Efficient Transaction Aggregation | Dataford Interview Questions

Instacart wants a compact summary of order activity across its stores. Given a large list of completed order records, aggregate each store's total revenue, number of orders, and number of unique customers without repeatedly scanning the input.

Formal Specification

Implement aggregate_store_metrics(transactions). The input is a list of dictionaries, where every record contains:

store_id: a string identifying the store
customer_id: a string identifying the customer
amount_cents: a non-negative integer order amount in cents

Return a dictionary keyed by store_id. Each value must be a dictionary with exactly these keys: total_revenue_cents, order_count, and unique_customer_count. Store results may appear in the order each store is first encountered.

Constraints

0 <= len(transactions) <= 10^6
Each record contains store_id, customer_id, and amount_cents
store_id and customer_id are strings
0 <= amount_cents <= 10^9
The input may contain repeated customers and repeated stores

Function Signature

def aggregate_store_metrics(transactions):

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