Use short key names in DIR objects on disk

This should save a bit of disk space. Previously, the full key name as
a string was used, and that led to a lot of repetition.

This is a backwards incompatible change.

3 files changed, 60 insertions, 5 deletions

diff --git a/NEWS b/NEWS
index ed097486..507c52f2 100644
--- a/NEWS
+++ b/NEWS
@@ -11,6 +11,11 @@ not use it unless you're willing to lose your backup.
 Version 1.20, not yet released
 ---------------------------------
 
+* Those using the experimental green-albatross repository format will
+  have to start over with fresh repositories. This release contains
+  backwards incompatible changes that mean existing repositories no
+  longer work. Sorry, but that's what experimental means.
+
 Minor changes:
 
 * Lars Wirzenius changed `obnam forget` so that if there is nothing to
diff --git a/obnamlib/fmt_ga/dirobj.py b/obnamlib/fmt_ga/dirobj.py
index da67abb9..f115872d 100644
--- a/obnamlib/fmt_ga/dirobj.py
+++ b/obnamlib/fmt_ga/dirobj.py
@@ -19,6 +19,33 @@
 import obnamlib
 
 
+_short_key_names = {
+    obnamlib.REPO_FILE_TEST_KEY: 'T',
+    obnamlib.REPO_FILE_USERNAME: 'U',
+    obnamlib.REPO_FILE_GROUPNAME: 'G',
+    obnamlib.REPO_FILE_SYMLINK_TARGET: 'S',
+    obnamlib.REPO_FILE_XATTR_BLOB: 'X',
+    obnamlib.REPO_FILE_MD5: '5',
+    obnamlib.REPO_FILE_MODE: 'M',
+    obnamlib.REPO_FILE_MTIME_SEC: 'ms',
+    obnamlib.REPO_FILE_MTIME_NSEC: 'mn',
+    obnamlib.REPO_FILE_ATIME_SEC: 'as',
+    obnamlib.REPO_FILE_ATIME_NSEC: 'an',
+    obnamlib.REPO_FILE_NLINK: 'N',
+    obnamlib.REPO_FILE_SIZE: 's',
+    obnamlib.REPO_FILE_UID: 'u',
+    obnamlib.REPO_FILE_GID: 'g',
+    obnamlib.REPO_FILE_BLOCKS: 'B',
+    obnamlib.REPO_FILE_DEV: 'D',
+    obnamlib.REPO_FILE_INO: 'I',
+}
+
+# Let's make sure we have no duplicate values.
+assert len(_short_key_names) == len(_short_key_names.values())
+
+_key_from_short = dict((v, k) for k, v in _short_key_names.items())
+
+
 class GADirectory(object):
 
     def __init__(self):
@@ -61,15 +88,30 @@ class GADirectory(object):
         return self._dict['metadata'].keys()
 
     def get_file_key(self, basename, key):
-        key_name = obnamlib.repo_key_name(key)
-        return self._dict['metadata'][basename].get(key_name)
+        short_name = self.get_short_key_name(key)
+        return self._dict['metadata'][basename].get(short_name)
 
     def set_file_key(self, basename, key, value):
-        key_name = obnamlib.repo_key_name(key)
-        old_value = self._dict['metadata'][basename].get(key_name, None)
+        short_name = self.get_short_key_name(key)
+        old_value = self._dict['metadata'][basename].get(short_name, None)
         if value != old_value:
             self._require_mutable()
-            self._dict['metadata'][basename][key_name] = value
+            self._dict['metadata'][basename][short_name] = value
+
+    def get_short_key_name(self, key):
+        '''Translate a key id to a short key name.
+
+        This is similar to obnamlib.repo_key_name, but the name is
+        guaranteed to be short, while still being unique. The length
+        matters, as these names will be used a lot.
+
+        '''
+
+        return _short_key_names[key]
+
+    def get_key_from_short_name(self, short_name):
+        '''Inverse of get_short_key_name.'''
+        return _key_from_short[short_name]
 
     def get_file_chunk_ids(self, basename):
         return self._dict['metadata'][basename]['chunk-ids']
diff --git a/obnamlib/fmt_ga/dirobj_tests.py b/obnamlib/fmt_ga/dirobj_tests.py
index b43324cc..7c223e3d 100644
--- a/obnamlib/fmt_ga/dirobj_tests.py
+++ b/obnamlib/fmt_ga/dirobj_tests.py
@@ -72,6 +72,14 @@ class GADirectoryTests(unittest.TestCase):
             obnamlib.GAImmutableError,
             dir_obj.remove_file, 'README')
 
+    def test_maps_file_key_to_short_name_and_back(self):
+        key = obnamlib.REPO_FILE_SIZE
+
+        dir_obj = obnamlib.GADirectory()
+        short = dir_obj.get_short_key_name(key)
+        key2 = dir_obj.get_key_from_short_name(short)
+        self.assertEqual(key, key2)
+
     def test_gets_file_key_when_unset(self):
         dir_obj = obnamlib.GADirectory()
         dir_obj.add_file('README')