(keep-indexed f)
(keep-indexed f coll)
Returns a lazy sequence of the non-nil results of (f index item). Note,
this means false return values will be included. f must be free of
side-effects. Returns a stateful transducer when no collection is
provided.
Source
(defn keep-indexed
"Returns a lazy sequence of the non-nil results of (f index item). Note,
this means false return values will be included. f must be free of
side-effects. Returns a stateful transducer when no collection is
provided."
{:added "1.2"
:static true}
([f]
(fn [rf]
(let [iv (volatile! -1)]
(fn
([] (rf))
([result] (rf result))
([result input]
(let [i (vswap! iv inc)
v (f i input)]
(if (nil? v)
result
(rf result v))))))))
([f coll]
(letfn [(keepi [idx coll]
(lazy-seq
(when-let [s (seq coll)]
(if (chunked-seq? s)
(let [c (chunk-first s)
size (count c)
b (chunk-buffer size)]
(dotimes [i size]
(let [x (f (+ idx i) (.nth c i))]
(when-not (nil? x)
(chunk-append b x))))
(chunk-cons (chunk b) (keepi (+ idx size) (chunk-rest s))))
(let [x (f idx (first s))]
(if (nil? x)
(keepi (inc idx) (rest s))
(cons x (keepi (inc idx) (rest s)))))))))]
(keepi 0 coll))))