2 files changed, 16 insertions, 0 deletions
diff --git a/core/fpdfapi/page/cpdf_contentmark.cpp b/core/fpdfapi/page/cpdf_contentmark.cpp
index 29d1bba210..1ff567d9e6 100644
--- a/core/fpdfapi/page/cpdf_contentmark.cpp
+++ b/core/fpdfapi/page/cpdf_contentmark.cpp
@@ -6,6 +6,7 @@
 
 #include "core/fpdfapi/page/cpdf_contentmark.h"
 
+#include <algorithm>
 #include <utility>
 
 #include "core/fpdfapi/parser/cpdf_dictionary.h"
@@ -79,6 +80,20 @@ void CPDF_ContentMark::DeleteLastMark() {
     m_pMarkData.Reset();
 }
 
+size_t CPDF_ContentMark::FindFirstDifference(
+    const CPDF_ContentMark* other) const {
+  if (m_pMarkData == other->m_pMarkData)
+    return CountItems();
+
+  size_t min_len = std::min(CountItems(), other->CountItems());
+
+  for (size_t i = 0; i < min_len; ++i) {
+    if (GetItem(i) != other->GetItem(i))
+      return i;
+  }
+  return min_len;
+}
+
 CPDF_ContentMark::MarkData::MarkData() {}
 
 CPDF_ContentMark::MarkData::MarkData(const MarkData& src)
diff --git a/core/fpdfapi/page/cpdf_contentmark.h b/core/fpdfapi/page/cpdf_contentmark.h
index 33180333af..8bbae52418 100644
--- a/core/fpdfapi/page/cpdf_contentmark.h
+++ b/core/fpdfapi/page/cpdf_contentmark.h
@@ -35,6 +35,7 @@ class CPDF_ContentMark {
                                  CPDF_Dictionary* pDict,
                                  const ByteString& property_name);
   void DeleteLastMark();
+  size_t FindFirstDifference(const CPDF_ContentMark* other) const;
 
  private:
   class MarkData : public Retainable {