/*
**  COMMAFMT.C
**
**  Public domain by Bob Stout
**
**  Notes:  1. Use static buffer to eliminate error checks on buffer overflow
**             and reduce code size.
**          2. By making the numeric argument a long and prototyping it before
**             use, passed numeric arguments will be implicitly cast to longs
**             thereby avoiding int overflow.
**          3. Use the thousands grouping and thousands separator from the
**             ANSI locale to make this more robust.
*/

#include <string.h>
#ifdef TEST
 #include <stdlib.h>
#endif

#define NUL '\0'

size_t commafmt(char   *buf,            /* Buffer for formatted string  */
                int     bufsize,        /* Size of buffer               */
                long    N)              /* Number to convert            */
{
        int len = 1, posn = 1, sign = 1;
        char *ptr = buf + bufsize - 1;

        if (2 > bufsize)
        {
ABORT:          *buf = NUL;
                return 0;
        }

        *ptr-- = NUL;
        --bufsize;
        if (0L > N)
        {
                sign = -1;
                N = -N;
        }

        for ( ; len <= bufsize; ++len, ++posn)
        {
                *ptr-- = (char)((N % 10L) + '0');
                if (0L == (N /= 10L))
                        break;
                if (0 == (posn % 3))
                {
                        *ptr-- = ',';
                        ++len;
                }
                if (len >= bufsize)
                        goto ABORT;
        }

        if (0 > sign)
        {
                if (0 == bufsize)
                        goto ABORT;
                *ptr-- = '-';
                ++len;
        }

        strcpy(buf, ++ptr);
        return (size_t)len;
}

#ifdef TEST

void main(int argc, char *argv[])
{
        size_t len;
        char buf[20];
        long N;

        N = strtol(argv[1], NULL, 10);
        len = commafmt(buf, 20, N);
        printf("%s converts to %s and returned %d\n", argv[1], buf, len);
}

#endif