Audit Blocked Merchants in Payouts

Problem Narrative

You’re building a compliance check for a fintech payout system processing tens of millions of transfers per day. Regulators require that payouts to blocked merchant IDs be detected quickly and deterministically. Your service receives (1) a list of blocked merchant IDs updated hourly and (2) a batch of payout merchant IDs for the last minute. You must report which blocked IDs appear in the payout batch, including how many times each appears.

Task

Implement a function:

• Input:
  • blocked: list[int] — merchant IDs that must not receive payouts
  • payouts: list[int] — merchant IDs present in a payout batch (may contain duplicates)
• Output:
  • Return a list of pairs [(merchant_id, count), ...] for every merchant_id in blocked that appears in payouts, where count is the number of occurrences in payouts.
  • The output must be sorted by merchant_id ascending.

Requirements

1. If a blocked ID does not appear in payouts, it must not be included.
2. Duplicates in blocked should be treated as a single ID (i.e., output each ID at most once).
3. The solution should be efficient for large inputs.

Constraints

• 0 <= blocked.length, payouts.length <= 2 * 10^5
• -10^9 <= merchant_id <= 10^9

Notes

• Aim for better than O(len(blocked) * len(payouts)).
• Returning an empty list is valid when there are no matches.