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Chunking 2M+ files a day for Code Search using Syntax Trees

Kevin Lu - July 30th, 2023

Updates:

Initializing any vector store requires chunking large documents for effective search.

Why can't we just embed entire files? Let's consider the file of our main API endpoint (opens in a new tab):

  1. Imports
  2. Constants declarations
  3. Helper functions
  4. Business logic for each webhook endpoint

If I search for “GitHub Action run”, it should match the section corresponding to the switch case block (opens in a new tab) that checks for the “check_runs completed” event. However, this is only 20 lines of code out of 400+ lines, so even a perfect search algorithm would only consider this a 5% match. However, if we chunk the 400 lines into 20 chunks of 20 lines each, it would match the correct switch case block.

How do we create 20-line chunks? One naive approach is to evenly break up the 400-line file into 20-line chunks.

However, this approach does not work. Semantically similar code will not stay together, and will have lost context. For instance, function headers could be separated from their implementation details and the docstrings.

Our current code chunking algorithm processes 2M+ files a day and is open-source (opens in a new tab)!

Constraints 🚧

Most chunkers for RAG-based (retrieval augmented generation) agents cap by token count. For simplicity, we decided to use character count, with a max of 1500.

This is because the average token to character ratio for code is ~1:5(300 tokens), and embedding models are capped at 512 tokens. Further, 1500 characters correspond approximately to 40 lines, roughly equivalent to a small to medium sized function or class.

The challenge is getting as close to 1500 characters as possible, while ensuring the chunks are semantically similar and the relevant context is connected.

Out of the Box Solution 📦

The easiest out-of-the-box solution for code chunking is Langchain’s recursive chunker (opens in a new tab). At a high level:

  1. Break the text using the top-level delimiter (firstly using classes, then function definitions, then function methods etc.)
  2. Loop through each section and greedily concatenate them until it breaks the character limit. For chunks that are too big, recursively chunk the section starting with the next-level delimiter.
delimiters = ["\nclass ", "\ndef ", "\n\tdef ", "\n\n", "\n", " ", ""]
def chunk(text: str, delimiter_index: int = 0, MAX_CHARS: int = 1500) -> list[str]:
	delimiter = delimiters[delimiter_index]
	new_chunks = []
	current_chunk = ""
	for section in text.split(delimiter):
		if len(section) > MAX_CHARS:
			# Section is too big, recursively chunk this section
			new_chunks.append(current_chunk)
			current_chunk = ""
			new_chunks.extend(chunk(section, delimiter_index + 1, MAX_CHARS)
		elif len(current_chunk) + len(section) > MAX_CHARS:
			# Current chunk is max size
			new_chunks.append(current_chunk)
			current_chunk = section
		else:
			# Concatenate section to current_chunk
			current_chunk += section
	return new_chunks

For each language, we would also use different delimiters.

Examples

For full files of the examples, see https://gist.github.com/kevinlu1248/ded3ea33dcd8a9bd08078f4c64eb9268 (opens in a new tab).

Example #1

Based on our on_check_suite.py file for handling GitHub Action runs. A bad split separates a string concatenation declaration from its contents. ❌

...
 
def on_check_suite(request: CheckRunCompleted):
    logger.info(f"Received check run completed event for {request.repository.full_name}")
    g = get_github_client(request.installation.id)
    repo = g.get_repo(request.repository.full_name)
    if not get_gha_enabled(repo):
        logger.info(f"Skipping github action for {request.repository.full_name} because it is not enabled")
        return None
    pr = repo.get_pull(request.check_run.pull_requests[0].number)
    num_pr_commits = len(list(pr.get_commits()))
    if num_pr_commits > 20:
        logger.info(f"Skipping github action for PR with {num_pr_commits} commits")
        return None
    logger.info(f"Running github action for PR with {num_pr_commits} commits")
    logs = download_logs(
        request.repository.full_name,
        request.check_run.run_id,
        request.installation.id
    )
    if not logs:
        return None
    logs = clean_logs(logs)
    extractor = GHAExtractor()
    logger.info(f"Extracting logs from {request.repository.full_name}, logs: {logs}")
    problematic_logs = extractor.gha_extract(logs)
    if problematic_logs.count("
") > 15:
        problematic_logs += "
 
========================================
 
There are a lot of errors. This is likely a larger issue with the PR and not a small linting/type-checking issue."
    comments = list(pr.get_issue_comments())
    if len(comments) >= 2 and problematic_logs == comments[-1].body and comments[-2].body == comments[-1].body:
        comment = pr.as_issue().create_comment(log_message.format(error_logs=problematic_logs) + "
 
I'm getting the same errors 3 times in a row, so I will stop working on fixing this PR.")
        logger.warning("Skipping logs because it is duplicated")
        raise Exception("Duplicate error logs")
    print(problematic_logs)
    comment = pr.as_issue().create_comment(log_message.format(error_logs=problematic_logs))
    on_comment(
        repo_full_name=request.repository.full_name,
        repo_description=request.repository.description,
        comment=problematic_logs,
        pr_path=None,
        pr_line_position=None,
        username=request.sender.login,
        installation_id=request.installation.id,
        pr_number=request.check_run.pull_requests[0].number,
        comment_id=comment.id,
        repo=repo,
    )
    return {"success": True}

Example #2

Based on BaseIndex.ts file from LlamaIndex declaring the ABC for vector stores. A bad split separates a class method from its header. ❌

...
 
export class IndexDict extends IndexStruct {
  nodesDict: Record<string, BaseNode> = {};
  docStore: Record<string, Document> = {}; // FIXME: this should be implemented in storageContext
  type: IndexStructType = IndexStructType.SIMPLE_DICT;
 
========================================
 
getSummary(): string {
    if (this.summary === undefined) {
      throw new Error("summary field of the index dict is not set");
    }
    return this.summary;
  }
 
  addNode(node: BaseNode, textId?: string) {
    const vectorId = textId ?? node.id_;
    this.nodesDict[vectorId] = node;
  }
 
  toJson(): Record<string, unknown> {
    return {
      ...super.toJson(),
      nodesDict: this.nodesDict,
      type: this.type,
    };
  }
}
 
...

Problems 🤔

However, it comes with some major problems:

  1. This chunker decently for Python but breaks curly-bracket-heavy languages like JS and XML-based languages like HTML in unexpected ways.
    • Further, str.split does not work well for these more complex syntaxes like JS and HTML.
    • E.g. Even for Python, it broke the problematic logs line by splitting problematic_logs += \" and the rest of the string
  2. Only 16 languages are currently supported, without support for JSX, Typescript, EJS and C#.
    • JSX/TSX makes up the majority of our userbase
  3. Langchain deletes important delimiters such as “def” and “class”.

Our Solution 🧠

The inherent problem is that iterative str.split with different delimiters is a primitive method for approximating concrete syntax trees (CST).

To solve this, we decided to just use CST parsers. But how do we get CST parsers for a large number of languages? Thankfully, the library tree-sitter (opens in a new tab) provides a standardized way to access 113 CST-parsers for programming languages and is fast (written in C) and dependency-free.

The new algorithm is fairly similar to the Langchain algorithm and is as follows:

  1. To chunk a parent node, we iterate through its children and greedily bundle them together. For each child node:
    1. If the current chunk is too big, we add that to our list of chunks and empty the bundle
    2. If the next child node is too big, we recursively chunk the child node and add it to the list of chunks
    3. Otherwise, concatenate the current chunk with the child node
  2. Post-process the final result by combining single-line chunks with the next chunk.
    1. This guarantees that there are no chunks that are too small since they yield less meaningful results
from tree_sitter import Node
 
def chunk_node(node: Node, text: str, MAX_CHARS: int = 1500) -> list[str]:
	new_chunks = []
	current_chunk = ""
	for child in node.children:
		if child.end_byte - child.start_byte > MAX_CHARS:
			new_chunks.append(current_chunk)
			current_chunk = ""
			new_chunks.extend(chunk_node(child, text, MAX_CHARS)
		elif len(current_chunk) + child.end_byte - child.start_byte > MAX_CHARS:
			new_chunks.append(current_chunk)
			current_chunk = text[node.start_byte:node.end_byte]
		else:
			current_chunk += text[node.start_byte:node.end_byte]
	return new_chunks

