From 7777ea84e7117a305a74450ae37ed3ecb7739583 Mon Sep 17 00:00:00 2001
From: m-holger <m.holger@qpdf.org>
Date: Sat, 31 Aug 2024 20:15:19 +0100
Subject: [PATCH] Add new method ObjTable::emplace_back

---
 libqpdf/qpdf/ObjTable.hh               | 29 ++++++++++++++++++++++++++
 libtests/obj_table.cc                  | 11 +++++++++-
 libtests/qtest/obj_table/obj_table.out | 14 ++++++++++++-
 3 files changed, 52 insertions(+), 2 deletions(-)

diff --git a/libqpdf/qpdf/ObjTable.hh b/libqpdf/qpdf/ObjTable.hh
index dd762214..3a36208d 100644
--- a/libqpdf/qpdf/ObjTable.hh
+++ b/libqpdf/qpdf/ObjTable.hh
@@ -28,6 +28,7 @@ template <class T>
 class ObjTable: public std::vector<T>
 {
   public:
+    using reference = T&;
     ObjTable() = default;
     ObjTable(const ObjTable&) = delete;
     ObjTable(ObjTable&&) = delete;
@@ -99,6 +100,18 @@ class ObjTable: public std::vector<T>
         return element(id);
     }
 
+    // emplace_back to the end of the vector. If there are any conflicting sparse elements, emplace
+    // them to the back of the vector before adding the current element.
+    template <class... Args>
+    inline T&
+    emplace_back(Args&&... args)
+    {
+        if (min_sparse == std::vector<T>::size()) {
+            return emplace_back_large(std::forward<Args&&...>(args...));
+        }
+        return std::vector<T>::emplace_back(std::forward<Args&&...>(args...));
+    }
+
     void
     resize(size_t a_size)
     {
@@ -168,6 +181,22 @@ class ObjTable: public std::vector<T>
         throw std::runtime_error("Impossibly large object id encountered accessing ObjTable");
         return element(0); // doesn't return
     }
+
+    // Must only be called by emplace_back. Separated out from emplace_back to keep inlined code
+    // tight.
+    template <class... Args>
+    T&
+    emplace_back_large(Args&&... args)
+    {
+        auto it = sparse_elements.begin();
+        auto end = sparse_elements.end();
+        while (it != end && it->first == std::vector<T>::size()) {
+            std::vector<T>::emplace_back(std::move(it->second));
+            it = sparse_elements.erase(it);
+        }
+        min_sparse = (it == end) ? std::numeric_limits<size_t>::max() : it->first;
+        return std::vector<T>::emplace_back(std::forward<Args&&...>(args...));
+    }
 };
 
 #endif // OBJTABLE_HH
diff --git a/libtests/obj_table.cc b/libtests/obj_table.cc
index 96ca9fb1..2b58eca7 100644
--- a/libtests/obj_table.cc
+++ b/libtests/obj_table.cc
@@ -2,6 +2,11 @@
 
 struct Test
 {
+    Test() = default;
+    Test(int value) :
+        value(value)
+    {
+    }
     int value{0};
 };
 
@@ -24,11 +29,15 @@ class Table: public ObjTable<Test>
             (*this)[i].value = 2 * i;
         }
         resize(100);
-        for (int i: {1, 99, 100, 105, 110, 120, 220}) {
+        for (int i: {1, 99, 100, 105, 110, 120, 205, 206, 207, 210}) {
             (*this)[i].value = 3 * i;
         }
         resize(200);
 
+        for (int i = 1; i < 10; ++i) {
+            emplace_back(i);
+        }
+
         forEach([](auto i, auto const& item) -> void {
             if (item.value) {
                 std::cout << std::to_string(i) << " : " << std::to_string(item.value) << "\n";
diff --git a/libtests/qtest/obj_table/obj_table.out b/libtests/qtest/obj_table/obj_table.out
index 6454c26b..c5642855 100644
--- a/libtests/qtest/obj_table/obj_table.out
+++ b/libtests/qtest/obj_table/obj_table.out
@@ -22,7 +22,19 @@
 199 : 398
 200 : 400
 201 : 402
-220 : 660
+202 : 1
+203 : 2
+204 : 3
+205 : 615
+206 : 618
+207 : 621
+208 : 4
+209 : 5
+210 : 630
+211 : 6
+212 : 7
+213 : 8
+214 : 9
 1000 : 2000
 1001 : 2002
 1002 : 2004