Use a more efficient checker for cached images

chunk.py

@@ -169,6 +169,7 @@ class ChunkRenderer(object):
         moredirs, dir2 = os.path.split(destdir)
         _, dir1 = os.path.split(moredirs)
         self.cachedir = os.path.join(cachedir, dir1, dir2)
+        self.dirbits = (dir1, dir2)
 
 
         if self.world.useBiomeData:
@@ -301,16 +302,12 @@ class ChunkRenderer(object):
         return self._digest
 
     def find_oldimage(self, cave):
-        # Get the name of the existing image. No way to do this but to look at
-        # all the files
+        # Get the name of the existing image.
         oldimg = oldimg_path = None
-        for filename in os.listdir(self.cachedir):
-            if filename.startswith("img.{0}.{1}.".format(self.blockid,
-                    "cave" if cave else "nocave")) and \
-                    filename.endswith(".png"):
-                oldimg = filename
-                oldimg_path = os.path.join(self.cachedir, oldimg)
-                break
+        key = ".".join((dirbits[0], dirbits[1], self.blockid, "cave" if cave else "nocave"))
+        if key in self.world.cached:
+             oldimg_path = self.world.cached[key]
+             _, oldimg = os.path.split(oldimg_path)
         return oldimg, oldimg_path
 
     def render_and_save(self, cave=False):

world.py

@@ -21,6 +21,7 @@ import Queue
 import sys
 import logging
 import cPickle
+import collections
 
 import numpy
 
@@ -107,6 +108,20 @@ class WorldRenderer(object):
             textures.prepareBiomeData(worlddir)
 
         self.chunklist = chunklist
+        self.cached = collections.defaultdict(dict)
+
+        # In order to avoid having to look up the cache file names in
+        # ChunkRenderer, get them all and store them here
+        for root, dirnames, filenames in os.walk('.'):
+            for filename in filenames:
+                if not filename.endswith('.png'):
+                    continue
+                dirname, dir_b = os.path.split(root)
+                _, dir_a = os.path.split(dirname)
+                _, x, z, cave, _ = filename.split('.', 4)
+                bits = '.'.join((dir_a, dir_b, x, z, cave))
+                self.cached[bits] = os.path.join(root, filename)
+
 
         #  stores Points Of Interest to be mapped with markers
         #  a list of dictionaries, see below for an example