Fix O(n^2) behaviour in parser.

Despite what the c++11 spec says, std::list::size() is
still O(n), not O(1).

R=dsinclair@chromium.org, ochang@chromium.org

Review URL: https://codereview.chromium.org/1763443003 .

2 files changed, 4 insertions, 10 deletions

diff --git a/core/include/fpdfapi/fpdf_page.h b/core/include/fpdfapi/fpdf_page.h
index 4c880525f1..7c27340525 100644
--- a/core/include/fpdfapi/fpdf_page.h
+++ b/core/include/fpdfapi/fpdf_page.h
@@ -7,7 +7,7 @@
 #ifndef CORE_INCLUDE_FPDFAPI_FPDF_PAGE_H_
 #define CORE_INCLUDE_FPDFAPI_FPDF_PAGE_H_
 
-#include <list>
+#include <deque>
 #include <memory>
 
 #include "core/include/fpdfapi/fpdf_parser.h"
@@ -27,9 +27,9 @@ class CPDF_ImageObject;
 #define PDFTRANS_ISOLATED 0x0200
 #define PDFTRANS_KNOCKOUT 0x0400
 
-class CPDF_PageObjectList : public std::list<std::unique_ptr<CPDF_PageObject>> {
+class CPDF_PageObjectList
+    : public std::deque<std::unique_ptr<CPDF_PageObject>> {
  public:
-  // Linear complexity, to be avoided except as needed by public APIs.
   CPDF_PageObject* GetPageObjectByIndex(int index);
 };
 
diff --git a/core/src/fpdfapi/fpdf_page/fpdf_page.cpp b/core/src/fpdfapi/fpdf_page/fpdf_page.cpp
index 301fca8969..77707624d2 100644
--- a/core/src/fpdfapi/fpdf_page/fpdf_page.cpp
+++ b/core/src/fpdfapi/fpdf_page/fpdf_page.cpp
@@ -462,13 +462,7 @@ void CPDF_FormObject::CalcBoundingBox() {
 CPDF_PageObject* CPDF_PageObjectList::GetPageObjectByIndex(int index) {
   if (index < 0 || index >= pdfium::CollectionSize<int>(*this))
     return nullptr;
-  int current = 0;
-  for (const auto& pObj : *this) {
-    if (index == current)
-      return pObj.get();
-    ++current;
-  }
-  return nullptr;
+  return (*this)[index].get();
 }
 
 CPDF_PageObjectHolder::CPDF_PageObjectHolder()