Tested against

• unit tests
• large local file
• large stream

Checked with

• cargo fmt
• cargo clippy
• cargo test
• manual tests

Assumptions

• it's legal to dispute a withdrawal, but all operations, including resolve and chargeback, must leave available funds nonnegative; otherwise, a transaction is dropped

Performance notes

• if the number of clients is expected to be near u16::MAX, it's better to use a plain array instead of a hash map
• same for transactions - 4B records is still something a server can usually handle
• at first glance, deposit and withdrawal transactions could have a separate type in the Rust type system, but since they're symmetrical, withdrawals are represented simply as transactions with negative amounts - the underlying decimal type is capable of storing the sign anyway, and it makes the structure footprint smaller
• total funds are not denormalized and stored in order to further minimize the memory footprint - total funds are trivially computable from available + held