Optimize file reads for password validation

Given a multi-gigabyte credentials file on disk with one record per line in the format user_id,salt,password_hash (hex), implement validate(user_id, password) -> bool. The file is unsorted and cannot be fully loaded into memory. Optimize space by only reading the content needed. Propose and implement: (a) a streaming approach that scans lines with constant memory and stops early when the user is found; (b) an optional offline indexing strategy (e.g., build a user_id -> byte offset index) to enable sublinear lookups on subsequent runs. Discuss trade-offs, file encoding/error handling, memory-mapped I/O vs buffered reads, and time/space complexity.