Example

Full chunks can be found at https://gist.github.com/kevinlu1248/49a72a1978868775109c5627677dc512 (opens in a new tab)

Example #1

Based on our on_check_suite.py file for handling GitHub Action runs. Correct splitting, also splitting before an if statement instead of separating the if-statement from the body. ✅

...
 
def on_check_suite(request: CheckRunCompleted):
    logger.info(f"Received check run completed event for {request.repository.full_name}")
    g = get_github_client(request.installation.id)
    repo = g.get_repo(request.repository.full_name)
    if not get_gha_enabled(repo):
        logger.info(f"Skipping github action for {request.repository.full_name} because it is not enabled")
        return None
    pr = repo.get_pull(request.check_run.pull_requests[0].number)
    num_pr_commits = len(list(pr.get_commits()))
    if num_pr_commits > 20:
        logger.info(f"Skipping github action for PR with {num_pr_commits} commits")
        return None
    logger.info(f"Running github action for PR with {num_pr_commits} commits")
    logs = download_logs(
        request.repository.full_name,
        request.check_run.run_id,
        request.installation.id
    )
    if not logs:
        return None
    logs = clean_logs(logs)
    extractor = GHAExtractor()
    logger.info(f"Extracting logs from {request.repository.full_name}, logs: {logs}")
    problematic_logs = extractor.gha_extract(logs)
    if problematic_logs.count("\n") > 15:
        problematic_logs += "\n\nThere are a lot of errors. This is likely a larger issue with the PR and not a small linting/type-checking issue."
    comments = list(pr.get_issue_comments())
 
==========
 
    if len(comments) >= 2 and problematic_logs == comments[-1].body and comments[-2].body == comments[-1].body:
        comment = pr.as_issue().create_comment(log_message.format(error_logs=problematic_logs) + "\n\nI'm getting the same errors 3 times in a row, so I will stop working on fixing this PR.")
        logger.warning("Skipping logs because it is duplicated")
        raise Exception("Duplicate error logs")
    print(problematic_logs)
    comment = pr.as_issue().create_comment(log_message.format(error_logs=problematic_logs))
    on_comment(
        repo_full_name=request.repository.full_name,
        repo_description=request.repository.description,
        comment=problematic_logs,
        pr_path=None,
        pr_line_position=None,
        username=request.sender.login,
        installation_id=request.installation.id,
        pr_number=request.check_run.pull_requests[0].number,
        comment_id=comment.id,
        repo=repo,
    )

Example #2

Based on BaseIndex.ts file from LlamaIndex declaring the ABC for vector stores. Our chunker correctly splits between exported classes and functions. ✅

...
 
export class IndexDict extends IndexStruct {
  nodesDict: Record<string, BaseNode> = {};
  docStore: Record<string, Document> = {}; // FIXME: this should be implemented in storageContext
  type: IndexStructType = IndexStructType.SIMPLE_DICT;
 
  getSummary(): string {
    if (this.summary === undefined) {
      throw new Error("summary field of the index dict is not set");
    }
    return this.summary;
  }
 
  addNode(node: BaseNode, textId?: string) {
    const vectorId = textId ?? node.id_;
    this.nodesDict[vectorId] = node;
  }
 
  toJson(): Record<string, unknown> {
    return {
      ...super.toJson(),
      nodesDict: this.nodesDict,
      type: this.type,
    };
  }
}
 
========================================
 
export function jsonToIndexStruct(json: any): IndexStruct {
  if (json.type === IndexStructType.LIST) {
    const indexList = new IndexList(json.indexId, json.summary);
    indexList.nodes = json.nodes;
    return indexList;
  } else if (json.type === IndexStructType.SIMPLE_DICT) {
    const indexDict = new IndexDict(json.indexId, json.summary);
    indexDict.nodesDict = json.nodesDict;
    return indexDict;
  } else {
    throw new Error(`Unknown index struct type: ${json.type}`);
  }
}
 
...

