Toy Payment Engine

To run the binary

$ # errors are printed to stderr
$ cargo run -- transactions.csv > accounts.csv

To run tests

$ cargo test

Additional assumptions

1. Disputes are only made against deposits. It's not clear to me how disputes with withdrawal are to be handled (can held amount be negative?). So, instead I decided to ignore (and report error) for disputes against withdrawals.

2. Transaction amounts are non negative.

3. A disputes can cause available amount to become negative.

4. It's not specified if a locked account can receive deposits and withdrawals. The only thing that is specified is that a chargeback causes and account be marked as locked.

5. A chargeback doesn't change the available amount.

Design notes

1. The code is divided into the following modules:

   1. csv: read from and write to CSVs, types for CSV records.
   2. ledger: maintain balances and disputes.
   3. model: data types to define the business models.
   4. lib: wrap everything else and define functions to drive the engine.
   5. main: drive the engine via lib.

2. Deposits and disputes are stored separately from balances, in memory. This makes it trivial to retrieve all the balances and print them.

3. The Balance for a Client is only created on first deposit. It suffices because none of the other transactions would have an effect on the Balance of a Client with no deposits.

4. Amounts are represented by rust_decimal::Decimal.

Remaining work

1. Implement a custom deserializer for Amount, to reject negative amounts from the input CSV.

2. Generate a large sample CSV to test against.

3. Look into writing property based tests with QuickCheck

4. If locked accounts are to not accept deposits and withdrawals, handle that in ledger.

5. Use something like failure, anyhow and such to handle errors instead of overloading std::io::Error everywhere.

6. Unit test the error cases in ledger.