⚡ Bolt: Optimize string concatenation in dl_github_archive.py

.jules/bolt.md

@@ -105,6 +105,9 @@
 ## 2024-05-24 - [Avoid `email.parser` for large package indexes]
 **Learning:** `email.parser.Parser` performs full RFC 822/2822 compliance checks which adds massive overhead. When parsing tens of thousands of machine-generated opkg package index blocks with predictable `Key: Value` line formats, standard string splitting and `.startswith()` checks provide a ~14x speedup.
 **Action:** When extracting a few specific headers from a trusted and uniform block format instead of parsing arbitrary emails, avoid `email.parser.Parser` and use fast native python string operations instead. Make sure to use `.strip()` when parsing values to correctly handle `\r\n` line endings.
+## 2024-05-14 - Python String Concatenation Optimization
+**Learning:** In Python, string concatenation within a loop using the `+=` operator involves creating a new string object and copying contents on each iteration because strings are immutable. This leads to O(N^2) performance degradation over many iterations.
+**Action:** Replace `+=` string concatenation inside loops with `list.append()` to collect the string parts, followed by `''.join(list)` outside the loop. This ensures an O(N) linear time complexity and avoids unnecessary allocations, providing significant performance speedups.
 
 ## 2026-05-22 - [Python String Splitting Memory Overhead]
 **Learning:** When parsing tens of thousands of blocks in a massive text file, using `.split()` to chunk the entire string creates an intermediate list containing all chunk strings simultaneously, leading to massive memory bloat (O(N) memory overhead in addition to the original string).

scripts/dl_github_archive.py

@@ -350,7 +350,7 @@ def _init_commit_ts(self):
         version_is_sha1sum = len(self.version) == 40
         if not version_is_sha1sum:
             apis.insert(0, apis.pop())
-        reasons = ''
+        reasons = []
         for api in apis:
             url = api['url']
             attr_path = api['attr_path']
@@ -364,8 +364,8 @@ def _init_commit_ts(self):
                 self.commit_ts_cache.set(url, ct)
                 return
             except Exception as e:
-                reasons += '\n' + ("  {}: {}".format(url, e))
-        raise self._error('Cannot fetch commit ts:{}'.format(reasons))
+                reasons.append('\n  {}: {}'.format(url, e))
+        raise self._error('Cannot fetch commit ts:{}'.format(''.join(reasons)))
 
     def _init_commit_ts_remote_get(self, url, attrpath):
         resp = self._make_request(url)