The Readings, One at a Time
Stardate 55144.4 · UES Magnanimous
Briefing
The other Magnanimous kept everything. Its sensor log is nineteen years long and Lt. Skree does not want it in memory all at once; Skree wants to walk it, one reading at a time, stopping whenever the pattern shows up.
Build a sequence that produces readings on demand: a start value, a step, and a count. It should hand out one value when asked and not before, so that asking for the first three of a million costs three. Skree describes this as "the difference between reading a book and being hit with it".
Mission objectives
ARCHIE checks all five every time you run diagnostics.
- •Produces start, start+step, ... for exactly count values
- •A count of zero produces nothing
- •It is lazy: values come one at a time on demand, not as a prebuilt list
- •The first three of a million cost three
- •It composes: the values can be doubled with a standard adaptor
Objective
Write readings(start, step, count) as a generator: it
yields start, then start + step, and so on, for exactly
count values. It must be lazy: a generator object, not a list.
Press Run Diagnostics and I will tell you which objectives you actually met.
Objective
Keep the struct as given and implement Iterator for it, so that
Readings::new(start, step, count) yields start, then
start + step, and so on, for exactly count values, then
None. Because it is a real iterator, .take(), .map()
and .collect() all work on it for free.
Press Run Diagnostics and I will tell you which objectives you actually met.
Ask Commander Raghunathan for a hint
Python: yield inside a loop makes a generator, and generators are lazy by nature. Rust: a struct holding current, step and remaining, and impl Iterator with a next that returns None when nothing remains.
Debrief
Skree is walking the other ship's log. So far it is identical to ours, entry for entry, up to a stardate nineteen years ago. Skree has asked for the next reading. Then the next. Skree has stopped saying "hm" and started writing things down.