Compute transaction fees from a CSV string

Parse a CSV of payment transactions and compute the **total processing fee** owed by each user, returned in **integer cents**. ## What to implement Implement `solution(csv_string)`. The input is a single CSV string; return a **dictionary mapping each user to their total fee, expressed as a whole number of cents** (an integer). ## Input format When non-empty, the CSV begins with the exact header line: ``` user,status,payment_provider,amount ``` Each subsequent line is one transaction with these fields: - **`user`** — the user the transaction belongs to. - **`status`** — one of `payment_completed`, `payment_failed`, `payment_pending`, `refund_completed`, `dispute_lost`, `dispute_won`. - **`payment_provider`** — one of `card`, `bank_transfer`, `wallet`. - **`amount`** — the transaction amount in **US dollars**, a non-negative decimal with at most 2 digits after the decimal point. ## Fee rules Compute the fee for each transaction based on its `status`: - **`payment_completed`, `payment_failed`, `payment_pending`** — the fee is a percentage of `amount` plus a fixed **$0.30**: `fee = amount × provider_rate + 0.30` (USD) where `provider_rate` depends on the payment provider: | provider | rate | |---|---| | `card` | 2.9% | | `bank_transfer` | 1.0% | | `wallet` | 1.5% | - **`dispute_lost`** — a flat **$15.00** fee. - **`refund_completed`** — **$0** (no fee). - **`dispute_won`** — **$0** (no fee). ## Rounding and accumulation - Round **each transaction's fee** to the nearest cent using **half-up** rounding, then convert it to an integer number of cents **before** adding it to that user's running total. - A user's returned value is the **sum of their per-transaction cent fees**. ## Output and edge cases - Return a **dictionary** mapping `user` → total fee **in cents** (integer). - **Include every user that appears** in the CSV, even if their total fee is `0` (for example, a user with only a `refund_completed` or `dispute_won` transaction still appears, mapped to `0`). - **Ignore blank lines** (and any row with no user). - If the input string is **empty or whitespace-only**, return an **empty dictionary** `{}`. ## Examples A `card` `payment_completed` of `100.00` produces `100.00 × 0.029 + 0.30 = 3.20` USD → **320** cents. A `wallet` `payment_completed` of `0.99` produces `0.99 × 0.015 + 0.30 = 0.31485` USD, which rounds half-up to `0.31` USD → **31** cents.

Compute the total payment-processing fee charged to each user, in **whole USD cents**, given a CSV of transactions and a currency-to-USD exchange-rate table. ## Function ``` solution(csv_string, exchange_rates) ``` ## Inputs - **`csv_string`** — a CSV string whose header (when non-empty) is exactly: `user,status,payment_provider,buyer_country,currency,amount` - **`exchange_rates`** — a dictionary where `exchange_rates[currency]` is the USD value of **1 unit** of that currency. ## Output Return a dictionary mapping each **`user`** to their **total fee in USD cents** (an integer). Every user that appears in the CSV must be present in the result — **even if their total fee is `0`**. If `csv_string` is empty or contains only whitespace, return `{}`. (Key order in the returned dictionary does not matter.) ## How to process each transaction **1. Convert the amount to USD:** ``` converted_usd = amount * exchange_rates[currency] ``` **2. Compute the transaction fee in USD based on `status`:** ### `payment_completed` Fee = `converted_usd * rate + 0.30`, where `rate` depends on the **provider** and **buyer_country**: | Provider | US / CA | GB | DE / FR / AT | All other countries | |-----------------|---------|-------|--------------|---------------------| | `card` | 2.9% | 2.5% | 2.3% | 3.1% | | `bank_transfer` | 1.2% | 1.1% | 1.0% | 1.4% | | `wallet` | 1.8% | 1.8% | 1.8% | 1.8% | (`wallet` is **1.8%** for any country.) ### `payment_failed` and `payment_pending` Use the **provider-only** (country-independent) rates from Part 1 on the converted USD amount: | Provider | Rate | |-----------------|-------| | `card` | 2.9% | | `bank_transfer` | 1.0% | | `wallet` | 1.5% | Fee = `converted_usd * rate + 0.30`. ### Other statuses - `refund_completed` → fee = **0 USD** - `dispute_won` → fee = **0 USD** - `dispute_lost` → fee = **15.00 USD flat** (no conversion, no percentage, no fixed add-on) **3. Round and accumulate:** Round **each transaction's fee** to the nearest cent using **half-up rounding** (e.g. 0.005 rounds up to 0.01), then add the resulting integer cents to that user's running total. Round per transaction — **before** adding to the total, not after summing. ## Notes - **Skip any row whose `user` field is empty** (after trimming surrounding whitespace), including fully blank lines. - Even users whose only transactions are `refund_completed`, `dispute_won`, or otherwise sum to `0` must still appear in the result with a total of `0`. ## 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`.