Greatest Common Divisor

gcd.c

#include <stdio.h>
#include <stdlib.h>

typedef const int INT;
typedef const char CHAR;
typedef CHAR *const PCHAR;
typedef char **const Ppchar;
extern int main( INT, Ppchar );

#define by( _ ) for( auto _; ; )
#define at( _ ) { return _; }
#define et( _ ) else at( _ )

static int o( INT X, INT Y ) at( ( 0 > X )? Y: 0 )
static int my( INT X, INT Y, Ppchar A ) at( main( X - Y, X + A ) )
static int gcd( INT X, INT Y ) at( X? gcd( Y % X, X ): Y )
extern int main( INT X, Ppchar A )
{
	if( A ) by( INT Y = -X ) by( PCHAR B = *A )
		if( 0 < Y )
			if( 1 < Y ) by( INT Z = Y >> 1 )
				at( gcd( my( 0, Z, A ), my( Z, Y, A ) ) )
			et( atoi( B ) )
		et( o( Y, main( printf( "%s:%+d\n", B, my( 1, X, A ) ), 0 ) ) )
	et( o( X, EXIT_FAILURE ) )
}

Tab-Size Preference: { default, 2, 3, 4, 6, 8 }