From 854983f275dd6b6f3f8185403f4a618e8bad8c81 Mon Sep 17 00:00:00 2001
From: Adam Coldrick <adam.coldrick@codethink.co.uk>
Date: Thu, 22 Jan 2015 10:58:53 +0000
Subject: Add a cache manager utilising pylru and pickle

---
 morphlib/sourceresolver.py | 51 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/morphlib/sourceresolver.py b/morphlib/sourceresolver.py
index d9ffd049..35f65729 100644
--- a/morphlib/sourceresolver.py
+++ b/morphlib/sourceresolver.py
@@ -17,11 +17,62 @@
 import cliapp
 
 import collections
+import cPickle
 import logging
+import pylru
 
 import morphlib
 
 
+class PickleCacheManager(object):
+    '''Cache manager for PyLRU that reads and writes to Pickle files.
+
+    The 'pickle' format is less than ideal in many ways and is actually
+    slower than JSON in Python. However, the data we need to cache is keyed
+    by tuples and in JSON a dict can only be keyed with strings. For now,
+    using 'pickle' seems to be the least worst option.
+
+    '''
+
+    def __init__(self, filename, size):
+        self.filename = filename
+        self.size = size
+
+    def _populate_cache_from_file(self, filename, cache):
+        try:
+            with open(filename, 'r') as f:
+                data = cPickle.load(f)
+            for key, value in data.iteritems():
+                cache[key] = value
+        except (EOFError, IOError, cPickle.PickleError) as e:
+            logging.warning('Failed to load cache %s: %s', self.filename, e)
+
+    def load_cache(self):
+        '''Create a pylru.lrucache object prepopulated with saved data.'''
+        cache = pylru.lrucache(self.size)
+        # There should be a more efficient way to do this, by hooking into
+        # the json module directly.
+        if os.path.exists(self.filename):
+            self._populate_cache_from_file(self.filename, cache)
+        return cache
+
+    def save_cache(self, cache):
+        '''Save the data from a pylru.lrucache object to disk.
+
+        Any changes that have been made by other instances or processes since
+        load_cache() was called will be overwritten.
+
+        '''
+        data = {}
+        for key, value in cache.items():
+            data[key] = value
+        try:
+            with open(self.filename, 'w') as f:
+                cPickle.dump(data, f)
+        except (IOError, cPickle.PickleError) as e:
+            logging.warning('Failed to save cache to %s: %s', self.filename, e)
+
+
 class SourceResolverError(cliapp.AppException):
     pass
 
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