Add `ErdosRenyi graph model for tests

build.gradle.kts

@@ -30,7 +30,10 @@ kotlin {
   sourceSets {
     val commonMain by getting { dependencies { implementation(kotlin("stdlib-common")) } }
     val commonTest by getting { dependencies { implementation(kotlin("test")) } }
-    all { languageSettings.optIn("kotlin.contracts.ExperimentalContracts") }
+    all {
+      languageSettings.optIn("kotlin.contracts.ExperimentalContracts")
+      languageSettings.optIn("kotlin.time.ExperimentalTime")
+    }
   }
 }
 

src/commonTest/kotlin/io/github/alexandrepiveteau/graphs/algorithms/KosarajuTests.kt

@@ -1,10 +1,13 @@
 package io.github.alexandrepiveteau.graphs.algorithms
 
+import io.github.alexandrepiveteau.graphs.DirectedGraph
 import io.github.alexandrepiveteau.graphs.VertexArray
 import io.github.alexandrepiveteau.graphs.arcTo
 import io.github.alexandrepiveteau.graphs.asIntArray
 import io.github.alexandrepiveteau.graphs.builder.buildDirectedGraph
+import io.github.alexandrepiveteau.graphs.builder.erdosRenyi
 import io.github.alexandrepiveteau.graphs.util.Repeats
+import kotlin.random.Random
 import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertNotEquals
@@ -125,4 +128,15 @@ class KosarajuTests {
     assertNotEquals(scc[map[graph[2]]], scc[map[graph[4]]])
     assertNotEquals(scc[map[graph[3]]], scc[map[graph[4]]])
   }
+
+  @Test
+  fun randomGraphsHaveTopologicalSortAfterScc() {
+    val random = Random(42)
+    repeat(Repeats) {
+      val graph = DirectedGraph.erdosRenyi(n = 1_000, p = 0.1, random = random)
+      val (scc, map) = graph.stronglyConnectedComponentsKosaraju()
+      val order = scc.topologicalSort()
+      assertEquals(map.values().asIntArray().distinct().count(), order.size)
+    }
+  }
 }

src/commonTest/kotlin/io/github/alexandrepiveteau/graphs/builder/ErdosRenyi.kt

@@ -0,0 +1,50 @@
+package io.github.alexandrepiveteau.graphs.builder
+
+import io.github.alexandrepiveteau.graphs.*
+import kotlin.random.Random
+
+/**
+ * Creates a random [UndirectedGraph] with [n] vertices and an edge between each pair of vertices
+ * with probability [p].
+ *
+ * @param n the number of vertices in the graph.
+ * @param p the probability of an edge between two vertices.
+ * @param random the [Random] instance to use.
+ */
+fun UndirectedGraph.Companion.erdosRenyi(
+    n: Int,
+    p: Double,
+    random: Random = Random,
+) = buildUndirectedGraph {
+  val vertices = VertexArray(n) { addVertex() }
+  for (i in 0 until n) {
+    for (j in i + 1 until n) {
+      if (random.nextDouble() < p) {
+        addEdge(vertices[i] edgeTo vertices[j])
+      }
+    }
+  }
+}
+
+/**
+ * Creates a random [DirectedGraph] with [n] vertices and an arc between each pair of vertices with
+ * probability [p].
+ *
+ * @param n the number of vertices in the graph.
+ * @param p the probability of an arc between two vertices.
+ * @param random the [Random] instance to use.
+ */
+fun DirectedGraph.Companion.erdosRenyi(
+    n: Int,
+    p: Double,
+    random: Random = Random,
+) = buildDirectedGraph {
+  val vertices = VertexArray(n) { addVertex() }
+  for (i in 0 until n) {
+    for (j in 0 until n) {
+      if (random.nextDouble() < p) {
+        addArc(vertices[i] arcTo vertices[j])
+      }
+    }
+  }
+}