added caching to chunks in RegionSet() currently disabled

This build is broken due to the mismatch in expected and actual error
condition for get_chunk. The C code still expects it to return None.

overviewer_core/cache.py

@@ -0,0 +1,62 @@
+#    This file is part of the Minecraft Overviewer.
+#
+#    Minecraft Overviewer is free software: you can redistribute it and/or
+#    modify it under the terms of the GNU General Public License as published
+#    by the Free Software Foundation, either version 3 of the License, or (at
+#    your option) any later version.
+#
+#    Minecraft Overviewer is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General
+#    Public License for more details.
+#
+#    You should have received a copy of the GNU General Public License along
+#    with the Overviewer.  If not, see <http://www.gnu.org/licenses/>.
+
+"""This module has supporting functions for the caching logic used in world.py.
+
+"""
+import functools
+
+def lru_cache(max_size=100):
+    """A quick-and-dirty LRU implementation.
+    Uses a dict to store mappings, and a list to store orderings.
+
+    Only supports positional arguments
+
+    """
+    def lru_decorator(fun):
+
+        cache = {}
+        lru_ordering = []
+        
+        @functools.wraps(fun)
+        def new_fun(*args):
+            try:
+                result = cache[args]
+            except KeyError:
+                # cache miss =(
+                new_fun.miss += 1
+                result = fun(*args)
+
+                # Insert into cache
+                cache[args] = result
+                lru_ordering.append(args)
+
+                if len(cache) > max_size:
+                    # Evict an item
+                    del cache[ lru_ordering.pop(0) ]
+
+            else:
+                # Move the result item to the end of the list
+                new_fun.hits += 1
+                position = lru_ordering.index(args)
+                lru_ordering.append(lru_ordering.pop(position))
+
+            return result
+
+        new_fun.hits = 0
+        new_fun.miss = 0
+        return new_fun
+
+    return lru_decorator