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Forward-fill readings within each sensor

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Forward-fill readings within each sensor

A readings DataFrame has sensor_id, reading_ts, and value (with gaps). Carry the last known value forward within each sensor, ordered by time — without leaking values across sensor boundaries. A leading gap (no prior value) stays missing.

Return the resulting value column (sorted by sensor_id, then reading_ts) as a list, with None for any value that's still missing.

Python 3.12, standard library only — no pip installs.

solution.py

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