Syllabus Lesson 106 of 141 · Past the tenth idea
Past the tenth idea

Prototype one: what changed while you were away

Why it matters An idea is cheap until it meets a test. A small honest prototype shows whether the unusual idea holds up, and it often shows the hard part nobody saw.

The card "the user comes back after a year away" gave the to-do app a feature that is not in the top ten: when you open a shared list, it first shows what changed since your last visit. Not every edit: the net change. If a task's owner went from Ana to Ben and back to Ana, nothing changed for you.

The edit log is long and lives on the server; every entry fetched costs. Your last visit was recent, so only the end of the log matters.

The data

log = [(1, "t1", "status", "todo"), (2, "t2", "owner", "ana"), (5, "t1", "status", "doing"),
       (6, "t2", "owner", "ben"), (7, "t2", "owner", "ana"), (8, "t3", "title", "New"), (9, "t1", "status", "done")]
snapshot = {("t1", "status"): "todo", ("t2", "owner"): "ana"}   # what you saw at time 4
changes_since(Tape(log), snapshot, 4)
# {("t1", "status"): ("todo", "done"), ("t3", "title"): (None, "New")}
Your turn

Write changes_since(log, snapshot, since). log is a Tape of (t, item, field, value) edits sorted by t (whole numbers, increasing). snapshot maps (item, field) to the value the user saw at time since. Return a dict from (item, field) to (old, new) for every field edited after since whose latest value differs from the snapshot; old is the snapshot value, or None if the field is new. An edit that is later put back, or that sets the same value, is not a change. Budget: the log has 50,300 edits and about 300 are after since; read at most 3,000 entries. len() is free.

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