github.com/bgentry/go@v0.0.0-20150121062915-6cf5a733d54d/src/lib9/fmt/pow10.c (about)

     1  /*
     2   * The authors of this software are Rob Pike and Ken Thompson,
     3   * with contributions from Mike Burrows and Sean Dorward.
     4   *
     5   *     Copyright (c) 2002-2006 by Lucent Technologies.
     6   *     Portions Copyright (c) 2004 Google Inc.
     7   * 
     8   * Permission to use, copy, modify, and distribute this software for any
     9   * purpose without fee is hereby granted, provided that this entire notice
    10   * is included in all copies of any software which is or includes a copy
    11   * or modification of this software and in all copies of the supporting
    12   * documentation for such software.
    13   * THIS SOFTWARE IS BEING PROVIDED "AS IS", WITHOUT ANY EXPRESS OR IMPLIED
    14   * WARRANTY.  IN PARTICULAR, NEITHER THE AUTHORS NOR LUCENT TECHNOLOGIES 
    15   * NOR GOOGLE INC MAKE ANY REPRESENTATION OR WARRANTY OF ANY KIND CONCERNING 
    16   * THE MERCHANTABILITY OF THIS SOFTWARE OR ITS FITNESS FOR ANY PARTICULAR PURPOSE.
    17   */
    18  
    19  #include <u.h>
    20  #include <libc.h>
    21  #include "fmtdef.h"
    22  
    23  /*
    24   * this table might overflow 127-bit exponent representations.
    25   * in that case, truncate it after 1.0e38.
    26   * it is important to get all one can from this
    27   * routine since it is used in atof to scale numbers.
    28   * the presumption is that C converts fp numbers better
    29   * than multipication of lower powers of 10.
    30   */
    31  
    32  static
    33  double	tab[] =
    34  {
    35  	1.0e0, 1.0e1, 1.0e2, 1.0e3, 1.0e4, 1.0e5, 1.0e6, 1.0e7, 1.0e8, 1.0e9,
    36  	1.0e10,1.0e11,1.0e12,1.0e13,1.0e14,1.0e15,1.0e16,1.0e17,1.0e18,1.0e19,
    37  	1.0e20,1.0e21,1.0e22,1.0e23,1.0e24,1.0e25,1.0e26,1.0e27,1.0e28,1.0e29,
    38  	1.0e30,1.0e31,1.0e32,1.0e33,1.0e34,1.0e35,1.0e36,1.0e37,1.0e38,1.0e39,
    39  	1.0e40,1.0e41,1.0e42,1.0e43,1.0e44,1.0e45,1.0e46,1.0e47,1.0e48,1.0e49,
    40  	1.0e50,1.0e51,1.0e52,1.0e53,1.0e54,1.0e55,1.0e56,1.0e57,1.0e58,1.0e59,
    41  	1.0e60,1.0e61,1.0e62,1.0e63,1.0e64,1.0e65,1.0e66,1.0e67,1.0e68,1.0e69,
    42  };
    43  
    44  double
    45  __fmtpow10(int n)
    46  {
    47  	int m;
    48  
    49  	if(n < 0) {
    50  		n = -n;
    51  		if(n < (int)(sizeof(tab)/sizeof(tab[0])))
    52  			return 1/tab[n];
    53  		m = n/2;
    54  		return __fmtpow10(-m) * __fmtpow10(m-n);
    55  	}
    56  	if(n < (int)(sizeof(tab)/sizeof(tab[0])))
    57  		return tab[n];
    58  	m = n/2;
    59  	return __fmtpow10(m) * __fmtpow10(n-m);
    60  }