Phlogiston/Forecasting Model

For all calculations, "now" or "today" is the most recent day in the data. Phlogiston makes two independent forecasts, one by points, using the Story Points field in Phabricator, and one by count of tasks.

Velocity over a time period is the number of resolved tasks (or sum of points) at the end minus resolved tasks at the beginning. If negative, it is counted as zero in this model.

Backlog growth over a time period is the total number of tasks, open and resolved (or sum of points), at the end minus at the beginning. If negative, it is counted as zero in this model.
 * 1) Calculate velocity per category
 * 2) Calculate velocity for the week ending today
 * 3) Make three forecasts of future velocity
 * 4) Optimistic is the average velocity of the highest 3 weeks within the last 3 months, with a minimum of 2
 * 5) Nominal is the average velocity per week over the last 3 weeks, with a minimum of 1
 * 6) Pessimistic is the average velocity of the worst 3 weeks in last 3 months, with a minimum of zero.
 * 7) Note: For teams that resolve tasks every 2 weeks, and thus have a bunch of 0 weeks, this will produce a falsely low forecast
 * 8) Count all negative weeks as zero before averaging or summing.
 * 9) Make three forecasts of backlog growth (As of 17 Mar 2016, work in progress not yet implemented in production).
 * 10) Optimistic is flat: 0 growth.
 * 11) Nominal is the average of the last three weeks.
 * 12) Pessimistic is the greater of
 * 13) the average backlog growth of the last three weeks
 * 14) the average weekly growth over the last three months.
 * 15) Count all negative weeks as zero before averaging or summing.
 * 16) Combine Velocity and backlog growth forecasts
 * 17) Optimistic is not changed
 * 18) Updated nominal velocity is nominal velocity - nominal backlog growth (not less than 0).
 * 19) Updated pessimistic velocity = pessimistic velocity - pessimistic backlog growth (not less than 0).
 * 20) Divide the total remaining backlog (per category) by each of the three velocities to determine # of weeks remaining.