Rest of the Algorithm 🤖

  1. Iterate through the list of languages until one of them successfully parses the code
  2. Chunk the code’s syntax tree root node
  3. If none of the languages hit, we use a naive chunker that takes 40 lines at a time with 15 lines of overlap in between (0.1% of cases)
language_names = ["python", "java", "cpp", "go", "rust", "ruby", "php"] # and more
 
# Installing the parsers
languages = {}
for language in LANGUAGE_NAMES:
   subprocess.run(f"git clone https://github.com/tree-sitter/tree-sitter-{language} cache/tree-sitter-{language}", shell=True)
  for language in LANGUAGE_NAMES:
      Language.build_library(f'cache/build/{language}.so', [f"cache/tree-sitter-{language}"])
  self.languages = {language: Language(f"cache/build/{language}.so", language) for language in LANGUAGE_NAMES}
 
def chunk(text: str, MAX_CHARS: int = 1500) -> list[str]:
	# Determining the language
	for language_name in language_names:
    language = languages[language_name]
    parser = Parser()
    parser.set_language(language)
    tree = parser.parse(bytes(text, "utf-8"))
    if not tree.root_node.children or tree.root_node.children[0].type != "ERROR":
        file_language = language
        break
    logger.warning(f"Not language {language_name}")
 
	# Smart chunker
	if file_language:
      return chunk_node(tree.root_node, text, max_chunk_size)
 
	# Naive algorithm
  source_lines = file_content.split('\n')
  num_lines = len(source_lines)
  logger.info(f"Number of lines: {num_lines}")
  chunks = []
  start_line = 0
  while start_line < num_lines and num_lines > overlap:
      end_line = min(start_line + chunk_size, num_lines)
      chunk = '\n'.join(source_lines[start_line:end_line])
      chunks.append(chunk)
      start_line += chunk_size - overlap
	return chunks

At Sweep, we currently installed Python, Java, C++, Go, Rust, Ruby, PHP, C#, Embedded Template (ERB & EJS), Markdown, Vue, and TSX. Also, note that C++ covers C and TSX covers JS, JSX and TS.

Pitfalls 🕳️

Unfortunately, tree-sitter is unreliable at times and many of the parsers are community-driven:

  • The TSX parser hangs when it doesn’t parse instead of returning an error
  • Further, the base language is written in C. Running it in production on our serverless architecture involves a convoluted method of caching C-compiled executables, moving it to executable directories and using a Python wrapper to call them.
  • Some parsers leave gaps in between children nodes. We solved this by coalescing
  • None of the parsers error out when they parse the wrong language and yield errors in different ways
    • Some of them have root nodes that are “ERROR” nodes while others have that as the first child

We worked around this by always defaulting to the naive chunker in cases of errors like these and prioritizing TSX last. We also prioritize the language corresponding to the file extension.

Future 🔮

This algorithm is currently embedded into our codebase but can be open-sourced as a standalone project or as part of Langchain. Although we lack the time to undertake this task, we are more than willing to help anyone interested in implementing it.

Edit: This algorithm is now integrated into LlamaIndex via https://github.com/jerryjliu/llama_index/pull/7100 (opens in a new tab).

Another problem is that code snippets far apart (in lines) may still need to share context. For example, a class method may need the context of the class header and long functions also need their function signatures. A possible improvement would be to somehow use a format like:

class Foo:
  ...
  def bar(self):
      pass

We can consider using universal ctags or the like for simpler and more universal parsing or train a custom spaCy sentencizer on manually annotated chunks, but that might be a bit over-engineered.

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