Convert big number to human readable format

human-numbers.cpp

/*
 * A simple way to format numbers as human readable strings.
 * E.g., 123456789 ==> 123 million
 *
 * Written by Christian Stigen Larsen
 * http://csl.sublevel3.org
 *
 * Placed in the public domain by the author, 2012
 */

#include <stdio.h>
#include <math.h>
#include <string.h>

/*
 * Short scale units
 * http://en.wikipedia.org/wiki/Short_scale
 */
static const char* short_scale[] = {
  "",
  "thousand",
  "million",
  "billion",
  "trillion",
  "quadrillion",
  "quintillion",
  "sextillion",
  "septillion"
};

/*
 * Long scale units
 * http://en.wikipedia.org/wiki/Short_scale
 */
static const char* long_scale[] = {
  "",
  "thousand",
  "million",
  "milliard",
  "billion",
  "billiard",
  "trillion",
  "trilliard",
  "quadrillion"
};

/*
 * Convert number to human readable string using
 * the given naming system.
 */
const char* scale(double n, int decimals = 1, const char* units[] = short_scale)
{
  /*
   * Number of digits in n is given by
   * 10^x = n ==> x = log(n)/log(10) = log_10(n).
   *
   * So 1000 would be 1 + floor(log_10(10^3)) = 4 digits.
   */
  int digits = n==0? 0 : 1+floor(log10l(fabs(n)));

  // determine base 10 exponential
  int exp = digits <= 4? 0 : 3*((digits-1)/3);

  // normalized number
  double m = n / powl(10, exp);

  // no decimals? then don't print any
  if ( m - static_cast<long>(n) == 0 )
    decimals = 0;

  // don't print unit for exp<3
  static char s[32];
  static const char* fmt[] = {"%1.*lf %s", "%1.*lf"};
  sprintf(s, fmt[exp<3], decimals, m, units[exp/3]);
  return s;
}

/*
 * Convert number to short scale representation
 */
const char* sscale(double n, int decimals = 1)
{
  static char s[32];
  strcpy(s, scale(n, decimals, short_scale));
  return s;
}

/*
 * Convert number to long scale representation
 */
const char* lscale(double n, int decimals = 1)
{
  static char s[32];
  strcpy(s, scale(n, decimals, long_scale));
  return s;
}

void test(double n)
{
  printf("%.lf in short scale is %s and %s in long scale\n", n, sscale(n), lscale(n));
}

int main()
{
  long numbers[] = {
    1000000000,
    0,
    1,
    12,
    123,
    1234,
    12345,
    123456,
    1234567,
    12345678,
    123456789,
    1234567890,
    12345678901,
    123456789012,
    1234567890123,
    12345678901234,
    123456789012345,
    1234567890123456,
    12345678901234567,
    123456789012345677,
    1234567890123456789
  };

  for ( size_t n=0; n<sizeof(numbers)/sizeof(long); ++n )
    test(numbers[n]);

  test(-1);
  test((double)12345.6789);
  test((double)123456789012345678910.223);
}