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(ns lazy.partition) | |
;; returns tuple: [(take-while pred coll) rest-of-coll] | |
(defn split-with [acc pred coll] | |
(lazy-seq | |
(let [h (first coll)] | |
(if (and h (pred h)) | |
(split-with (lazy-cat acc [h]) pred (rest coll)) | |
[acc coll])))) | |
(defn lazy-partition-by [f coll] | |
(lazy-seq | |
(when-let [s (seq coll)] | |
(let [v (f (first s)) | |
;; Need coordination here, since we don't know if | |
;; the car has been realised by the time we want to start | |
;; consuming cdr | |
sp (delay (split-with () #(= (f %) v) s))] | |
(cons (first @sp) | |
(lazy-partition-by f (second @sp))))))) | |
(let [big-seq (repeatedly 1000000 #(rand-int 10)) | |
very-lazy-seq (lazy-partition-by even? big-seq)] | |
(assert (instance? clojure.lang.LazySeq very-lazy-seq)) | |
(assert (every? #(instance? clojure.lang.LazySeq %) (take 10 very-lazy-seq)))) | |
;; BROKEN | |
;; (let [big-seq-two (mapcat (fn [_] (repeat (rand-int 100) (rand-int 10))) (range 10000))] | |
;; (lazy-partition-by odd? big-seq-two)) | |
;; ---------------------------------------------------------------------------------------- | |
;; mutation saves (??) the day | |
(defn take-while! [pred coll] | |
(lazy-seq | |
(let [h (first @coll)] | |
(when (and h (pred h)) | |
(swap! coll next) | |
(cons h (take-while! pred coll)))))) | |
(defn lazy-partition-helper [f coll] | |
(lazy-seq | |
(when-let [h (-> @coll seq first)] | |
(let [v (f h) | |
subseq (take-while! #(= (f %) v) coll)] | |
#_(prn :coll @coll) | |
(cons subseq | |
(do (when-not (realized? subseq) (dorun subseq)) | |
(lazy-partition-helper f coll))))))) | |
(defn lazy-partition-by-2 [f coll] | |
(lazy-partition-helper f (atom coll))) | |
(let [big-seq (repeatedly 1000000 #(rand-int 10)) | |
very-lazy-seq (lazy-partition-by-2 even? big-seq)] | |
(assert (instance? clojure.lang.LazySeq very-lazy-seq)) | |
(assert (every? #(instance? clojure.lang.LazySeq %) (take 10 very-lazy-seq)))) | |
(let [big-seq-two (mapcat (fn [_] (repeat (rand-int 10000) (rand-int 10))) | |
(range 10000)) | |
very-lazy-seq-two (lazy-partition-by-2 odd? big-seq-two)] | |
(assert (instance? clojure.lang.LazySeq very-lazy-seq-two)) | |
(assert (every? #(instance? clojure.lang.LazySeq %) (take 10 very-lazy-seq-two)))) | |
Version 2 works at the cost of introducing an atom into the mix.
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Overflows the stack if the subsequences are large!