From 70bd7d69a4a490b0da337a0d9d8b57541993fe25 Mon Sep 17 00:00:00 2001
From: Gillu13 <gillu13@users.noreply.github.com>
Date: Wed, 29 Jan 2020 23:37:57 +0100
Subject: [PATCH 1/2] add isAcyclic function in the GraphProperty class to
 check wether a graph is a DAG

---
 src/Property/GraphProperty.php       | 43 ++++++++++++++++++++++++++
 tests/Property/PropertyGraphTest.php | 46 ++++++++++++++++++++++++++++
 2 files changed, 89 insertions(+)

diff --git a/src/Property/GraphProperty.php b/src/Property/GraphProperty.php
index d83b7a6..eea099a 100644
--- a/src/Property/GraphProperty.php
+++ b/src/Property/GraphProperty.php
@@ -48,4 +48,47 @@ public function isTrivial()
     {
         return ($this->graph->getEdges()->isEmpty() && \count($this->graph->getVertices()) === 1);
     }
+    
+    /**
+     * checks whether this graph is acyclic (directed graph with no cycles)
+     * using the Kahn algorithm
+     * 
+     * @return boolean
+     * @link https://en.wikipedia.org/wiki/Directed_acyclic_graph
+     */
+    public function isAcyclic()
+    {
+        $vertices = $this->graph->getVertices();
+        $nVertices = count($vertices);
+        $visited = 0;
+        $inDegree = [];
+        $stack=[];
+
+        foreach($vertices as $vert){
+            $deg=count($vert->getEdgesIn());
+            $inDegree[$vert->getId()]=$deg;
+            if($deg==0){
+                $stack[]=$vert;
+            }
+        }
+
+        while(!(empty($stack))){
+            $n = array_pop($stack);
+            $visited++;
+            foreach($n->getEdgesOut() as $e){
+                $m = $e->getVertexEnd();
+                $inDegree[$m->getId()]--;
+                if($inDegree[$m->getId()]==0){
+                    $stack[]=$m;
+                }
+            }
+        }
+        
+        if($visited==$nVertices){
+            return true;
+        }
+        else{
+            return false;
+        }
+    }
 }
diff --git a/tests/Property/PropertyGraphTest.php b/tests/Property/PropertyGraphTest.php
index 32e0374..66cc3da 100644
--- a/tests/Property/PropertyGraphTest.php
+++ b/tests/Property/PropertyGraphTest.php
@@ -14,6 +14,7 @@ public function testEmptyIsEdgeless()
         $this->assertTrue($alg->isNull());
         $this->assertTrue($alg->isEdgeless());
         $this->assertFalse($alg->isTrivial());
+        $this->assertTrue($alg->isAcyclic());
     }
 
     public function testSingleVertexIsTrivial()
@@ -26,5 +27,50 @@ public function testSingleVertexIsTrivial()
         $this->assertFalse($alg->isNull());
         $this->assertTrue($alg->isEdgeless());
         $this->assertTrue($alg->isTrivial());
+        $this->assertTrue($alg->isAcyclic());
+    }
+    
+    public function testUndirectedIsAcyclic()
+    {
+        $graph = new Graph();
+        $graph->createVertex(1)->createEdge($graph->createVertex(2));
+        
+        $alg = new GraphProperty($graph);
+        
+        $this->assertFalse($alg->isNull());
+        $this->assertFalse($alg->isEdgeless());
+        $this->assertFalse($alg->isTrivial());
+        $this->assertFalse($alg->isAcyclic());
+    }
+    
+    public function testGraphSimpleIsAcyclic()
+    {
+        $graph = new Graph();
+        $graph->createVertex(1)->createEdgeTo($graph->createVertex(2));
+        
+        $alg = new GraphProperty($graph);
+        
+        $this->assertFalse($alg->isNull());
+        $this->assertFalse($alg->isEdgeless());
+        $this->assertFalse($alg->isTrivial());
+        $this->assertTrue($alg->isAcyclic());
+    }
+    
+    public function testGraphWithCycleIsAcyclic()
+    {
+        $graph = new Graph();
+        $vertexOne = $graph->createVertex(1);
+        $vertexTwo = $graph->createVertex(2);
+        $vertexThree = $graph->createVertex(3);
+        $vertexOne->createEdgeTo($vertexTwo);
+        $vertexTwo->createEdgeTo($vertexThree);
+        $vertexThree->createEdgeTo($vertexOne);
+        
+        $alg = new GraphProperty($graph);
+        
+        $this->assertFalse($alg->isNull());
+        $this->assertFalse($alg->isEdgeless());
+        $this->assertFalse($alg->isTrivial());
+        $this->assertFalse($alg->isAcyclic());
     }
 }

From b2fb6a8fa86e0d69badda9dcd0fb5abbc068c3ff Mon Sep 17 00:00:00 2001
From: Gillu13 <gillu13@users.noreply.github.com>
Date: Fri, 31 Jan 2020 00:16:47 +0100
Subject: [PATCH 2/2] change array GraphProperty/isAcyclic (PHP 5.3 compliance)

---
 src/Property/GraphProperty.php | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/Property/GraphProperty.php b/src/Property/GraphProperty.php
index eea099a..6132433 100644
--- a/src/Property/GraphProperty.php
+++ b/src/Property/GraphProperty.php
@@ -61,14 +61,14 @@ public function isAcyclic()
         $vertices = $this->graph->getVertices();
         $nVertices = count($vertices);
         $visited = 0;
-        $inDegree = [];
-        $stack=[];
+        $inDegree = array();
+        $stack = array();
 
         foreach($vertices as $vert){
             $deg=count($vert->getEdgesIn());
             $inDegree[$vert->getId()]=$deg;
             if($deg==0){
-                $stack[]=$vert;
+                \array_push($stack,$vert);
             }
         }
 
@@ -79,7 +79,7 @@ public function isAcyclic()
                 $m = $e->getVertexEnd();
                 $inDegree[$m->getId()]--;
                 if($inDegree[$m->getId()]==0){
-                    $stack[]=$m;
+                    \array_push($stack,$m);
                 }
             }
         }