Hi,
On Jan 28, 7:26 am, braver <delivera...@gmail.com> wrote:
> I'd like to partition a positive numeric sequence into subsequences so
> that each increasing subsequence is the longest possible among its
> neighbors. Here's the closest I got so far:
>
> (use 'clojure.contrib.seq-utils)
> (defn clis [s] (->> s (into [-1]) (partition 2 1) (partition-by (fn
> [[x y]] (< x y))) (map #(map second %))))
>
> -1 here plays the role of minus infinity. Prepending it with into
> assumes we supply the original sequence as a vector; would be nice to
> relax that and allow either a seq or a vector. Here's a sample
> output:
>
> (clis [3 2 1 2 3 0 1 2 3 4 0 1 1 2])
> => ((3) (2 1) (2 3) (0) (1 2 3 4) (0) (1) (1) (2))
>
> In between decreasing and increasing, it assigns the middle element
> wrongly. Reassigning it via an elaborate (partition 2 ...) with
> regluing between halves seems too heavy... Anything smarter?
lazy-seq to the rescue:
user=> (defn clis
[coll]
(letfn [(step
[prev s]
(let [fst (first s)
lst (peek prev)]
(if (or (nil? s) (and lst (<= fst lst)))
(cons prev (clis s))
(recur (conj prev fst) (next s)))))]
(lazy-seq
(when-let [s (seq coll)]
(step [] s)))))
user=> (clis [3 2 1 2 3 0 1 2 3 4 0 1 1 2])
([3] [2] [1 2 3] [0 1 2 3 4] [0 1] [1 2])
Hope this helps.
Sincerely
Meikel
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