Set MP4 chapters only if modified

An equality operator is added for the chapters. The chapters are
only written to the file if they were really modified, so just
reading the chapters without modifying them will not affect
the save operation.

tests/test_mp4.cpp

@@ -34,6 +34,7 @@
 #include "mp4atom.h"
 #include "mp4file.h"
 #include "mp4itemfactory.h"
+#include "mp4chapterholder.h"
 #include "plainfile.h"
 #include <cppunit/extensions/HelperMacros.h>
 #include "utils.h"
@@ -69,6 +70,56 @@ namespace
   };
 
   CustomItemFactory CustomItemFactory::factory;
+
+  class MockChapterList : public MP4::ChapterHolder {
+  public:
+    static const MP4::ChapterList mockChapters;
+
+    bool read(TagLib::File *)
+    {
+      chapterList = mockChapters;
+      ++readCount;
+      return true;
+    }
+
+    bool write(TagLib::File *)
+    {
+      ++writeCount;
+      return true;
+    }
+
+    int readCount = 0;
+    int writeCount = 0;
+  };
+
+  const MP4::ChapterList MockChapterList::mockChapters = {
+    MP4::Chapter("Mock", 123)
+  };
+
+  class MockChapterFile : public PlainFile {
+  public:
+    explicit MockChapterFile(FileName name) : PlainFile(name)
+    {
+    }
+
+    MP4::ChapterList chapters()
+    {
+      return getChaptersLazy(chapterList, this);
+    }
+
+    void setChapters(const MP4::ChapterList& chapters)
+    {
+      setChaptersLazy(chapterList, chapters);
+    }
+
+    bool save() override
+    {
+      return MP4::saveChaptersIfModified(chapterList, this);
+    }
+
+    std::unique_ptr<MockChapterList> chapterList;
+  };
+
 }  // namespace
 
 class TestMP4 : public CppUnit::TestFixture
@@ -121,6 +172,7 @@ class TestMP4 : public CppUnit::TestFixture
   CPPUNIT_TEST(testQTChapterListSingleEmptyTitleNotStripped);
   CPPUNIT_TEST(testNeroAndQTChaptersAreIndependent);
   CPPUNIT_TEST(testNeroChaptersAloneWhenNoQT);
+  CPPUNIT_TEST(testLazyReadingAndWritingChapters);
   CPPUNIT_TEST_SUITE_END();
 
 public:
@@ -1634,6 +1686,105 @@ public:
     }
   }
 
+  void testLazyReadingAndWritingChapters()
+  {
+    // No reads or writes if chapters are not used
+    {
+      MockChapterFile f(TEST_FILE_PATH_C("no-tags.m4a"));
+      f.save();
+      CPPUNIT_ASSERT(!f.chapterList);
+    }
+    // Do not read if already read, do not write if not modified
+    {
+      MockChapterFile f(TEST_FILE_PATH_C("no-tags.m4a"));
+      auto chapters = f.chapters();
+      CPPUNIT_ASSERT(chapters == MockChapterList::mockChapters);
+      CPPUNIT_ASSERT(f.chapterList);
+      CPPUNIT_ASSERT_EQUAL(1, f.chapterList->readCount);
+      chapters = f.chapters();
+      CPPUNIT_ASSERT(chapters == MockChapterList::mockChapters);
+      CPPUNIT_ASSERT_EQUAL(1, f.chapterList->readCount);
+      f.save();
+      CPPUNIT_ASSERT_EQUAL(1, f.chapterList->readCount);
+      CPPUNIT_ASSERT_EQUAL(0, f.chapterList->writeCount);
+    }
+    // Do not write if not modified
+    {
+      MockChapterFile f(TEST_FILE_PATH_C("no-tags.m4a"));
+      auto chapters = f.chapters();
+      CPPUNIT_ASSERT(chapters == MockChapterList::mockChapters);
+      CPPUNIT_ASSERT(f.chapterList);
+      CPPUNIT_ASSERT_EQUAL(1, f.chapterList->readCount);
+      f.setChapters(MockChapterList::mockChapters);
+      f.save();
+      CPPUNIT_ASSERT_EQUAL(1, f.chapterList->readCount);
+      CPPUNIT_ASSERT_EQUAL(0, f.chapterList->writeCount);
+    }
+    // Write if set without being read before
+    {
+      MockChapterFile f(TEST_FILE_PATH_C("no-tags.m4a"));
+      f.setChapters(MP4::ChapterList());
+      f.save();
+      CPPUNIT_ASSERT(f.chapterList);
+      CPPUNIT_ASSERT_EQUAL(0, f.chapterList->readCount);
+      CPPUNIT_ASSERT_EQUAL(1, f.chapterList->writeCount);
+    }
+    // Do write if modified
+    {
+      MockChapterFile f(TEST_FILE_PATH_C("no-tags.m4a"));
+      CPPUNIT_ASSERT(f.chapters() == MockChapterList::mockChapters);
+      CPPUNIT_ASSERT(f.chapterList);
+      CPPUNIT_ASSERT_EQUAL(1, f.chapterList->readCount);
+      const auto chapters1 = MP4::ChapterList({
+        MP4::Chapter("Chapter 1", 0),
+      });
+      f.setChapters(chapters1);
+      CPPUNIT_ASSERT(f.chapters() == chapters1);
+      f.save();
+      CPPUNIT_ASSERT(f.chapters() == chapters1);
+      CPPUNIT_ASSERT_EQUAL(1, f.chapterList->readCount);
+      CPPUNIT_ASSERT_EQUAL(1, f.chapterList->writeCount);
+      f.setChapters(chapters1);
+      f.save();
+      CPPUNIT_ASSERT_EQUAL(1, f.chapterList->writeCount);
+      auto chapters2 = MP4::ChapterList({
+        MP4::Chapter("Chapter 1", 0),
+        MP4::Chapter("Chapter 2", 1),
+      });
+      f.setChapters(chapters2);
+      CPPUNIT_ASSERT(f.chapters() == chapters2);
+      f.save();
+      CPPUNIT_ASSERT(f.chapters() == chapters2);
+      CPPUNIT_ASSERT_EQUAL(2, f.chapterList->writeCount);
+      chapters2 = MP4::ChapterList({
+        MP4::Chapter("Chapter 1", 0),
+        MP4::Chapter("Chapter 2", 2),
+      });
+      f.setChapters(chapters2);
+      f.save();
+      CPPUNIT_ASSERT_EQUAL(3, f.chapterList->writeCount);
+      f.setChapters(chapters2);
+      CPPUNIT_ASSERT(f.chapters() == chapters2);
+      f.save();
+      CPPUNIT_ASSERT(f.chapters() == chapters2);
+      CPPUNIT_ASSERT_EQUAL(3, f.chapterList->writeCount);
+      const auto chapters3 = MP4::ChapterList({
+        MP4::Chapter("Chapter 1", 0),
+        MP4::Chapter("Chapter 3", 2),
+      });
+      f.setChapters(chapters3);
+      CPPUNIT_ASSERT(f.chapters() == chapters3);
+      f.save();
+      CPPUNIT_ASSERT(f.chapters() == chapters3);
+      CPPUNIT_ASSERT_EQUAL(4, f.chapterList->writeCount);
+      f.setChapters(MP4::ChapterList());
+      CPPUNIT_ASSERT(f.chapters().isEmpty());
+      f.save();
+      CPPUNIT_ASSERT(f.chapters().isEmpty());
+      CPPUNIT_ASSERT_EQUAL(5, f.chapterList->writeCount);
+    }
+  }
+
 };
 
 CPPUNIT_TEST_SUITE_REGISTRATION(TestMP4);