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# req-030
Schist must implement the procedure described below as
`calculate_bucket_balance`.
Source: req-029
Satisfied by:
- task-001 (desktop interface for inserting transactions)
- task-006 (SMS interface for inserting transactions)
- task-054 (Web interface for inserting transactions)
- req-109 (transactions have a `timestamp`)
- req-107 (transactions have a `bucket_id`)
- req-108 (transactions have an `amount`)
- req-027 (buckets have a `balance`)
- req-110 (buckets have a `balance_cache_key`)
- req-111 (buckets have an `id`)
- req-098 (buckets have a `name`)
- req-035 (implement `is_bucket_balance_up_to_date`)
- req-037 (implement `update_bucket_drips`)
- dml-drip (drips have a `bucket_id` and `amount`)
## 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}`.
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