Graph editor can fail to warn of akrasia horizon violation

[dreeves]

Desiderata

Preview Give feedback

1. Confirm / firm up the replicata
2. Bee or Dreev deal with https://github.com/beeminder/beeminder/issues/1643
3. TBD

Via Nicky and RPerce. Widely confirmed.

NICKY: Looks like you were trying to set a break from 9th July to 10th July, on the 3rd. So it's correct that you can't make that change: it's within the akrasia horizon of 7 days.

The bug is that the graph going red and saying "error" doesn't happen when you're on the cusp of the akrasia horizon.

Replicata

1. Try to set a break on a goal that will kick in 6 days from now (e.g., on Sunday try to set a break for next Sunday). [wait, but Sunday to Sunday is 7 days]
2. Expect to see the error flagged up on the graph preview (e.g. as would happen if on Sunday you tried to set a break for next Saturday).
3. Press submit and receive an error.

Working as expected (violating the akrasia horizon):

Edge-case where the graph doesn't warn you but the API won't accept it:

Cognata

• Can't extend flat spot past the akrasia horizon #232
• Graph editor surprise when deleting penultimate row: changes final rate #236
• Different off-by-one error with akrasia horizon #258
• Graph editor won't allow goal to end sooner #196
• Red line that hugs the pink zone erroneously gives error #164
• https://github.com/beeminder/beeminder/issues/1643
• Graph editor failing most goal changes at the moment because of Rails 5.2 upgrade thing #261

Verbata: akrasia horizon, graph editor, user confusion, baneful bugs,

[dreeves]

Bee and I discussed this and found the place in the Beebody code (related to roadall in the API) where this needs to be fixed:

today = Date.today
eps = 0.000000001 # floating point epsilon
# diff the roads and return list of days w/in AKH at which the road is easier
t = (0..7).map{|v| today + v}
  .map{|v| {date: v, from: road_front_center_at(from, v, goal.siru),
                     to:   road_front_center_at(to,   v, goal.siru)}}
  .select{|v| goal.yaw==1 ? v[:to] + eps < v[:from] : v[:from] + eps < v[:to]}

t.empty? ? [nil, true] :
  ["Error: graph matrix can't get easier in akrasia horizon"]

It's the above that sometimes happens when it shouldn't.

def road_front_center_at(road,date,siru)
  # has same sketchiness as roadfunc: returns frontmost point on discontinuities
  start = road[0][:end]
  val = road[0][:endv]
  if date >= start
    road[1..-1].each{ |v|
      if (v[:type]=="ratedate" ||
          v[:type]=="valuedate") && v[:end] < date
        val = road_segment_endv(v,start,val,siru)
        start = v[:end]
      else
        val += road_segment_rate(v,start,val,siru) * (date - start)
        break
      end
    }
  end
  val
end

def road_segment_rate(v,start,startv,siru)
  # need to normalize rate we are comparing to daily rate:
  # dailyrate is rate/siru * siday
  v[:type]=="ratedate" || v[:type]=="ratevalue" ?
    (v[:rate]/siru.to_f)*1.day.seconds :
    (v[:endv]-startv)/(v[:end]-start).to_f
end

def road_segment_endv(v,start,startv,siru)
  # need to normalize rate we are comparing to daily rate:
  # dailyrate is rate/siru * siday
  v[:type]=="ratevalue"|| v[:type]=="valuedate" ?
    v[:endv] :
    startv + ((v[:rate]/siru.to_f)*1.day.seconds) * (v[:end]-start)
end