New implementation of modification_queue...

...that preserves order when priorities are equal.

Fixes bug #20196.

changelog

@@ -44,6 +44,7 @@ Version 1.11.0+svn:
    * Added [terrain_type] max_light= and min_light=.
    * Standardize weapon filters, supporting special=, [and], [or], and [not]
      wherever weapons can be filtered.
+   * Image path functions again evaluated left-to-right. Fixes bug #20196.
  * Miscellaneous and bug fixes:
    * Fix invalid memory access crash resulting from deleting all saved games
      in the Load Game dialog

src/image_modifications.cpp

@@ -35,6 +35,43 @@ static lg::log_domain log_display("display");
 namespace image {
 
 
+/** Adds @a mod to the queue (unless mod is NULL). */
+void modification_queue::push(modification * mod)
+{
+	// Null pointers do not get stored. (Shouldn't happen, but just in case.)
+	if ( mod != NULL )
+		priorities_[mod->priority()].push_back(mod);
+}
+
+/** Removes the top element from the queue */
+void modification_queue::pop()
+{
+	map_type::iterator top_pair = priorities_.begin();
+	std::vector<modification *> & top_vector = top_pair->second;
+
+	// Erase the top element.
+	top_vector.erase(top_vector.begin());
+	if ( top_vector.empty() )
+		// We need to keep the map clean.
+		priorities_.erase(top_pair);
+}
+
+/** Returns the number of elements in the queue. */
+size_t modification_queue::size() const
+{
+	size_t count = 0;
+	BOOST_FOREACH ( const map_type::value_type & pair, priorities_ )
+		count += pair.second.size();
+	return count;
+}
+
+/** Returns the top element in the queue . */
+modification * modification_queue::top() const
+{
+	return priorities_.begin()->second.front();
+}
+
+
 namespace {
 
 /// A function used to parse modification arguments
@@ -113,13 +150,6 @@ modification_queue modification::decode(const std::string& encoded_mods)
 	return mods;
 }
 
-/// Compares two modification pointers, providing descending priority order
-bool mod_ptr_comparator_::operator()(const modification* a,
-				     const modification* b) const
-{
-	return a->priority() < b->priority();
-}
-
 surface rc_modification::operator()(const surface& src) const
 {
 	// unchecked

src/image_modifications.hpp

@@ -24,11 +24,32 @@
 namespace image {
 
 class modification;
-struct mod_ptr_comparator_;
-/// A priority queue used to enforce using the rc modifications first
-typedef std::priority_queue<modification*,
-			    std::vector<modification*>,
-			    mod_ptr_comparator_> modification_queue;
+
+
+/// A modified priority queue used to order image modifications.
+/// The priorities for this queue are to order modifications by priority(),
+/// then by the order they are added to the queue.
+class modification_queue {
+	// Invariant for this class:
+	// At the beginning and end of each member function call, there
+	// are no empty vectors in priorities_.
+public:
+	modification_queue() {}
+	~modification_queue() {}
+
+	bool empty() const  { return priorities_.empty(); }
+	void push(modification * mod);
+	void pop();
+	size_t size() const;
+	modification * top() const;
+
+private: // data
+	/// Map from a mod's priority() to the mods having that priority.
+	typedef std::map<int, std::vector<modification *>, std::greater<int> > map_type;
+	/// Map from a mod's priority() to the mods having that priority.
+	map_type priorities_;
+};
+
 
 /// Base abstract class for an image-path modification
 class modification
@@ -81,13 +102,6 @@ public:
 	virtual int priority() const { return 0; }
 };
 
-/// A functor for comparing modification pointers
-struct mod_ptr_comparator_
-{
-	/// Provides a descending priority ordering
-	bool operator()(const modification* a, const modification* b) const;
-};
-
 /**
  * Recolor (RC/TC/PAL) modification.
  * It is used not only for color-range-based recoloring ("~RC(magenta>teal)")