Solution to level 17 in Untrusted: http://alex.nisnevich.com/untrusted/

untrusted-lvl17-solution.js

/***************
 * pointers.js *
 ***************
 *
 * You! How are you still alive?
 *
 * Well, no matter. Good luck getting through this
 * maze of rooms - you'll never see me or the Algorithm again!
 */

function startLevel(map) {
    function shuffle(o){ //v1.0 [http://bit.ly/1l6LGQT]
        for(var j, x, i = o.length; i; j = Math.floor(Math.random() * i),
            x = o[--i], o[i] = o[j], o[j] = x);
        return o;
    };

    map.createFromGrid(
        ['+++++++++++++++++++++++++++++++++++++++++++++',
         '++o *++++o *++++o *++++o *++++o *++++o *+++++',
         '+* @ o++*   o++*   o++*   o++*   o++*   o++++',
         '++o *++++o *++++o *++++o *++++o *++++o *+++++',
         '+++++++++++++++++++++++++++++++++++++++++++++',
         '+++++* o++++* o++++* o++++* o++++* o++++* o++',
         '++++o   *++o   *++o   *++o   *++o   *++o   *+',
         '+++++* o++++* o++++* o++++* o++++* o++++* o++',
         '+++++++++++++++++++++++++++++++++++++++++++++',
         '++o *++++o *++++o *++++o *++++o *++++o *+++++',
         '+*   o++*   o++*   o++*   o++*   o++*   o++++',
         '++o *++++o *++++o *++++o *++++o *++++o *+++++',
         '+++++++++++++++++++++++++++++++++++++++++++++',
         '+++++* o++++* o++++* o++++* o++++* o++++* o++',
         '++++o   *++o   *++o   *++o   *++o   *++o   *+',
         '+++++* o++++* o++++* o++++* o++++* o++++* o++',
         '+++++++++++++++++++++++++++++++++++++++++++++',
         '++o *++++o *++++o *++++o *++++o *++++o *+++++',
         '+*   o++*   o++*   o++*   o++*   o++* E o++++',
         '++o *++++o *++++o *++++o *++++o *++++o *+++++',
         '+++++++++++++++++++++++++++++++++++++++++++++'],
        {
            '@': 'player',
            'E': 'exit',
            '+': 'block',
            'o': 'teleporter',
            '*': 'trap',
        }, 2, 2);

    var canvas = map.getCanvasContext();

    var teleportersAndTraps = map.getDynamicObjects();
    teleportersAndTraps = shuffle(teleportersAndTraps);

    for (i = 0; i < teleportersAndTraps.length; i+=2) {
        var t1 = teleportersAndTraps[i];
        var t2 = teleportersAndTraps[i+1];

        // Point each teleporter to either another teleporter
        // or a trap
        if (t1.getType() == 'teleporter') {
            t1.setTarget(t2);
        }
        if (t2.getType() == 'teleporter') {
            t2.setTarget(t1);
        }

       if(i+2 >= teleportersAndTraps.length)
       {
         var q=map.getCanvasCoords,c=map.getCanvasContext()
         var ENTRY=0, EXIT=5+4*6, ports=[],t=teleportersAndTraps;
         function Line(t1,t2,s,w)
         {
           var a=q(t1),b=q(t2);
           c.beginPath(); c.moveTo(a.x,a.y); c.lineTo(b.x,b.y); 
           c.strokeStyle=s; c.lineWidth=w; c.stroke();
         }
         function Mark(t,text,color)
         {
           var a=q(t)
           c.font='italic 30pt Arial'
           c.fillStyle=color;
           c.textBaseline='middle'; c.textAlign='center'
           c.fillText(text, a.x,a.y)
         }
         function GetGroupno(t)
         {
           var ygroup = Math.floor((t.getY()-3) / 4),
               xgroup = Math.floor((t.getX() - (ygroup%2?5:2))/7);
           return xgroup + 6*ygroup;
         }
         // Collect puzzle edges
         for(var n=0; n<t.length; ++n)
           if(t[n  ].getType()=='teleporter'
           && t[n^1].getType()=='teleporter')
           {
             ports.push( {a:t[n], b:t[n^1], g:GetGroupno(t[n])} );
             Line(t[n], t[n^1], '#222',1);
           }
         // Figure out which teleports are neighbors
         var groups = []
         for(var n=0; n<6*5; ++n) groups.push([])
         for(var n=0; n<ports.length; ++n)
           groups[ports[n].g].push(n)
         // Solve the puzzle
         var heads = [], solution = [], g = groups[ENTRY]
         for(var n=0; n<g.length; ++n)
         {
           var new_head = {h:[], m:{}}
           new_head.h.push(g[n])
           new_head.m[g[n]] = true;
           heads.push(new_head)
         }
         while(solution.length==0 && heads.length!=0)
         {
           var nextheads = []
           // Go through each head.
           // Stop when we find a head connected to EXIT.
           // Add all other end's neighbors to new heads list.
           for(var n=0; n<heads.length; ++n)
           {
             var h = heads[n];
             var last = h.h[h.h.length-1], tgt = last^1;
             var target_group = ports[tgt].g;
             Line(ports[last].a, ports[last].b, '#021',2);
             if( target_group == EXIT)
             {
               solution = h.h;
               break;
             }
             for(var m=0; m<groups[target_group].length; ++m)
             {
               var o = groups[target_group][m]
               // Have we been to "o" yet?
               if(typeof h.m[o] == 'undefined')
               {
                 var new_head = {h:[], m:{}}
                 for(var r=0; r<h.h.length; ++r)
                   { new_head.h.push( h.h[r] );
                     new_head.m[ h.h[r] ] = true; }
                 new_head.h.push(o);
                 new_head.m[o]=true;
                 nextheads.push(new_head)
               }
             }
           }
           heads = nextheads
         }
         // Show the solution
         for(var n=0; n<solution.length; ++n)
         {
           var s = solution[n];
           Line( ports[s].a, ports[s].b, 'gold', 3)
           Mark(ports[s].a, ""+(n+1)+"", 'white');
         }
         //if(solution.length==0) throw "No solution!";
       }       
    }
}

function validateLevel(map) {
    map.validateExactlyXManyObjects(1, 'exit');
}