Encode a list of strings specially

Dict keys are strings, handling them specially makes things go
a bit faster. ./serialise-speed went from 71/s to 97/s.

1 files changed, 38 insertions, 6 deletions

diff --git a/obnamlib/obj_serialiser.py b/obnamlib/obj_serialiser.py
index 3bcdb470..0eb70b5a 100644
--- a/obnamlib/obj_serialiser.py
+++ b/obnamlib/obj_serialiser.py
@@ -146,22 +146,54 @@ def _next_object(pos, length):
 # Dicts.
 
 def _serialise_dict(obj):
-    pairs = ''.join(
-        _serialise_str(key) + serialise_object(value)
-        for key, value in obj.iteritems())
-    return _DICT + _serialise_length(len(pairs)) + pairs
+    keys = obj.keys()
+    str_keys = [key for key in keys if type(obj[key]) is str]
+    other_keys = [key for key in keys if key not in str_keys]
+    parts = []
+    parts.append(_serialise_str_list(str_keys))
+    parts.append(_serialise_str_list([obj[key] for key in str_keys]))
+    parts.append(_serialise_str_list(other_keys))
+    for key in other_keys:
+        parts.append(serialise_object(obj[key]))
+
+    encoded = ''.join(parts)
+    return _DICT + _serialise_length(len(encoded)) + encoded
 
 
 def _deserialise_dict(serialised):
     result = {}
     pos = 0
-    while pos < len(serialised):
-        key, pos = _deserialise_prefix(serialised, pos)
+
+    str_keys, pos = _deserialise_str_list(serialised, pos)
+    str_values, pos = _deserialise_str_list(serialised, pos)
+    result.update(zip(str_keys, str_values))
+
+    other_keys, pos = _deserialise_str_list(serialised, pos)
+    for key in other_keys:
         value, pos = _deserialise_prefix(serialised, pos)
         result[key] = value
+
     return result
 
 
+def _serialise_str_list(strings):
+    n = len(strings)
+    encoded_lengths = struct.pack('!' + 'Q' * n, *[len(s) for s in strings])
+    return _serialise_integer(n) + encoded_lengths + ''.join(strings)
+
+
+def _deserialise_str_list(serialised, pos):
+    n, pos = _deserialise_prefix(serialised, pos)
+    int_size = struct.calcsize('!Q')
+    lengths = struct.unpack('!' + 'Q' * n, serialised[pos:pos + n*int_size])
+    pos += n * int_size
+    strings = []
+    for i in range(n):
+        strings.append(serialised[pos:pos+lengths[i]])
+        pos += lengths[i]
+    return strings, pos
+
+
 def _deserialise_prefix(serialised, pos):
     length = _extract_length(serialised, pos)
     end = _next_object(pos, length)