/* origin: OpenBSD /usr/src/lib/libm/src/ld80/s_log1pl.c */
/*
 * Copyright (c) 2008 Stephen L. Moshier <steve@moshier.net>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */
/*
 *      Relative error logarithm
 *      Natural logarithm of 1+x, long double precision
 *
 *
 * SYNOPSIS:
 *
 * long double x, y, log1pl();
 *
 * y = log1pl( x );
 *
 *
 * DESCRIPTION:
 *
 * Returns the base e (2.718...) logarithm of 1+x.
 *
 * The argument 1+x is separated into its exponent and fractional
 * parts.  If the exponent is between -1 and +1, the logarithm
 * of the fraction is approximated by
 *
 *     log(1+x) = x - 0.5 x^2 + x^3 P(x)/Q(x).
 *
 * Otherwise, setting  z = 2(x-1)/x+1),
 *
 *     log(x) = z + z^3 P(z)/Q(z).
 *
 *
 * ACCURACY:
 *
 *                      Relative error:
 * arithmetic   domain     # trials      peak         rms
 *    IEEE     -1.0, 9.0    100000      8.2e-20    2.5e-20
 */

#include "libm.h"

#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
long double log1pl(long double x) {
  return log1p(x);
}
#elif LDBL_MANT_DIG == 64 && LDBL_MAX_EXP == 16384
/* Coefficients for log(1+x) = x - x^2 / 2 + x^3 P(x)/Q(x)
 * 1/sqrt(2) <= x < sqrt(2)
 * Theoretical peak relative error = 2.32e-20
 */
static const long double P[] = {
    4.5270000862445199635215E-5L, 4.9854102823193375972212E-1L,
    6.5787325942061044846969E0L,  2.9911919328553073277375E1L,
    6.0949667980987787057556E1L,  5.7112963590585538103336E1L,
    2.0039553499201281259648E1L,
};
static const long double Q[] = {
    /* 1.0000000000000000000000E0,*/
    1.5062909083469192043167E1L, 8.3047565967967209469434E1L,
    2.2176239823732856465394E2L, 3.0909872225312059774938E2L,
    2.1642788614495947685003E2L, 6.0118660497603843919306E1L,
};

/* Coefficients for log(x) = z + z^3 P(z^2)/Q(z^2),
 * where z = 2(x-1)/(x+1)
 * 1/sqrt(2) <= x < sqrt(2)
 * Theoretical peak relative error = 6.16e-22
 */
static const long double R[4] = {
    1.9757429581415468984296E-3L, -7.1990767473014147232598E-1L,
    1.0777257190312272158094E1L, -3.5717684488096787370998E1L,
};
static const long double S[4] = {
    /* 1.00000000000000000000E0L,*/
    -2.6201045551331104417768E1L, 1.9361891836232102174846E2L,
    -4.2861221385716144629696E2L,
};
static const long double C1 = 6.9314575195312500000000E-1L;
static const long double C2 = 1.4286068203094172321215E-6L;

#define SQRTH 0.70710678118654752440L

long double log1pl(long double xm1) {
  long double x, y, z;
  int e;

  if (isnan(xm1))
    return xm1;
  if (xm1 == INFINITY)
    return xm1;
  if (xm1 == 0.0)
    return xm1;

  x = xm1 + 1.0;

  /* Test for domain errors.  */
  if (x <= 0.0) {
    if (x == 0.0)
      return -1 / (x * x); /* -inf with divbyzero */
    return 0 / 0.0f;       /* nan with invalid */
  }

  /* Separate mantissa from exponent.
     Use frexp so that denormal numbers will be handled properly.  */
  x = frexpl(x, &e);

  /* logarithm using log(x) = z + z^3 P(z)/Q(z),
     where z = 2(x-1)/x+1)  */
  if (e > 2 || e < -2) {
    if (x < SQRTH) { /* 2(2x-1)/(2x+1) */
      e -= 1;
      z = x - 0.5;
      y = 0.5 * z + 0.5;
    } else { /*  2 (x-1)/(x+1)   */
      z = x - 0.5;
      z -= 0.5;
      y = 0.5 * x + 0.5;
    }
    x = z / y;
    z = x * x;
    z = x * (z * __polevll(z, R, 3) / __p1evll(z, S, 3));
    z = z + e * C2;
    z = z + x;
    z = z + e * C1;
    return z;
  }

  /* logarithm using log(1+x) = x - .5x**2 + x**3 P(x)/Q(x) */
  if (x < SQRTH) {
    e -= 1;
    if (e != 0)
      x = 2.0 * x - 1.0;
    else
      x = xm1;
  } else {
    if (e != 0)
      x = x - 1.0;
    else
      x = xm1;
  }
  z = x * x;
  y = x * (z * __polevll(x, P, 6) / __p1evll(x, Q, 6));
  y = y + e * C2;
  z = y - 0.5 * z;
  z = z + x;
  z = z + e * C1;
  return z;
}
#elif LDBL_MANT_DIG == 113 && LDBL_MAX_EXP == 16384
// TODO: broken implementation to make things compile
long double log1pl(long double x) {
  return log1p(x);
}
#endif