Improve detection of old questions

[Azquelt]

Cases to be considered:

Base state, API returns the most recent X questions

• Q3
• Q2
• Q1

New question asked, should see Q4 as new question and send message accordingly

• Q4
• Q3
• Q2

Question Q5 added to list (e.g. because a relevant tag was added), should be considered as a new question:

• Q3
• Q5
• Q2

Question Q2 removed from list (e.g. tag removed or question deleted), should not see Q0 as a new question:

• Q3
• Q1
• Q0

[Azquelt]

Also note that we have occasionally observed that questions appear in the API some time after they are created or updated.

[Azquelt]

Potential approach:

Keep a created date cutoff

• only ever increases

Keep a list of questions (ID + createdDate) seen since the cutoff

• add all questions seen to this
• remove any questions not after the cutoff

Operations:

• filter old questions (newQuestions, oldState)
• compute new state (newQuestions, oldState)

ComputeNewState:

• compute new cutoff as being max(earliest createdDate from the newQuestions, oldState.cutoff)
• create new list of questions (ID + createdDate) by merging oldState questions with newQuestions
• remove any questions which were not created after the cutoff