15 files changed, 95 insertions, 8 deletions
diff --git a/requirements/assumptions/ass-021.md b/requirements/assumptions/ass-021.md
new file mode 100644
index 0000000..fffac36
--- /dev/null
+++ b/requirements/assumptions/ass-021.md
@@ -0,0 +1,4 @@
+# ass-021
+
+There is no feasible SMS UI for creating, updating and deleting buckets or pipes
+which any user would ever choose to use.
diff --git a/requirements/catalogue/req-068.md b/requirements/catalogue/req-068.md
index 7e61976..d744c3d 100644
--- a/requirements/catalogue/req-068.md
+++ b/requirements/catalogue/req-068.md
@@ -3,6 +3,6 @@
 Schist must have a desktop interface for creating, updating and deleting
 buckets.
 
-Sources: req-056, req-000, req-025
+Sources: req-056, req-000, req-025, req-089
 
 Satisfied by: task-030
diff --git a/requirements/catalogue/req-069.md b/requirements/catalogue/req-069.md
index 8e9f559..7b56720 100644
--- a/requirements/catalogue/req-069.md
+++ b/requirements/catalogue/req-069.md
@@ -2,6 +2,6 @@
 
 Schist must have a desktop interface for creating, updating and deleting pipes.
 
-Sources: req-038, req-000, req-025
+Sources: req-038, req-000, req-025, req-089
 
 Satisfied by: task-029
diff --git a/requirements/catalogue/req-085.md b/requirements/catalogue/req-085.md
index a5d1940..c8ce9af 100644
--- a/requirements/catalogue/req-085.md
+++ b/requirements/catalogue/req-085.md
@@ -6,3 +6,5 @@ planned incomings and outgoings, so that my incomings and outgoings are roughly
 equal in the long run.
 
 Sources: req-025, req-000
+
+Satisfied by: req-088
diff --git a/requirements/catalogue/req-086.md b/requirements/catalogue/req-086.md
index 2d5af8e..40acb29 100644
--- a/requirements/catalogue/req-086.md
+++ b/requirements/catalogue/req-086.md
@@ -2,6 +2,6 @@
 
 Schist must have a Web interface for creating, updating and deleting buckets.
 
-Source: req-056
+Sources: req-056, req-090
 
 Satisfied by: task-055
diff --git a/requirements/catalogue/req-087.md b/requirements/catalogue/req-087.md
index 79c33ea..f3e7748 100644
--- a/requirements/catalogue/req-087.md
+++ b/requirements/catalogue/req-087.md
@@ -1,7 +1,7 @@
-# req-086
+# req-087
 
-Schist must have a Web interface for creating, updating and deleting buckets.
+Schist must have a Web interface for creating, updating and deleting pipes.
 
-Source: req-038
+Sources: req-038, req-090
 
 Satisfied by: task-056
diff --git a/requirements/catalogue/req-088.md b/requirements/catalogue/req-088.md
new file mode 100644
index 0000000..a1791fa
--- /dev/null
+++ b/requirements/catalogue/req-088.md
@@ -0,0 +1,36 @@
+# req-088
+
+As a user, when I'm creating, updating and deleting my buckets and pipes, I want
+to see the result of the procedure,
+`calculate_long_term_expected_monthly_tank_change`, described below,
+prominently, so that I see a prominent number which summarises the long-term
+difference between my planned incomings and outgoings.
+
+Source: req-085
+
+Satisfied by: ass-009, req-089, req-090, ass-021, dml-pipe
+
+## calculate_long_term_expected_monthly_tank_change
+
+Explanation: the least common multiple of a day, a week, a month and a year is
+28 years. This procedure works by summing what the tank draw of each pipe over
+28 years would be, and then dividing by 28 x 12 to get the long-term monthly
+tank draw.
+
+1. Let `factor` be `12 x 28`, that is, `336`.
+2. Let `tank_change_over_28_years` be the sum of
+   `calculate_tank_draw_for_pipe_over_28_years(pipe)` for each pipe, `pipe`.
+3. Return `tank_change_over_28_years / factor`.
+
+## calculate_tank_draw_for_pipe_over_28_years
+
+Inputs:
+
+- `pipe`: a pipe
+
+1. If `pipe.period` is `day`, let `factor` be `(365 x 3 + 366) x 7`, that is,
+   `10,227`.
+2. If `pipe.period` is `week`, let `factor` be `365 x 3 + 366`, that is, `1461`.
+3. If `pipe.period` is `month`, let `factor` be `12 x 28`, that is, `336`.
+4. If `pipe.period` is `year`, let `factor` be `28`.
+5. Return `pipe.amount x factor`.
diff --git a/requirements/catalogue/req-089.md b/requirements/catalogue/req-089.md
new file mode 100644
index 0000000..349d4d9
--- /dev/null
+++ b/requirements/catalogue/req-089.md
@@ -0,0 +1,11 @@
+# req-089
+
+As a user, when I'm creating, updating and deleting my buckets and pipes in
+Schist's desktop UI, I want to see the result of the procedure,
+`calculate_long_term_expected_monthly_tank_change`, as described in req-088,
+prominently, so that I see a prominent number which summarises the long-term
+difference between my planned incomings and outgoings.
+
+Source: req-088
+
+Satisfied by: req-068, req-069, task-057
diff --git a/requirements/catalogue/req-090.md b/requirements/catalogue/req-090.md
new file mode 100644
index 0000000..5b72fca
--- /dev/null
+++ b/requirements/catalogue/req-090.md
@@ -0,0 +1,11 @@
+# req-090
+
+As a user, when I'm creating, updating and deleting my buckets and pipes in
+Schist's Web UI, I want to see the result of the procedure,
+`calculate_long_term_expected_monthly_tank_change`, as described in req-088,
+prominently, so that I see a prominent number which summarises the long-term
+difference between my planned incomings and outgoings.
+
+Source: req-088
+
+Satisfied by: req-086, req-087, task-058
diff --git a/requirements/data_models/dml-pipe.md b/requirements/data_models/dml-pipe.md
index d889b0a..7a6825d 100644
--- a/requirements/data_models/dml-pipe.md
+++ b/requirements/data_models/dml-pipe.md
@@ -18,4 +18,4 @@ Constraints:
   - `month`
   - `year`
 
