integer to string conversion

[Bill Poser]

One way to put an integer into a character string is to use sprintf(3),
e.g.:	sprintf(string,"%d",foo);

If you are doing a lot of conversions it may be worthwhile to
use a special-purpose function.

/*
 * This is a fast subroutine for converting a binary
 * integer into a decimal ASCII string. It is very much
 * like the itoa routine described on p.60 of K&R, but differs
 * from it in several respects. One is the order of arguments.
 * The name of the string in which to write is given first in order
 * to be consistent with sprintf and kindred functions.
 *	 
 * The other differences are intended to speed up the function.
 * These include:
 *	 
 * 	Use of registers for frequently accessed variables.
 *
 *	Use of a divide, two shifts, an add and a subtract in place of a
 *	divide and a C modulus operation. Interestingly, on this machine
 *	replacing the modulus with a multiplication and a subtraction only
 *	cut the time a little, but replacing the multiplication with
 *	two shifts and an add cut the time substantially. This optimization
 *	should work on other machines using the MC68020 processor. Your
 *	mileage may vary on machines using processors whose relative
 * 	instruction times are different.
 *
 * 	Use of pointer arithmetic rather than array indexing.
 *
 *	Inline code for reversal of the string (which is generated backwards).
 *	This not only eliminates the function call overhead, but it also
 *	eliminates the need to detect the end of the string using strlen
 *	since we know its location already.
 *
 * Using itoa(s,n) is almost 20 % faster than using sprintf(s,"%d",n).
 * Testing on conversions of the 1e6 integers from -500000 to 499999
 * on an unloaded HP 9000/350 showed an average of 53.2 microseconds
 * per conversion for sprintf() and 43.8 microseconds per conversion
 * for this implementation of itoa. (Figures are after subtraction
 * of test loop overhead of 1.1 seconds.)
 *
 * Author:	William J. Poser (Department of Linguistics, Stanford U.)
 * 
 */

void
itoa(string,k)
char *string;				/* String in which to write */
int k;					/* Integer to convert */
{
   register int temp;
   register char *s;
   register char *p;
   register int n;
   register int q;
   register int r;
   int sign;
   
   n = k;			/* Copy integer into register */
   p = s = string;		/* Copy pointer into registers */
   
   if( (sign = n) < 0) n = -n;	/* Make n positive */
   
   /* Do conversion */

   do {
      q = n / 10;
      r = (q << 3) + (q << 1);	/* Multiply by 10 (8x + 2x = 10x) */
      *s++ = n + '0' - r;
   } while (( n = q) > 0);
   
   if(sign < 0) *s++ = '-';	/* If negative, add sign */
   
   *s = '\0';			/* Null terminate string */
   
   s--;				/* Point s at last non-null char */
   
   /*
    * Now we reverse the string. When we begin, p points at the first
    * character of the string, s at the last non-null character.
    */

   while(p < s){
      temp = *p;
      *p++ = *s;
      *s-- = temp;
   }

   return;
}