On Tuesday, May 17, 2016 at 5:47:06 AM UTC-4, Simon Brooke wrote:
>
> I'm having trouble with writing a function
>
> 1. in idiomatic clojure
> 2. which doesn't blow the stack
>
> The problem is I have a time series of events e.g.
>
> ({:idhistory 78758272, :timestamp #inst
> "2016-03-31T19:34:27.313000000-00:00", :nameid 5637, :stringvalue nil,
> :value 8000.0}
> {:idhistory 78756591, :timestamp #inst
> "2016-03-31T19:33:31.697000000-00:00", :nameid 5637, :stringvalue nil,
> :value 7368.0}
> {:idhistory 78754249, :timestamp #inst
> "2016-03-31T19:32:17.100000000-00:00", :nameid 5637, :stringvalue nil,
> :value 6316.0}
> {:idhistory 78753165, :timestamp #inst
> "2016-03-31T19:31:41.843000000-00:00", :nameid 5637, :stringvalue nil,
> :value 5263.0}
> {:idhistory 78751187, :timestamp #inst
> "2016-03-31T19:30:36.213000000-00:00", :nameid 5637, :stringvalue nil,
> :value 4211.0}
> {:idhistory 78749476, :timestamp #inst
> "2016-03-31T19:29:41.363000000-00:00", :nameid 5637, :stringvalue nil,
> :value 3158.0} ...)
>
> which is to say, each event is a map, and each event has two critical
> keys, :timestamp and :value. The series is sorted in descending order by
> timestamp, i.e. most recent event first. These series are of up to millions
> of events; the average length of the series is about half a million events.
> However, many contain successive events at which the value does not change,
> and where the value doesn't change I want to retain only the first event.
>
> So far what I've got is:
>
> (defn consolidate-events
> "Return a time series like this `series`, but without those events whose
> value is
> identical to the value of the preceding event."
> [series]
> (let [[car cadr & cddr] series]
> (cond
> (empty? series) series
> (=
> (get-value-for-event car)
> (get-value-for-event cadr)) (consolidate-events (rest series))
> true (cons car (consolidate-events (rest series))))))
>
>
The most idiomatic solution would probably be to change the last four lines
to:
(=
(get-value-for-event car)
(get-value-for-event cadr)) (recur (rest series))
true (cons car (lazy-seq (consolidate-events (rest series)))))))
The penultimate line's call is in tail position so you can recur there, and
wrapping the recursive call on the final line in "lazy-seq" defers its
invocation until the caller consumes the next item in the returned
sequence. By then of course the original function call has returned. The
effect is similar to trampoline: the recursion turns into a
constant-stack-space iteration under the hood.
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