You need to use a buffer to defer calls to the reduced function
(defn drop-last [n coll]
(reducer coll
(fn [f1]
(let [buffer
(atom clojure.lang.PersistentQueue/EMPTY)]
(fn self
([] (f1))
([ret x]
(let [b (swap! buffer conj x)]
(if (<= (count @buffer) n)
ret
(do
(swap! buffer pop)
(f1 ret (peek b)))))))))))
An array or a ring buffer should be used instead of the atom and
persistent queue combo to reduce allocation.
take-last is harder because you can't know when the reduction is over when
using #'reducer, so you have to implement CollReduce yourself:
(defn take-last [n coll]
(reify clojure.core.protocols.CollReduce
(coll-reduce [this f1]
(clojure.core.protocols/coll-reduce this f1 (f1)))
(coll-reduce [_ f1 init]
(clojure.core.protocols/coll-reduce
(clojure.core.protocols/coll-reduce
coll
(fn [q x]
(let [q (conj q x)]
(if (<= (count q) n)
q
(pop q)))) clojure.lang.PersistentQueue/EMPTY)
f1 init))))
again, use of a mutable array/buffer would be preferable.
hth,
Christophe
On Thu, Aug 8, 2013 at 1:00 PM, Jozef Wagner <[email protected]> wrote:
> Is it possible to implement efficient butlast (and drop-last, take-last)
> with reducers? The only solution I can think of needs additional reduce to
> compute count, which may often be undesirable.
>
> Or is it OK to say that reducers are not designed for such cases?
>
> JW
>
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