Add space after // in single-line comments (#854)

* Add recipe to enforce space after // in single-line comments

* fix review comments

* Use ListUtils.map as suggested by reviewer

* fix review comments: use @Getter fields, skip block comments, update copyright year

* Invert instanceof check and combine with isMultiline guard

* Skip noinspection and region/endregion IDE comments

---------

Co-authored-by: Tim te Beek <tim@moderne.io>

Author: avimta

src/main/java/org/openrewrite/staticanalysis/SingleLineCommentSpacing.java

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2026 the original author or authors.
+ * <p>
+ * Licensed under the Moderne Source Available License (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * https://docs.moderne.io/licensing/moderne-source-available-license
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.openrewrite.staticanalysis;
+
+import lombok.Getter;
+import org.openrewrite.ExecutionContext;
+import org.openrewrite.Recipe;
+import org.openrewrite.TreeVisitor;
+import org.openrewrite.internal.ListUtils;
+import org.openrewrite.java.JavaIsoVisitor;
+import org.openrewrite.java.tree.Space;
+import org.openrewrite.java.tree.TextComment;
+
+import java.time.Duration;
+import java.util.Set;
+
+import static java.util.Collections.singleton;
+
+@Getter
+public class SingleLineCommentSpacing extends Recipe {
+
+    final String displayName = "Add space after // in single-line comments";
+
+    final String description = "Ensures there is exactly one space after // in single-line comments when missing.";
+
+    final Set<String> tags = singleton("formatting");
+
+    final Duration estimatedEffortPerOccurrence = Duration.ofMinutes(1);
+
+    @Override
+    public TreeVisitor<?, ExecutionContext> getVisitor() {
+        return new JavaIsoVisitor<ExecutionContext>() {
+
+            @Override
+            public Space visitSpace(Space space, Space.Location loc, ExecutionContext ctx) {
+                Space s = super.visitSpace(space, loc, ctx);
+
+                return s.withComments(ListUtils.map(s.getComments(), comment -> {
+                    if (!(comment instanceof TextComment) || ((TextComment) comment).isMultiline()) {
+                        return comment;
+                    }
+                    TextComment tc = (TextComment) comment;
+                    String text = tc.getText();
+                    if (text.isEmpty() || text.startsWith(" ") ||
+                            text.startsWith("language=") ||
+                            text.startsWith("noinspection") ||
+                            text.startsWith("region") ||
+                            text.startsWith("endregion")) {
+                        return comment;
+                    }
+                    return tc.withText(" " + text);
+                }));
+            }
+        };
+    }
+}