Pivot data without date libraries

Q: Pivot data without date libraries

This question evaluates a candidate's competency in data manipulation and transformation, specifically pivoting tall datasets into wide views, handling missing metric values, ordering ISO date strings via string operations, and meeting algorithmic efficiency constraints (O(n log n)).

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Question

Transform a tall dataset into a wide, pivoted view without using any date/time libraries. Input: a list of records (date, store_id, metric, value) where date is an ISO 8601 string YYYY-MM-DD, metric ∈ {"sales", "refunds", "visits"}, and value is a non-negative integer. Output: for each (store_id, date), produce one row with columns [store_id, date, sales, refunds, visits], filling missing metric values with 0 and ordering rows by ascending date within each store. Requirements: (

Implement this in code (any language), but you may not import date/time packages; rely only on string operations for ordering dates. (
Ensure the solution is O(n log n) or better with respect to the number of records n. (
Explain how you would handle malformed dates, duplicate (store_id, date, metric) entries, and very large inputs that do not fit in memory.

Pivot data without date libraries

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