Flesh out OrderedGroups.

1. Add the ability to bulk remove elements.
2. Add accessor for elements_to_group.
3. Early exit in Remove if there are no elements.

Change-Id: I3df1f00de05338a42e9907423b674469a022d3bc

include/ceres/ordered_groups.h

@@ -33,6 +33,7 @@
 
 #include <map>
 #include <set>
+#include <vector>
 #include "ceres/internal/port.h"
 
 namespace ceres {
@@ -103,6 +104,20 @@ class OrderedGroups {
     return true;
   }
 
+  // Bulk remove elements. The return value indicates the number of
+  // elements successfully removed.
+  int Remove(const vector<T>& elements) {
+    if (NumElements() == 0 || elements.size() == 0) {
+      return 0;
+    }
+
+    int num_removed = 0;
+    for (int i = 0; i < elements.size(); ++i) {
+      num_removed += Remove(elements[i]);
+    }
+    return num_removed;
+  }
+
   // Reverse the order of the groups in place.
   void Reverse() {
     typename map<int, set<T> >::reverse_iterator it =
@@ -160,6 +175,10 @@ class OrderedGroups {
     return group_to_elements_;
   }
 
+  const map<T, int>& element_to_group() const {
+    return element_to_group_;
+  }
+
  private:
   map<int, set<T> > group_to_elements_;
   map<T, int> element_to_group_;