⚡ Bolt: Optimize yEnc decoding latency using C-backed translations

verify_nzb.py

@@ -115,18 +115,34 @@ def _parse_yenc_attrs(line: bytes) -> dict[str, str]:
     return attrs
 
 
+# Precomputed translation tables for fast C-backed yEnc decoding.
+# yEnc shifts all characters by 42. Escaped characters are shifted by an additional 64.
+# Normal character: (char - 42) % 256
+_YENC_DECODE_TABLE = bytes((i - 42) % 256 for i in range(256))
+# Escaped character: (char - 64 - 42) % 256 = (char - 106) % 256
+_YENC_ESCAPE_TABLE = bytes((i - 106) % 256 for i in range(256))
+
 def _decode_yenc_lines(lines: Iterable[bytes]) -> bytes:
     decoded = bytearray()
     for line in lines:
+        escape_pos = line.find(b'=')
+        if escape_pos == -1:
+            decoded.extend(line.translate(_YENC_DECODE_TABLE))
+            continue
+
         index = 0
-        while index < len(line):
-            byte = line[index]
-            if byte == 61:
-                index += 1
-                if index >= len(line):
-                    raise ValueError("dangling yEnc escape")
-                byte = (line[index] - 64) % 256
-            decoded.append((byte - 42) % 256)
+        while True:
+            escape_pos = line.find(b'=', index)
+            if escape_pos == -1:
+                decoded.extend(line[index:].translate(_YENC_DECODE_TABLE))
+                break
+
+            decoded.extend(line[index:escape_pos].translate(_YENC_DECODE_TABLE))
+            index = escape_pos + 1
+            if index >= len(line):
+                raise ValueError("dangling yEnc escape")
+
+            decoded.append(_YENC_ESCAPE_TABLE[line[index]])
             index += 1
     return bytes(decoded)