# 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 I know summarises what the
long-term difference between what my incomings and outgoings will be in the
future if I control what I do with my money.

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`.