Fix StringIndexOutOfBoundsException when resolving a self $ref '#'

src/main/java/io/vertx/json/schema/impl/JsonRef.java

@@ -320,7 +320,9 @@ private static JsonObject resolveUri(String uri, Map<String, JsonObject> anchors
     //  [prefix, path]
     final String[] parts = uri.split("#", 2);
 
-    final boolean hashPresent = parts.length == 2 && parts[1] != null;
+    // an empty fragment ("#" or "id#") is equivalent to no fragment at all: the uri refers
+    // to the schema identified by the prefix, e.g.: the root schema when the prefix is empty
+    final boolean hashPresent = parts.length == 2 && parts[1] != null && !parts[1].isEmpty();
 
     final String prefix = parts[0];
     final String path = hashPresent ? parts[1] : null;

src/test/java/io/vertx/tests/impl/RefTest.java

@@ -203,6 +203,83 @@ public void testCase2() {
       "string");
   }
 
+  @Test
+  public void testRefToRoot() {
+    JsonObject schema = new JsonObject()
+      .put("type", "object")
+      .put("properties", new JsonObject()
+        .put("children", new JsonObject()
+          .put("type", "array")
+          .put("items", new JsonObject()
+            .put("$ref", "#"))));
+
+    JsonObject resolved = JsonRef.resolve(schema);
+
+    JsonObject items = resolved
+      .getJsonObject("properties")
+      .getJsonObject("children")
+      .getJsonObject("items");
+    assertEquals("object", items.getString("type"));
+    // the ref points back to the root schema itself
+    assertNotNull(
+      items
+        .getJsonObject("properties")
+        .getJsonObject("children")
+        .getJsonObject("items"));
+  }
+
+  @Test
+  public void testRefToRootWithId() {
+    JsonObject schema = new JsonObject()
+      .put("$id", "http://www.example.com/tree/")
+      .put("type", "object")
+      .put("properties", new JsonObject()
+        .put("children", new JsonObject()
+          .put("type", "array")
+          .put("items", new JsonObject()
+            .put("$ref", "#"))));
+
+    JsonObject resolved = JsonRef.resolve(schema);
+
+    JsonObject items = resolved
+      .getJsonObject("properties")
+      .getJsonObject("children")
+      .getJsonObject("items");
+    assertEquals("object", items.getString("type"));
+    assertNotNull(
+      items
+        .getJsonObject("properties")
+        .getJsonObject("children")
+        .getJsonObject("items"));
+  }
+
+  @Test
+  public void testRefToRootThroughRepository() {
+    SchemaRepository repo =
+      SchemaRepository.create(new JsonSchemaOptions().setBaseUri("http://vertx.io").setDraft(Draft.DRAFT4));
+    JsonObject schema = new JsonObject()
+      .put("type", "object")
+      .put("properties", new JsonObject()
+        .put("children", new JsonObject()
+          .put("type", "array")
+          .put("items", new JsonObject()
+            .put("$ref", "#"))));
+    repo.dereference(JsonSchema.of(schema.copy()));
+
+    JsonObject resolved = repo.resolve(schema);
+
+    JsonObject items = resolved
+      .getJsonObject("properties")
+      .getJsonObject("children")
+      .getJsonObject("items");
+    assertEquals("object", items.getString("type"));
+    assertNotNull(
+      items
+        .getJsonObject("properties")
+        .getJsonObject("children")
+        .getJsonObject("items"));
+  }
+
   @Test
   void testSerialization() {
     SchemaRepository repo =