/*	topological sort
 *	input is sequence of pairs of items (blank-free strings)
 *	nonidentical pair is a directed edge in graph
 *	identical pair merely indicates presence of node
 *	output is ordered list of items consistent with
 *	the partial ordering specified by the graph
*/
#include "stdio.h"

/*	the nodelist always has an empty element at the end to
 *	make it easy to grow in natural order
 *	states of the "live" field:*/
#define DEAD 0	/* already printed*/
#define LIVE 1	/* not yet printed*/
#define VISITED 2	/*used only in findloop()*/

struct nodelist {
	struct nodelist *nextnode;
	struct predlist *inedges;
	char *name;
	int live;
} firstnode = {NULL, NULL, NULL, DEAD};

/*	a predecessor list tells all the immediate
 *	predecessors of a given node
*/
struct predlist {
	struct predlist *nextpred;
	struct nodelist *pred;
};

struct nodelist *index();
struct nodelist *findloop();
struct nodelist *mark();
char *malloc();
char *empty = "";

/*	the first for loop reads in the graph,
 *	the second prints out the ordering
*/
main(argc,argv)
char **argv;
{
	register struct predlist *t;
	FILE *input = stdin;
	register struct nodelist *i, *j;
	int x;
	char precedes[50], follows[50];
	if(argc>1) {
		input = fopen(argv[1],"r");
		if(input==NULL)
			error("cannot open ", argv[1]);
	}
	for(;;) {
		x = fscanf(input,"%s%s",precedes, follows);
		if(x==EOF)
			break;
		if(x!=2)
			error("odd data",empty);
		i = index(precedes);
		j = index(follows);
		if(i==j||present(i,j)) 
			continue;
		t = (struct predlist *)malloc(sizeof(struct predlist));
		t->nextpred = j->inedges;
		t->pred = i;
		j->inedges = t;
	}
	for(;;) {
		x = 0;	/*anything LIVE on this sweep?*/
		for(i= &firstnode; i->nextnode!=NULL; i=i->nextnode) {
			if(i->live==LIVE) {
				x = 1;
				if(!anypred(i))
					break;
			}
		}
		if(x==0)
			break;
		if(i->nextnode==NULL)
			i = findloop();
		printf("%s\n",i->name);
		i->live = DEAD;
	}
}

/*	is i present on j's predecessor list?
*/
present(i,j)
struct nodelist *i, *j;
{
	register struct predlist *t;
	for(t=j->inedges; t!=NULL; t=t->nextpred)
		if(t->pred==i)
			return(1);
	return(0);
}

/*	is there any live predecessor for i?
*/
anypred(i)
struct nodelist *i;
{
	register struct predlist *t;
	for(t=i->inedges; t!=NULL; t=t->nextpred)
		if(t->pred->live==LIVE)
			return(1);
	return(0);
}

/*	turn a string into a node pointer
*/
struct nodelist *
index(s)
register char *s;
{
	register struct nodelist *i;
	register char *t;
	for(i= &firstnode; i->nextnode!=NULL; i=i->nextnode)
		if(cmp(s,i->name))
			return(i);
	for(t=s; *t; t++) ;
	t = malloc((unsigned)(t+1-s));
	i->nextnode = (struct nodelist *)malloc(sizeof(struct nodelist));
	if(i->nextnode==NULL||t==NULL)
		error("too many items",empty);
	i->name = t;
	i->live = LIVE;
	i->nextnode->nextnode = NULL;
	i->nextnode->inedges = NULL;
	i->nextnode->live = DEAD;
	while(*t++ = *s++);
	return(i);
}

cmp(s,t)
register char *s, *t;
{
	while(*s==*t) {
		if(*s==0)
			return(1);
		s++;
		t++;
	}
	return(0);
}

error(s,t)
char *s, *t;
{
	note(s,t);
	exit(1);
}

note(s,t)
char *s,*t;
{
	fprintf(stderr,"tsort: %s%s\n",s,t);
}

/*	given that there is a cycle, find some
 *	node in it
*/
struct nodelist *
findloop()
{
	register struct nodelist *i, *j;
	for(i= &firstnode; i->nextnode!=NULL; i=i->nextnode)
		if(i->live==LIVE)
			break;
	note("cycle in data",empty);
	i = mark(i);
	if(i==NULL)
		error("program error",empty);
	for(j= &firstnode; j->nextnode!=NULL; j=j->nextnode)
		if(j->live==VISITED)
			j->live = LIVE;
	return(i);
}

/*	depth-first search of LIVE predecessors
 *	to find some element of a cycle;
 *	VISITED is a temporary state recording the
 *	visits of the search
*/
struct nodelist *
mark(i)
register struct nodelist *i;
{
	register struct nodelist *j;
	register struct predlist *t;
	if(i->live==DEAD)
		return(NULL);
	if(i->live==VISITED)
		return(i);
	i->live = VISITED;
	for(t=i->inedges; t!=NULL; t=t->nextpred) {
		j = mark(t->pred);
		if(j!=NULL) {
			note(i->name,empty);
			return(j);
		}
	}
	return(NULL);
}
