remove an edge when there's cycles

index.js

@@ -16,6 +16,10 @@ function toposort(graph) {
   let s = sources(g);
   let l = [];
 
+  for (let c of g.cycles()) {
+    g.removeExistingEdge(c.pop(), c.shift())
+  }
+
   while (s.length) {
     let n = s.shift(); // vs pop?
     l.push(n);
@@ -28,10 +32,6 @@ function toposort(graph) {
     }
   }
 
-  if (g.edgeCount()) {
-    throw new Error('cycle detected in graph, topological sort is impossible');
-  }
-
   return l;
 }
 

test/index.js

@@ -13,17 +13,16 @@ describe('toposort(graph)', function () {
     assert.isArray(toposort(graph));
   });
 
-  it('should throw when the graph has a cycle', function () {
-    // a -> b -> c -> b* (cycle)
+  it('should handle graphs that have a cycle', function () {
+    // a -> b -> c -> d -> b* (cycle)
     var graph = new Graph(
       [ [ 'a', 'b' ] ],
       [ [ 'b', 'c' ] ],
-      [ [ 'c', 'b' ] ]
+      [ [ 'c', 'd' ] ],
+      [ [ 'd', 'b' ] ]
     );
 
-    assert.throws(function () {
-      toposort(graph);
-    });
+    assert.deepEqual(toposort(graph), ['a', 'b', 'c', 'd'])
   });
 
   it('should correctly sort a simple tree', function () {