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))))))
Obviously, with millions of events or even merely hundreds of thousands, a
recursive function blows the stack. Furthermore, this one isn't even tail
call optimisable. I tried creating an inner function which I naively
thought should be tail call optimisable, but it fails 'Can only recur from
tail position':
(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]
(remove
nil?
(let [inner (fn [series]
(let [[car cadr & cddr] series]
(if
(not (empty? series))
;; then
(cons
(if
(= (get-value-for-event car)
(get-value-for-event cadr))
;; then
nil
;; else
car)
(if
(not (empty? series))
(recur (rest series)))))))]
(inner series))))
Test for the function is as follows:
(deftest consolidate-events-test
(testing "consolidate-events"
(let [s1 [{:timestamp #inst "2016-03-31T19:34:27.313000000-00:00",
:value 8000.0}
{:timestamp #inst "2016-03-31T19:33:31.697000000-00:00",
:value 7368.0}
{:timestamp #inst "2016-03-31T19:32:17.100000000-00:00",
:value 6316.0}
{:timestamp #inst "2016-03-31T19:31:41.843000000-00:00",
:value 5263.0}
{:timestamp #inst "2016-03-31T19:30:36.213000000-00:00",
:value 4211.0}
{:timestamp #inst "2016-03-31T19:29:41.363000000-00:00",
:value 3158.0}]
s2 [{:timestamp #inst "2016-03-31T19:34:27.313000000-00:00",
:value 8000.0}
{:timestamp #inst "2016-03-31T19:33:31.697000000-00:00",
:value 7368.0}
{:timestamp #inst "2016-03-31T19:33:17.100000000-00:00",
:value 6316.0}
{:timestamp #inst "2016-03-31T19:32:27.100000000-00:00",
:value 6316.0}
{:timestamp #inst "2016-03-31T19:32:17.100000000-00:00",
:value 6316.0}
{:timestamp #inst "2016-03-31T19:31:41.843000000-00:00",
:value 5263.0}
{:timestamp #inst "2016-03-31T19:30:36.213000000-00:00",
:value 4211.0}
{:timestamp #inst "2016-03-31T19:29:41.363000000-00:00",
:value 3158.0}]]
(is (= s1 (consolidate-events s1)) "There are no events in s1 that
can be consolidated")
(is (= s1 (consolidate-events s2)) "When consolidated, s2 = s1")
(is (not (= s2 (consolidate-events s2))) "When consolidated, s2 no
longer equals s2"))))
Any help gratefully accepted!
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