add req-030

3 files changed, 75 insertions, 6 deletions

diff --git a/requirements/catalogue/req-030.md b/requirements/catalogue/req-030.md
new file mode 100644
index 0000000..38961be
--- /dev/null
+++ b/requirements/catalogue/req-030.md
@@ -0,0 +1,47 @@
+# 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
diff --git a/requirements/data_models/dml-bucket.md b/requirements/data_models/dml-bucket.md
index 0a7960a..3e33028 100644
--- a/requirements/data_models/dml-bucket.md
+++ b/requirements/data_models/dml-bucket.md
@@ -2,10 +2,18 @@
 
 A bucket represents a real category of spending, like beer or holidays.
 
-| Field   | Type   | Description                                           |
-| ------- | ------ | ----------------------------------------------------- |
-| id      | string |                                                       |
-| name    | string | A short description of the real category of spending. |
-| balance | number | The amount of money currently in the bucket.          |
+| Field             | Type   | Description                                           |
+| ----------------- | ------ | ----------------------------------------------------- |
+| name              | string | A short description of the real category of spending. |
+| balance           | number | The amount of money currently in the bucket.          |
+| balance_cache_key | string | A cache key for the balance. See notes.               |
 
-Sources: req-027, req-028
+Constraints:
+
+-   `name` must be unique.
+
+Notes:
+
+- `balance_cache_key` must be of the format: `<drip_id>:<transaction_id>`.
+
+Sources: req-027, req-028, req-030
diff --git a/requirements/data_models/dml-drip.md b/requirements/data_models/dml-drip.md
new file mode 100644
index 0000000..a421180
--- /dev/null
+++ b/requirements/data_models/dml-drip.md
@@ -0,0 +1,14 @@
+# dml-drip
+
+A drip represents a single daily transaction from the tank into a bucket as a
+result of a pipe.
+
+| Field     | Type   | Description                                         |
+| --------- | ------ | --------------------------------------------------- |
+| id        | string | A unique ID.                                        |
+| bucket_id | string | The ID of the bucket into which this drip drap.     |
+| pipe_id   | string | The ID of the pipe which from which this drip drap. |
+| date      | date   | The date on which this drip drap.                   |
+| amount    | number | The amount of money which dripped.                  |
+
+Sources: req-030