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[unix-history] / usr / src / cmd / tsort.c

/*      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
*/

struct nodelist {
       struct nodelist *nextnode;
       struct predlist *inedges;
       char *name;
       enum {DEAD, LIVE, ONCE, TWICE} 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;
       register struct predlist *p;
       for(i= &firstnode; i->nextnode!=NULL; i=i->nextnode)
               if(i->live==LIVE)
                       break;
       note("cycle in reverse order",empty);
       while(i->live==LIVE) {
               i->live = ONCE;
               for(p=i->inedges; ; p=p->nextpred) {
                       if(p==NULL)
                               error("error 1");
                       i = p->pred;
                       if(i->live!=DEAD)
                               break;
               }
       }
       while(i->live==ONCE) {
               i->live = TWICE;
               note(i->name,empty);
               for(p=i->inedges; ; p=p->nextpred) {
                       if(p==NULL)
                               error("error 2");
                       i = p->pred;
                       if(i->live!=DEAD)
                               break;
               }
       }
       for(j= &firstnode; j->nextnode!=NULL; j=j->nextnode)
               if(j->live!=DEAD)
                       j->live = LIVE;
       return(i);
}