 + y * (a2 + y * (a3 + y * (a4 + y * a5))));
        p2 = b0 + y * (b1 + y * (b2 + y * (b3 + y * (b4 + y))));
        r += half * y + y * p1/p2;
        break;
      case 1:
    p1 = g0 + y * (g1 + y * (g2 + y * (g3 + y * (g4 + y * (g5 + y * g6)))));
    p2 = h0 + y * (h1 + y * (h2 + y * (h3 + y * (h4 + y * (h5 + y)))));
    p = tt + y * p1/p2;
        r += (tf + p);
        break;
      case 2:
 p1 = y * (u0 + y * (u1 + y * (u2 + y * (u3 + y * (u4 + y * (u5 + y * u6))))));
      p2 = v0 + y * (v1 + y * (v2 + y * (v3 + y * (v4 + y * (v5 + y)))));
        r += (-half * y + p1 / p2);
      }
    }
  else if (ix < 0x40028000) /* 8.0 */
    {
      /* x < 8.0 */
      i = (int) x;
      t = zero;
      y = x - (double) i;
  p = y *
     (s0 + y * (s1 + y * (s2 + y * (s3 + y * (s4 + y * (s5 + y * s6))))));
  q = r0 + y * (r1 + y * (r2 + y * (r3 + y * (r4 + y * (r5 + y * (r6 + y))))));
      r = half * y + p / q;
      z = one;                /* lgamma(1+s) = log(s) + lgamma(s) */
      switch (i)
      {
      case 7:
        z *= (y + 6.0); /* FALLTHRU */
      case 6:
        z *= (y + 5.0); /* FALLTHRU */
      case 5:
        z *= (y + 4.0); /* FALLTHRU */
      case 4:
        z *= (y + 3.0); /* FALLTHRU */
      case 3:
        z *= (y + 2.0); /* FALLTHRU */
        r += __ieee754_logl (z);
        break;
      }
    }
  else if (ix < 0x40418000) /* 2^66 */
    {
      /* 8.0 <= x < 2**66 */
      t = __ieee754_logl (x);
      z = one / x;
      y = z * z;
      w = w0 + z * (w1
          + y * (w2 + y * (w3 + y * (w4 + y * (w5 + y * (w6 + y * w7))))));
      r = (x - half) * (t - one) + w;
    }
  else
    /* 2**66 <= x <= inf */
    r = x * (__ieee754_logl (x) - one);
  if (se & 0x8000)
    r = nadj - r;
  return r;
}

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