From f1303fe37b0f143d87c5d03ea28c898a95a57830 Mon Sep 17 00:00:00 2001
From: Oshgnacknak <osh@oshgnacknak.de>
Date: Sun, 5 Sep 2021 22:44:37 +0200
Subject: [PATCH] Introduce MinimumSpanningForestAlgorithm and implement
 Kruskal

---
 src/main/java/h07/algorithm/Kruskal.java      | 62 +++++++++++++++++++
 .../MinimumSpanningForestAlgorithm.java       | 11 ++++
 src/main/java/h07/algorithm/Pair.java         | 11 ++++
 3 files changed, 84 insertions(+)
 create mode 100644 src/main/java/h07/algorithm/Kruskal.java
 create mode 100644 src/main/java/h07/algorithm/MinimumSpanningForestAlgorithm.java
 create mode 100644 src/main/java/h07/algorithm/Pair.java

diff --git a/src/main/java/h07/algorithm/Kruskal.java b/src/main/java/h07/algorithm/Kruskal.java
new file mode 100644
index 0000000..77aabae
--- /dev/null
+++ b/src/main/java/h07/algorithm/Kruskal.java
@@ -0,0 +1,62 @@
+package h07.algorithm;
+
+import aud.exam.prep.Pair;
+import h07.graph.DirectedGraph;
+import h07.graph.DirectedGraphFactory;
+
+import java.util.*;
+
+public class Kruskal<V, A> implements MinimumSpanningForestAlgorithm<V, A> {
+
+    @Override
+    public DirectedGraph<V, A> minimumSpanningForest(DirectedGraph<V, A> graph, Comparator<? super A> comparator, DirectedGraphFactory<V, A> factory) {
+        var output = factory.createDirectedGraph();
+        var sets = new ArrayList<Set<V>>();
+
+        for (V node : graph.getAllNodes()) {
+            output.addNode(node);
+
+            var set = new HashSet<V>();
+            set.add(node);
+            sets.add(set);
+        }
+
+        var egdes = graph.getAllNodes()
+            .stream()
+            .flatMap(n ->
+                graph.getChildrenForNode(n)
+                    .stream()
+                    .map(c ->
+                        new Pair<>(n, c)))
+            .sorted(Comparator.comparing(
+                p -> graph.getArcWeightBetween(p.fst, p.snd),
+                comparator));
+
+        egdes.forEach(p -> {
+            var s1 = sets
+                .stream()
+                .filter(s -> s.contains(p.fst))
+                .findFirst()
+                .orElseThrow();
+
+            if (s1.contains(p.snd)) {
+                return;
+            }
+
+            var s2 = sets
+                .stream()
+                .filter(s -> s.contains(p.snd))
+                .findFirst()
+                .orElseThrow();
+
+            s1.addAll(s2);
+            sets.remove(s2);
+
+            var arc = graph.getArcWeightBetween(p.fst, p.snd);
+            output.connectNodes(p.fst, arc, p.snd);
+            output.connectNodes(p.snd, arc, p.fst);
+        });
+
+        return output;
+    }
+}
diff --git a/src/main/java/h07/algorithm/MinimumSpanningForestAlgorithm.java b/src/main/java/h07/algorithm/MinimumSpanningForestAlgorithm.java
new file mode 100644
index 0000000..72b78be
--- /dev/null
+++ b/src/main/java/h07/algorithm/MinimumSpanningForestAlgorithm.java
@@ -0,0 +1,11 @@
+package h07.algorithm;
+
+import h07.graph.DirectedGraph;
+import h07.graph.DirectedGraphFactory;
+
+import java.util.Comparator;
+
+public interface MinimumSpanningForestAlgorithm<V, A> {
+
+    DirectedGraph<V, A> minimumSpanningForest(DirectedGraph<V, A> graph, Comparator<? super A> comparator, DirectedGraphFactory<V, A> factory);
+}
diff --git a/src/main/java/h07/algorithm/Pair.java b/src/main/java/h07/algorithm/Pair.java
new file mode 100644
index 0000000..71c4184
--- /dev/null
+++ b/src/main/java/h07/algorithm/Pair.java
@@ -0,0 +1,11 @@
+package h07.algorithm;
+
+public class Pair<T, S> {
+    public T fst;
+    public S snd;
+
+    public Pair(T fst, S snd) {
+        this.fst = fst;
+        this.snd = snd;
+    }
+}