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+# req-030
+
+Schist must implement the procedure described below as `calculate_bucket_balance`.
+
+## 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 sub-procedure,
+   `is_bucket_balance_up_to_date(bucket)`, as described below.
+6. If `is_up_to_date = TRUE`, then return `bucket.balance`.
+7. Let `transaction_sum` be the sum over the property, `amount`, of all
+   transactions `t` such that `t.bucket_id = bucket.id`.
+8. Let `drip_sum` be the sum over the property, `amount`, of all drips `d` such
+   that `d.bucket_id = bucket.id`.
+9. Return `drip_sum - transaction_sum`.
+
+## is_bucket_balance_up_to_date
+
+Inputs:
+
+-   A bucket, `bucket`
+
+1. Let `cache_key = bucket.balance_cache_key`.
+2. If `cache_key` is not in the format, `<string>:<string>`, then return FALSE.
+3. Let `drip_id` be the part of `cache_key` before the `:`.
+4. If there is no drip with the ID, `drip_id`, then return FALSE.
+5. Let `drip` be the drip with the ID, `drip_id`.
+6. If there is a drip, `drip_2`, such that `drip_2.date > drip.date` and
+   `drip.bucket_id = bucket.id`, then return FALSE.
+7. Let `transaction_id` be the part of `cache_key` after the `:`.
+8. If there is no transaction with the ID, `transaction_id`, then return FALSE.
+9. Let `transaction` be the transaction with the ID, `transaction_id`.
+10. If there is a transaction, `transaction_2`, such that
+    `transaction_2.date > transaction.date` and
+    `transaction.bucket_id = bucket.id`, then return FALSE.
+11. Return TRUE.
+
+Source: req-029
+
+Satisfied by: dml-bucket, dml-drip