Remove the length field from the index entries in data pack files.

After the last index entry, we put an extra entry (id of 0, offset
pointing to where the next entry would start) so we can compute
the size of the last index entry.

This saves 4 bytes per item in data pack files, which is about 10k per
locale pak file.  When compressed, this saves about 237k.

BUG=76285

Review URL: http://codereview.chromium.org/7604012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@96415 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 14 insertions, 7 deletions

diff --git a/tools/data_pack/data_pack.py b/tools/data_pack/data_pack.py
index 571842e..ffd2cb2 100755
--- a/tools/data_pack/data_pack.py
+++ b/tools/data_pack/data_pack.py
@@ -9,7 +9,7 @@ See base/pack_file* for details.
 
 import struct
 
-FILE_FORMAT_VERSION = 2
+FILE_FORMAT_VERSION = 3
 HEADER_LENGTH = 2 * 4  # Two uint32s. (file version and number of entries)
 
 class WrongFileVersion(Exception):
@@ -26,13 +26,17 @@ def ReadDataPack(input_file):
     raise WrongFileVersion
 
   resources = {}
+  if num_entries == 0:
+    return resources
+
   # Read the index and data.
   data = data[HEADER_LENGTH:]
-  kIndexEntrySize = 2 + 2 * 4  # Each entry is 1 uint16 and 2 uint32s.
+  kIndexEntrySize = 2 + 4  # Each entry is a uint16 and a uint32.
   for _ in range(num_entries):
-    id, offset, length = struct.unpack("<HII", data[:kIndexEntrySize])
+    id, offset = struct.unpack("<HI", data[:kIndexEntrySize])
     data = data[kIndexEntrySize:]
-    resources[id] = original_data[offset:offset + length]
+    next_id, next_offset = struct.unpack("<HI", data[:kIndexEntrySize])
+    resources[id] = original_data[offset:next_offset]
 
   return resources
 
@@ -44,15 +48,18 @@ def WriteDataPack(resources, output_file):
   # Write file header.
   file.write(struct.pack("<II", FILE_FORMAT_VERSION, len(ids)))
 
-  # Each entry is 1 uint16 and 2 uint32s.
-  index_length = len(ids) * (2 + 2 * 4)
+  # Each entry is a uint16 and a uint32. We have one extra entry for the last
+  # item.
+  index_length = (len(ids) + 1) * (2 + 4)
 
   # Write index.
   data_offset = HEADER_LENGTH + index_length
   for id in ids:
-    file.write(struct.pack("<HII", id, data_offset, len(resources[id])))
+    file.write(struct.pack("<HI", id, data_offset))
     data_offset += len(resources[id])
 
+  file.write(struct.pack("<HI", 0, data_offset))
+
   # Write data.
   for id in ids:
     file.write(resources[id])