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Compute transaction fees from a CSV string

Quick Overview

Evaluates CSV parsing, numeric conversions and rounding, map lookups with fallbacks, transactional fee computation and per-merchant aggregation in the Coding & Algorithms category for Data Engineer roles.

Compute transaction fees from a CSV string

Company: Stripe

Role: Data Engineer

Category: Coding & Algorithms

Difficulty: hard

Interview Round: Technical Screen

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).

Quick Answer: Evaluates CSV parsing, numeric conversions and rounding, map lookups with fallbacks, transactional fee computation and per-merchant aggregation in the Coding & Algorithms category for Data Engineer roles.

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Part 1: Parse a transaction CSV string and compute each user's fees

You are given a CSV string with header user,status,payment_provider,amount. Each row is one transaction in USD. Compute the total fee charged to each user and return a dictionary mapping user -> total fee in cents. Fee rules: - payment_completed, payment_failed, payment_pending: fee = amount * provider_rate + 0.30 USD - card: 2.9% - bank_transfer: 1.0% - wallet: 1.5% - refund_completed: 0 USD - dispute_lost: 15.00 USD flat - dispute_won: 0 USD Round each transaction fee to the nearest cent using half-up rounding before adding it to the user's total. Ignore blank lines. Return every user that appears in the CSV, even if that user's total fee is 0. If the input string is empty or whitespace only, return {}.

Constraints

0 <= number of transactions <= 100000
The CSV has the exact header user,status,payment_provider,amount when non-empty
amount is a non-negative decimal number with at most 2 digits after the decimal point
status is one of payment_completed, payment_failed, payment_pending, refund_completed, dispute_lost, dispute_won
payment_provider is one of card, bank_transfer, wallet

Examples

Input: "user,status,payment_provider,amount\nalice,payment_completed,card,100.00\nbob,payment_failed,bank_transfer,50.00\nalice,refund_completed,card,20.00\nbob,dispute_lost,wallet,80.00"

Expected Output: {"alice": 320, "bob": 1580}

Explanation: alice pays 100.00 * 2.9% + 0.30 = 3.20 => 320 cents. Her refund adds 0. bob pays 50.00 * 1.0% + 0.30 = 0.80 => 80 cents, plus a lost dispute fee of 1500 cents.

Input: "user,status,payment_provider,amount\nalice,payment_pending,wallet,10.00\nalice,payment_completed,wallet,0.99\ncarol,dispute_won,card,42.00"

Expected Output: {"alice": 76, "carol": 0}

Explanation: alice pays 45 cents for 10.00 via wallet and 31 cents for 0.99 via wallet after rounding, totaling 76. carol appears in the CSV, so she is included with fee 0.

Input: ""

Expected Output: {}

Explanation: Empty input produces an empty result.

Input: "user,status,payment_provider,amount\n\nzoe,refund_completed,card,10.00\n"

Expected Output: {"zoe": 0}

Explanation: Blank lines are ignored, and refunds have no fee.

Hints

Use the CSV header to access fields by name instead of relying on column positions.
Avoid floating-point errors by using Decimal or by converting each rounded fee into cents before summing.

Part 2: Compute fees in USD for completed transactions using country-specific rates

You are given: 1. A CSV string with header user,status,payment_provider,buyer_country,currency,amount 2. A dictionary exchange_rates where exchange_rates[currency] is the USD value of 1 unit of that currency Compute the total fee charged to each user and return a dictionary mapping user -> total fee in USD cents. First convert every amount to USD: converted_usd = amount * exchange_rates[currency] Fee rules: - payment_completed: - card: US/CA 2.9%, GB 2.5%, DE/FR/AT 2.3%, all other countries 3.1% - bank_transfer: US/CA 1.2%, GB 1.1%, DE/FR/AT 1.0%, all other countries 1.4% - wallet: any country 1.8% - fee = converted_usd * rate + 0.30 USD - payment_failed, payment_pending: use the Part 1 provider-only rates on the converted USD amount - card: 2.9% - bank_transfer: 1.0% - wallet: 1.5% - plus 0.30 USD fixed fee - refund_completed: 0 USD - dispute_lost: 15.00 USD flat - dispute_won: 0 USD Round each transaction fee to the nearest cent using half-up rounding before adding it to the user's total. Ignore blank lines. Return every user that appears in the CSV, even if that user's total fee is 0. If the CSV input is empty or whitespace only, return {}.

Constraints

0 <= number of transactions <= 100000
The CSV has the exact header user,status,payment_provider,buyer_country,currency,amount when non-empty
exchange_rates contains every currency used in the CSV
amount is a non-negative decimal number
payment_provider is one of card, bank_transfer, wallet
status is one of payment_completed, payment_failed, payment_pending, refund_completed, dispute_lost, dispute_won

Examples

Input: ("user,status,payment_provider,buyer_country,currency,amount\nalice,payment_completed,card,DE,EUR,100.00\nbob,payment_pending,bank_transfer,US,USD,50.00\nalice,dispute_lost,wallet,GB,GBP,80.00", {"USD": 1.0, "EUR": 1.10, "GBP": 1.25})

Expected Output: {"alice": 1783, "bob": 80}

Explanation: alice: 100 EUR -> 110 USD, DE card rate 2.3%, so fee is 110 * 0.023 + 0.30 = 2.83 => 283 cents, plus 1500 cents for dispute_lost. bob: 50 USD pending bank transfer uses Part 1 logic, 50 * 1.0% + 0.30 = 0.80 => 80 cents.

Input: ("user,status,payment_provider,buyer_country,currency,amount\nu1,payment_completed,card,JP,JPY,10000\nu1,payment_completed,wallet,FR,EUR,10.00\nu2,refund_completed,card,US,USD,9.99", {"USD": 1.0, "EUR": 1.10, "JPY": 0.0068})

Expected Output: {"u1": 291, "u2": 0}

Explanation: 10000 JPY -> 68 USD, JP card uses the default 3.1% rate, so fee is 68 * 0.031 + 0.30 = 2.408 => 241 cents. 10 EUR -> 11 USD, wallet completed fee is 11 * 1.8% + 0.30 = 0.498 => 50 cents. Total for u1 is 291.

Input: ("", {"USD": 1.0})

Expected Output: {}

Explanation: Empty CSV input produces an empty result.

Input: ("user,status,payment_provider,buyer_country,currency,amount\nsam,payment_completed,bank_transfer,GB,GBP,0.99\nsam,dispute_won,card,US,USD,20.00", {"USD": 1.0, "GBP": 1.25})

Expected Output: {"sam": 31}

Explanation: 0.99 GBP -> 1.2375 USD. GB bank transfer completed fee is 1.2375 * 1.1% + 0.30 = 0.3136125 => 31 cents after half-up rounding. dispute_won adds 0.

Hints

Use one lookup table for currency conversion and another lookup table for the payment_completed country-specific rates.
Only payment_completed changes from Part 1. The other statuses keep the same logic after converting the amount to USD.