Process a Large File Line by Line | Dataford Interview Questions

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Problem

Fractal analytics pipelines may receive log files that are too large to load into memory. Write a function that processes an iterable of log lines one at a time and summarizes lines containing a target keyword.

A line matches when the keyword appears as a case-sensitive substring. Return the total number of matching lines and the zero-based line numbers of the first three matches. The input may be a file object, generator, or list, so your solution must not convert the entire iterable to a list.

Formal Specification

Implement process_log_lines(lines, keyword).

lines is an iterable of strings. Each string represents one line and may include a trailing newline character.
keyword is a non-empty string.
Return a dictionary with this exact structure: {"matching_count": integer, "first_matches": list of integers}.
first_matches contains at most three zero-based line numbers, in ascending order.

Constraints

1 <= number of lines <= 10^7
1 <= len(keyword) <= 100
Each line contains at most 10^6 characters
The iterable may be single-use and should be consumed only once

Function Signature

def process_log_lines(lines, keyword):