# req-030

Schist must implement the procedure described below as
`calculate_bucket_balance`.

Source: req-029

Satisfied by: task-001, task-006, task-054

## calculate_bucket_balance

Inputs:

- A bucket name, `name`

1. If no bucket exists with the name, `name`, then return empty.
2. If more than one bucket exists with the name, `name`, then panic.
3. Let `bucket` be the bucket with the given name.
4. Let `cache_key = bucket.balance_cache_key`.
5. Let `is_up_to_date` be the result of the procedure,
   `is_bucket_balance_up_to_date(bucket)`, as described in req-035.
6. If `is_up_to_date = TRUE`, then return `bucket.balance`.
7. Carry out the procedure, `update_bucket_drips(bucket)`, as described in
   req-037.
8. Let `latest_transaction` be the transaction `t` with the most recent
   `t.timestamp` such that `t.bucket_id = bucket.id`.
9. Let `transaction_sum` be the sum over the property, `amount`, of all
   transactions `t` such that `t.bucket_id = bucket.id`.
10. Let `latest_drip` be the drip `d` with the most recent `d.timestamp` such
    that `d.bucket_id = bucket.id`.
11. Let `drip_sum` be the sum over the property, `amount`, of all drips `d` such
    that `d.bucket_id = bucket.id`.
12. Let `balance` be `drip_sum - transaction_sum`.
13. Set `bucket.balance` to `balance`.
14. Set `bucket.balance_cache_key` to
    `{latest_drip.id}:{latest_transaction.id}`.