Tried myself and my first transducer for fun:
(defn partition-when
[f]
(fn [rf]
(let [a (java.util.ArrayList.)
fval (volatile! false)]
(fn
([] (rf))
([result]
(let [result (if (.isEmpty a)
result
(let [v (vec (.toArray a))]
;;clear first!
(.clear a)
(unreduced (rf result v))))]
(rf result)))
([result input]
(if-not (and (f input) @fval)
(do
(vreset! fval true)
(.add a input)
result)
(let [v (vec (.toArray a))]
(.clear a)
(let [ret (rf result v)]
(when-not (reduced? ret)
(.add a input))
ret))))))))
(into [] (partition-when
#(.startsWith % ">>"))
[">> 1" ">> 2" "22" ">> 3"])
Based on partition-by (on master branch). Would be interesting how fast it
is compared to the other implementations.
Any comments appreciated.
Cheers
On Tuesday, March 3, 2015 at 2:46:29 PM UTC-5, Frank wrote:
>
> Hi all,
>
> for some tests I need a function which starts a new partition each time a
> predicate returns true, for instance:
>
> (partition-when
> (fn [s] (.startsWith s ">>"))
> [">> 1" "2" "3" ">> 4" "5" "6"])
> :=> [[">> 1" "2" "3"] [">> 4" "5" "6"]]
>
> Since I haven't found a built-in function, I copied, pasted, and modified
> the core function partition-by:
>
> (defn partition-when
> [f coll]
> (lazy-seq
> (when-let [s (seq coll)]
> (let [fst (first s)
> run (cons fst (take-while #(not (f %)) (next s)))]
> (cons run (partition-when f (seq (drop (count run) s))))))))
>
> Is there a better (more idiomatic) way to achieve the same result?
>
> Thank you in advance.
>
> Frank
>
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