/************************************************************************
 This ANSI C program prints all (n choose m) = n!/m!/(n-m)! combinations
 of 'n' different items on the standard output.
 usage:
    choose n m [nitems]
 where 1 <= m <= n are two integers, and the items can optionally be
 given 1-character representations. If the 'nitems' are not provided
 explicitly, the alphabetical letters 'a', 'b', 'c' etc are chosen.
 Richard J Mathar, 11/23/2003

 examples:

   choose 10 4 
 prints 210 lines
abcd
abce
abcf
abcg
abch
abci
abcj
abde
abdf
abdg
...
   choose 10 3 
 prints 120 lines
abc
abd
abe
abf
abg
abh
abi
abj
acd
ace
...
fhj
fij
ghi
ghj
gij
hij

  choose 8 4 AEIOULMN 
 prints  70 lines
AEIO
AEIU
AEIL
AEIM
AEIN
AEOU
AEOL
AEOM
AEON
AEUL
AEUM
..
OUMN
OLMN
ULMN
*******************************************************************/

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

void combin(const char const *letts,int mark, int n, int mleft,
		int *taken)
{
	if( mleft > n -mark )
		return ;
	else if( mleft > 0)
	{
		taken[mark]=1 ;
		combin(letts,mark+1,n,mleft-1,taken) ;
		taken[mark]=0 ;
		combin(letts,mark+1,n,mleft,taken) ;
	}
	else
	{
		for(int i=0 ; i < n; i++)
			if( taken[i] )
				printf("%c",letts[i]) ;
		printf("\n") ;
	}
}

int main(int argc, char *argv[])
{
	int n,m ,*taken;
	char * letts ;
	if (argc < 3 )
	{
		printf("usage: %s n m [letters]\n",argv[0]) ;
		exit(EXIT_FAILURE) ;
	}
	n=atoi(argv[1]) ;
	m=atoi(argv[2]) ;
	if( m > n)
	{
		printf("Second argument %d must be <= first one %d\n",m,n) ;
		exit(EXIT_FAILURE) ;
	}
	if (argc == 3 )
	{
		letts = malloc(n*sizeof(char)) ;
		for(int i=0 ; i < n; i++)
			letts[i] = i+'a' ;
	}
	else
	{
		if( n != strlen(argv[3]) )
		{
			printf("Third argument does not have %d characters\n",n) ;
			exit(EXIT_FAILURE) ;
		}
		letts = argv[3] ;
	}
	taken=malloc(n*sizeof(int)) ;
	combin(letts,0,n,m,taken) ;
	free(taken) ;
	if( argc == 3)
		free(letts) ;
}