nuttx/libc/math/lib_libexpi.c

/****************************************************************************
 * libc/math/lib_libexpi.c
 *
 * This file is a part of NuttX:
 *
 *   Copyright (C) 2012 Gregory Nutt. All rights reserved.
 *   Ported by: Darcy Gong
 *
 * It derives from the Rhombs OS math library by Nick Johnson which has
 * a compatibile, MIT-style license:
 *
 * Copyright (C) 2009-2011 Nick Johnson <nickbjohnson4224 at gmail.com>
 *
 * 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.
 *
 ****************************************************************************/

/****************************************************************************
 * Included Files
 ****************************************************************************/

#include <nuttx/config.h>
#include <nuttx/compiler.h>

#include <sys/types.h>
#include <math.h>

/****************************************************************************
 * Public Functions
 ****************************************************************************/

/****************************************************************************
 * Pre-processor Definitions
 ****************************************************************************/

#define M_E2    (M_E    * M_E)
#define M_E4    (M_E2   * M_E2)
#define M_E8    (M_E4   * M_E4)
#define M_E16   (M_E8   * M_E8)
#define M_E32   (M_E16  * M_E16)
#define M_E64   (M_E32  * M_E32)
#define M_E128  (M_E64  * M_E64)
#define M_E256  (M_E128 * M_E128)
#define M_E512  (M_E256 * M_E256)

/****************************************************************************
 * Private Data
 ****************************************************************************/

static const double g_expi_square_tbl[] =
{
  M_E,          /* e^1 */
  M_E2,         /* e^2 */
  M_E4,         /* e^4 */
  M_E8,         /* e^8 */
  M_E16,        /* e^16 */
  M_E32,        /* e^32 */
  M_E64,        /* e^64 */
  M_E128,       /* e^128 */
  M_E256,       /* e^256 */
  M_E512        /* e^512 */
};

/****************************************************************************
 * Public Functions
 ****************************************************************************/

double lib_expi(size_t n)
{
  size_t i;
  double val;

  /* The largest calculable value for n is floor(ln(DBL_MAX)) */

  if (n > 709)
    {
      return INFINITY;
    }

  val = 1.0;

  for (i = 0; n != 0; i++)
    {
      if ((n & (1 << i)) != 0)
        {
          n   &= ~(1 << i);
          val *= g_expi_square_tbl[i];
        }
    }

  return val;
}