content-hash-cache-patternlisted

# Content-Hash File Cache Pattern Cache expensive file processing results (PDF parsing, text extraction, image analysis) using SHA-256 content hashes as cache keys. Unlike path-based caching, this approach survives file moves/renames and auto-invalidates when content changes. ## When to Activate - Building file processing pipelines (PDF, images, text extraction) - Processing cost is high and same files are processed repeatedly - Need a `--cache/--no-cache` CLI option - Want to add caching to existing pure functions without modifying them ## Core Pattern ### 1. Content-Hash Based Cache Key Use file content (not path) as the cache key: ```python import hashlib from pathlib import Path _HASH_CHUNK_SIZE = 65536 # 64KB chunks for large files def compute_file_hash(path: Path) -> str: """SHA-256 of file contents (chunked for large files).""" if not path.is_file(): raise FileNotFoundError(f"File not found: {path}") sha256 = hashlib.sha256() with open(path, "rb") as f: while True: chunk = f.read(_HASH_CHUNK_SIZE) if not chunk: break sha256.update(chunk) return sha256.hexdigest() ``` **Why content hash?** File rename/move = cache hit. Content change = automatic invalidation. No index file needed. ### 2. Frozen Dataclass for Cache Entry ```python from dataclasses import dataclass @dataclass(frozen=True, slots=True) class CacheEntry: file_hash: str source_path: str document: