⚡ Bolt: Replace string concatenation in loops with list join for O(N) performance

Author: CoffeeMethod

.jules/bolt.md

@@ -1,3 +1,6 @@
 ## 2024-05-24 - [Regex Compilation Overhead]
 **Learning:** Python`s `re.compile()` has some internal caching, but doing it in a tight loop across multiple text chunks or items still introduces measurable overhead.
 **Action:** When a method applies dictionary/lexicon replacements via regular expressions, explicitly cache the compiled regex objects in a class attribute or closure, rather than compiling them on every call.
+## 2024-05-24 - [String Concatenation Bottleneck]
+**Learning:** Using `+=` for string concatenation inside loops (like iterating over PDF pages or large document segments) forces Python to create a new string object each time, leading to O(N²) time complexity.
+**Action:** Always accumulate strings in a list using `.append()` and combine them once at the end using `"".join(list)` to maintain O(N) performance.

kokoro_engine.py

@@ -433,28 +433,29 @@ def extract_text_from_file(self, fpath):
         if not os.path.exists(fpath):
             raise FileNotFoundError("File does not exist.")
 
-        text_data = ""
+        text_data_list = []
         lower_path = fpath.lower()
 
+        # Performance optimization: Use list accumulation instead of += for O(N) string building
         if lower_path.endswith(".pdf"):
             reader = pypdf.PdfReader(fpath)
             for page in reader.pages:
                 extracted = page.extract_text()
                 if extracted:
-                    text_data += extracted + "\n\n"
+                    text_data_list.append(extracted + "\n\n")
 
         elif lower_path.endswith(".epub"):
             book = epub.read_epub(fpath, options={'ignore_ncx': True})
             for item in book.get_items():
                 if item.get_type() == ebooklib.ITEM_DOCUMENT:
                     soup = BeautifulSoup(item.get_content(), 'html.parser')
-                    text_data += soup.get_text(separator='\n\n') + "\n\n"
+                    text_data_list.append(soup.get_text(separator='\n\n') + "\n\n")
         else:
             # Assume text based
             with open(fpath, "r", encoding="utf-8") as f:
-                text_data = f.read()
+                text_data_list.append(f.read())
 
-        return text_data
+        return "".join(text_data_list)
 
     def parse_multispeaker_text(self, text):
         """
@@ -890,9 +891,11 @@ async def playback_loop():
             else:
                 first_remaining_idx = 0
 
-            remaining_text = ""
+            # Performance optimization: Use list accumulation instead of += for O(N) string building
+            remaining_text_list = []
             for i in range(first_remaining_idx, total_segments):
-                remaining_text += all_text_segments[i][0] + "\n\n"
+                remaining_text_list.append(all_text_segments[i][0] + "\n\n")
+            remaining_text = "".join(remaining_text_list)
 
             if remaining_text:
                 rem_path = os.path.join(config['out_dir'], f"{config['filename']}_{config['time_id']}_remaining.txt")