Prevent edit if nothing was formatted (google#120)

Handle trivial cases, where input equals output, to prevent dirty flags in active editors.

Author: sormuras

eclipse_plugin/src/com/google/googlejavaformat/java/GoogleJavaFormatter.java

@@ -82,10 +82,16 @@ private TextEdit formatInternal(int kind, String source, IRegion[] regions, int
         default:
           throw new IllegalArgumentException(String.format("Unknown snippet kind: %d", kind));
       }
-      return editFromReplacements(
+      List<Replacement> replacements =
           new SnippetFormatter()
               .format(
-                  snippetKind, source, rangesFromRegions(regions), initialIndent, includeComments));
+                  snippetKind, source, rangesFromRegions(regions), initialIndent, includeComments);
+      if (idempotent(source, regions, replacements)) {
+        // Do not create edits if there's no diff.
+        return null;
+      }
+      // Convert replacements to text edits.
+      return editFromReplacements(replacements);
     } catch (IllegalArgumentException | FormatterException exception) {
       // Do not format on errors.
       return null;
@@ -100,6 +106,28 @@ private List<Range<Integer>> rangesFromRegions(IRegion[] regions) {
     return ranges;
   }
 
+  /** @return {@code true} if input and output texts are equal, else {@code false}. */
+  private boolean idempotent(String source, IRegion[] regions, List<Replacement> replacements) {
+    // This implementation only checks for single replacement.
+    if (replacements.size() == 1) {
+      Replacement replacement = replacements.get(0);
+      String output = replacement.getReplacementString();
+      // Entire source case: input = output, nothing changed.
+      if (output.equals(source)) {
+        return true;
+      }
+      // Single region and single replacement case: if they are equal, nothing changed.
+      if (regions.length == 1) {
+        Range<Integer> range = replacement.getReplaceRange();
+        String snippet = source.substring(range.lowerEndpoint(), range.upperEndpoint());
+        if (output.equals(snippet)) {
+          return true;
+        }
+      }
+    }
+    return false;
+  }
+
   private TextEdit editFromReplacements(List<Replacement> replacements) {
     // Split the replacements that cross line boundaries.
     TextEdit edit = new MultiTextEdit();