LZ77 Compression: Now 12.5% faster!

src/libWiiPy/archive/lz77.py

@@ -5,6 +5,7 @@
 
 import io
 from dataclasses import dataclass as _dataclass
+from typing import List as _List
 
 
 _LZ_MIN_DISTANCE = 0x01   # Minimum distance for each reference.
@@ -20,8 +21,9 @@ class _LZNode:
     weight: int = 0
 
 
-def _compress_compare_bytes(buffer: bytes, offset1: int, offset2: int, abs_len_max: int) -> int:
-    # Compare bytes up to the maximum length we can match.
+def _compress_compare_bytes(buffer: _List[int], offset1: int, offset2: int, abs_len_max: int) -> int:
+    # Compare bytes up to the maximum length we can match. Start by comparing the first 3 bytes, since that's the
+    # minimum match length and this allows for a more optimized early exit.
     num_matched = 0
     while num_matched < abs_len_max:
         if buffer[offset1 + num_matched] != buffer[offset2 + num_matched]:
@@ -30,9 +32,9 @@ def _compress_compare_bytes(buffer: bytes, offset1: int, offset2: int, abs_len_m
     return num_matched
 
 
-def _compress_search_matches(buffer: bytes, pos: int) -> (int, int):
+def _compress_search_matches_optimized(buffer: _List[int], pos: int) -> (int, int):
     bytes_left = len(buffer) - pos
-    global _LZ_MAX_DISTANCE, _LZ_MAX_LENGTH, _LZ_MIN_DISTANCE
+    global _LZ_MAX_DISTANCE, _LZ_MIN_LENGTH, _LZ_MAX_LENGTH, _LZ_MIN_DISTANCE
     # Default to only looking back 4096 bytes, unless we've moved fewer than 4096 bytes, in which case we should
     # only look as far back as we've gone.
     max_dist = min(_LZ_MAX_DISTANCE, pos)
@@ -52,7 +54,7 @@ def _compress_search_matches(buffer: bytes, pos: int) -> (int, int):
     return biggest_match, biggest_match_pos
 
 
-def _compress_search_matches_greedy(buffer: bytes, pos: int) -> (int, int):
+def _compress_search_matches_greedy(buffer: _List[int], pos: int) -> (int, int):
     # Finds and returns the first valid match, rather that finding the best one.
     bytes_left = len(buffer) - pos
     global _LZ_MAX_DISTANCE, _LZ_MAX_LENGTH, _LZ_MIN_DISTANCE
@@ -90,22 +92,23 @@ def _compress_lz77_optimized(data: bytes) -> bytes:
     # Iterate over the uncompressed data, starting from the end.
     pos = len(data)
     global _LZ_MAX_LENGTH, _LZ_MIN_LENGTH, _LZ_MIN_DISTANCE
+    data_list = list(data)
     while pos:
         pos -= 1
         node = nodes[pos]
         # Limit the maximum search length when we're near the end of the file.
-        max_search_len = min(_LZ_MAX_LENGTH, len(data) - pos)
+        max_search_len = min(_LZ_MAX_LENGTH, len(data_list) - pos)
         if max_search_len < _LZ_MIN_DISTANCE:
             max_search_len = 1
         # Initialize as 1 for each, since that's all we could use if we weren't compressing.
         length, dist = 1, 1
         if max_search_len >= _LZ_MIN_LENGTH:
-            length, dist = _compress_search_matches(data, pos)
+            length, dist = _compress_search_matches_optimized(data_list, pos)
         # Treat as direct bytes if it's too short to copy.
         if length == 0 or length < _LZ_MIN_LENGTH:
             length = 1
         # If the node goes to the end of the file, the weight is the cost of the node.
-        if (pos + length) == len(data):
+        if (pos + length) == len(data_list):
             node.len = length
             node.dist = dist
             node.weight = _compress_get_node_cost(length)
@@ -173,6 +176,7 @@ def _compress_lz77_greedy(data: bytes) -> bytes:
         buffer.write(len(data).to_bytes(3, 'little'))
 
         src_pos = 0
+        data_list = list(data)
         while src_pos < len(data):
             head = 0
             head_pos = buffer.tell()
@@ -180,7 +184,7 @@ def _compress_lz77_greedy(data: bytes) -> bytes:
 
             i = 0
             while i < 8 and src_pos < len(data):
-                length, dist = _compress_search_matches_greedy(data, src_pos)
+                length, dist = _compress_search_matches_greedy(data_list, src_pos)
                 # This is a reference node.
                 if length >= _LZ_MIN_LENGTH:
                     encoded = (((length - _LZ_MIN_LENGTH) & 0xF) << 12) | ((dist - _LZ_MIN_DISTANCE) & 0xFFF)