Compute transaction fees from a CSV string

You are given a CSV string representing a list of payment transactions. Write a function that parses the CSV and returns a CSV string with the total processing fee per merchant.

Input CSV

The input has a header and the following columns:

• transaction_id (string)
• merchant_id (string) — the “person” you should aggregate fees for
• status (string) — one of:
  • payment_completed , payment_pending , payment_failed ,
  • refund_completed ,
  • dispute_won , dispute_lost
• payment_provider (string) — e.g., card , bank_transfer , paypal
• buyer_country (string, ISO-2) — e.g., US , DE , FR
• currency (string, ISO-3) — e.g., USD , EUR
• amount (decimal) — transaction amount in the given currency

Assume:

• The CSV may contain extra whitespace; you should handle it.
• All amounts are non-negative.

Fee rules

You are also given (as in-memory dictionaries/maps):

1. base_rate_by_provider[payment_provider] -> decimal
   Used for all statuses except payment_completed .
2. completed_rate_by_provider_and_country[(payment_provider, buyer_country)] -> decimal
   Used only when status = 'payment_completed' .
3. fx_to_usd[currency] -> decimal
   Multiply an amount in currency by this factor to convert to USD.

Fee calculation per transaction:

• If status = 'refund_completed' : fee is 0 (no refund fee).
• If status = 'payment_completed' :
  • Convert amount to USD: amount_usd = amount * fx_to_usd[currency] .
  • fee_usd = amount_usd * completed_rate + 0.30 where completed_rate comes from completed_rate_by_provider_and_country .
• For all other statuses ( payment_pending , payment_failed , dispute_won , dispute_lost ):
  • Convert amount to USD: amount_usd = amount * fx_to_usd[currency] .
  • fee_usd = amount_usd * base_rate + 0.30 where base_rate comes from base_rate_by_provider .

Lookups and fallbacks:

• If (payment_provider, buyer_country) is missing for a completed payment, fall back to base_rate_by_provider[payment_provider] .
• If a payment_provider is unknown, treat its rate as 0 (still apply the $0.30 fixed fee for non-refunds).
• If currency is unknown, assume fx_to_usd[currency] = 1.0 .

Required output

Return a CSV string with header:

merchant_id,total_fee_usd

Where total_fee_usd is the sum of fee_usd across that merchant’s transactions, rounded to 2 decimals. Order rows by merchant_id ascending.

What to implement

Implement a function (in a language of your choice):

compute_fees_per_merchant(csv_str, base_rate_by_provider, completed_rate_by_provider_and_country, fx_to_usd) -> csv_str

Discuss time complexity and how you’d test edge cases (e.g., empty input, malformed rows, missing lookup keys).