Introduce PluginDependencyTracker

scm-webapp/src/main/java/sonia/scm/plugin/PluginDependencyTracker.java

@@ -0,0 +1,42 @@
+package sonia.scm.plugin;
+
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+
+import static sonia.scm.ScmConstraintViolationException.Builder.doThrow;
+
+class PluginDependencyTracker {
+
+  private final Map<String, Collection<String>> plugins = new HashMap<>();
+
+  void addInstalled(PluginDescriptor plugin) {
+    plugin.getDependencies().forEach(dependency -> addDependency(plugin.getInformation().getName(), dependency));
+  }
+
+  void removeInstalled(PluginDescriptor plugin) {
+    doThrow()
+      .violation("Plugin is needed as a dependency for other plugins", "plugin")
+      .when(!mayUninstall(plugin.getInformation().getName()));
+    plugin.getDependencies().forEach(dependency -> removeDependency(plugin.getInformation().getName(), dependency));
+  }
+
+  boolean mayUninstall(String name) {
+    return plugins.computeIfAbsent(name, x -> new HashSet<>()).isEmpty();
+  }
+
+  private void addDependency(String from, String to) {
+    plugins.computeIfAbsent(to, name -> createInitialDependencyCollection(from));
+  }
+
+  private void removeDependency(String from, String to) {
+    plugins.get(to).remove(from);
+  }
+
+  private Collection<String> createInitialDependencyCollection(String from) {
+    Collection<String> dependencies = new HashSet<>();
+    dependencies.add(from);
+    return dependencies;
+  }
+}