-Sources: req-017, req-037
+Sources: req-017, req-037, req-088
diff --git a/requirements/tasks/epic-000.md b/requirements/tasks/epic-000.md
index 5c66ee0..2ba3f62 100644
--- a/requirements/tasks/epic-000.md
+++ b/requirements/tasks/epic-000.md
@@ -13,5 +13,6 @@ Status: not ready
 - [ ] task-001 (inserting transactions)
 - [ ] task-029 (CRUDing pipes)
 - [ ] task-030 (CRUDing buckets)
+- [ ] task-057 (long-term expected monthly tank change)
 - [ ] task-013 (config & caches)
 - [ ] task-051 (test for accurate bucket balances)
diff --git a/requirements/tasks/epic-005.md b/requirements/tasks/epic-005.md
index fb477a4..4dd54b6 100644
--- a/requirements/tasks/epic-005.md
+++ b/requirements/tasks/epic-005.md
@@ -15,5 +15,6 @@ Blocked by: epic-006
 - [ ] task-054 (inserting transactions)
 - [ ] task-055 (CRUDing buckets)
 - [ ] task-055 (CRUDing pipes)
+- [ ] task-058 (long-term expected monthly tank change)
 - [ ] task-017 (hook up REST API)
 - [ ] task-052 (test for accurate bucket balances)
diff --git a/requirements/tasks/task-028.md b/requirements/tasks/task-028.md
index 7008eb7..5e97b0e 100644
--- a/requirements/tasks/task-028.md
+++ b/requirements/tasks/task-028.md
@@ -12,7 +12,8 @@ The designs should include interfaces for:
 - [ ] Creating buckets
 - [ ] Updating buckets
 - [ ] Deleting buckets
+- [ ] Viewing the long-term expected monthly tank change
 
-Sources: task-029, task-030
+Sources: task-029, task-030, task-058
 
 Epic: epic-000
diff --git a/requirements/tasks/task-057.md b/requirements/tasks/task-057.md
new file mode 100644
index 0000000..46db06c
--- /dev/null
+++ b/requirements/tasks/task-057.md
@@ -0,0 +1,10 @@
+# task-057
+
+Schist's desktop UI should display prominently the result of the procedure,
+`calculate_long_term_expected_monthly_tank_change`, as described in req-088, in
+any view from which the user may be thinking about how to create, update or
+delete their buckets or pipes.
+
+Source: req-089
+
+Epic: epic-000
diff --git a/requirements/tasks/task-058.md b/requirements/tasks/task-058.md
new file mode 100644
index 0000000..a4fc2fb
--- /dev/null
+++ b/requirements/tasks/task-058.md
@@ -0,0 +1,10 @@
+# task-057
+
+Schist's Web UI should display prominently the result of the procedure,
+`calculate_long_term_expected_monthly_tank_change`, as described in req-088, in
+any view from which the user may be thinking about how to create, update or
+delete their buckets or pipes.
+
+Source: req-088
+
+Epic: epic-005