Kahn topology sort

scala/src/main/scala/ch43_topology_sort/GraphTopology.scala

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+package ch43_topology_sort
+
+import scala.collection.mutable
+
+class GraphTopology(vertex: Int) {
+
+  //define the graph
+  val adjacency = new Array[mutable.MutableList[Int]](vertex)
+  for (i <- Range(0, vertex)) {
+    adjacency(i) = new mutable.MutableList[Int]()
+  }
+
+  def addEdge(startIndex: Int, targetIndex: Int) = {
+    adjacency(startIndex) += targetIndex
+  }
+
+  def topologySortByKahn(): Array[Int] = {
+    val seq = new mutable.ArrayBuffer[Int]()
+    //inDgrees contains all the inDegree for a given node
+    val inDegrees = new Array[Int](vertex)
+    for (i <- Range(0, vertex)) {
+      for (j <- adjacency(i).indices) {
+        val index = adjacency(i).get(j).get
+        inDegrees(index) += 1
+      }
+    }
+
+    val queue = new mutable.Queue[Int]()
+    for (i <- inDegrees.indices) {
+      if (inDegrees(i) == 0) {
+        // means there is no inDegree for this node,
+        // this could be the starting point of the dependency graph
+        queue += i
+      }
+    }
+
+    //start to navigating the graph from the starting point
+    while (queue.nonEmpty) {
+      val index = queue.dequeue()
+
+      //push to the result
+      seq += index
+
+      for (i <- adjacency(index).indices) {
+        val inDegreeIndex = adjacency(index).get(i).get
+        inDegrees(inDegreeIndex) -= 1
+
+        if (inDegrees(inDegreeIndex) == 0) {
+          queue += inDegreeIndex
+        }
+      }
+    }
+
+    seq.toArray
+  }
+}
+
+

scala/src/test/scala/ch43_topology_sort/GraphTopologyTest.scala

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+package ch43_topology_sort
+
+import org.scalatest.{FlatSpec, Matchers}
+
+class GraphTopologyTest extends FlatSpec with Matchers {
+
+  behavior of "GraphTopologyTest"
+
+  /*
+     a -> b
+     |    |
+    \|/  \|/
+     c -> d -> e
+   */
+  it should "topologySortByKahn - 1" in {
+    val nodes = Array("a", "b", "c", "d", "e")
+    val graphTopology = new GraphTopology(nodes.length)
+    graphTopology.addEdge(0, 1)
+    graphTopology.addEdge(0, 2)
+    graphTopology.addEdge(1, 3)
+    graphTopology.addEdge(2, 3)
+    graphTopology.addEdge(3, 4)
+
+    val seq = graphTopology.topologySortByKahn()
+    seq.map(nodes(_)).mkString(",") should equal("a,b,c,d,e")
+  }
+
+  /*
+     a -> d ---
+          |    |
+         \|/  \|/
+          e -> c
+   */
+  it should "topologySortByKahn - 2" in {
+    val nodes = Array("a", "b", "c", "d", "e")
+    val graphTopology = new GraphTopology(nodes.length)
+    graphTopology.addEdge(0, 3)
+    graphTopology.addEdge(3, 4)
+    graphTopology.addEdge(3, 2)
+    graphTopology.addEdge(4, 2)
+
+    val seq = graphTopology.topologySortByKahn()
+    seq.map(nodes(_)).mkString(",") should equal("a,b,d,e,c")
+  }
+}