Compute account balances with rejection and overdraft

You are given a list of transaction records as strings. Each record has the format:

account_id,timestamp,currency,amount

• account_id: string identifier
• timestamp: integer (not guaranteed sorted)
• currency: string like USD, EUR
• amount: integer (can be positive for credit or negative for debit)

Maintain balances per (account_id, currency).

Part 1 — Final balances

Process all transactions and return the final balance for each (account_id, currency).

• Exclude entries whose final balance is exactly 0.
• Output can be in any order (unless you choose to sort).

Part 2 — Reject transactions that would go negative

Now process transactions in increasing timestamp order (tie-breaker can be input order).

A transaction is rejected if applying it would make that account’s balance for that currency fall below 0.

• Rejected transactions have no effect.
• Return final non-zero balances as in Part 1.

Part 3 — Use a platform account for insufficient funds

Add support for a special “platform” account that can cover shortfalls. You are given platform_account_id as a parameter.

Processing is still in timestamp order. For a debit transaction (negative amount) on account A in currency C:

1. If A has enough balance in C, apply it normally.
2. Otherwise, attempt to cover the shortfall using the platform account’s balance in the same currency C:

• Deduct all remaining funds from A (bringing it to 0).
• Deduct the remaining required amount from the platform account.

1. If the platform account also lacks sufficient funds in C, reject the transaction (no balances change).

Credits (positive amount) always apply.

Return final non-zero balances for all accounts (including the platform account if non-zero).

Constraints (you may assume)

• Up to ~10^5 transactions
• timestamp fits in 64-bit integer
• amount fits in 64-bit signed integer