On Tue, Aug 4, 2009 at 5:50 PM, Rich Hickey <richhic...@gmail.com> wrote:
> On Aug 4, 4:31 pm, John Harrop <jharrop...@gmail.com> wrote:
> > What about things like:
> >
> > (persistent!
> > (reduce
> > (fn [x [i v]] (assoc! x i v))
> > (transient (vec (repeat 0 (reduce max (map first
> > coll-of-index-val-pairs)))))
> > coll-of-index-val-pairs))
> >
>
> Yes, that's completely fine intended usage, as the return value of the
> reducing fn becomes an argument to the next call.
>
> > which is just the transientification of
> >
>
> Nice word - transientification.
Thanks.
Of course, this makes me think of ways to possibly speed up other
operations. For example:
(defn vmap* [fun vect]
(let [c (count vect)]
(loop [out (transient []) i 0]
(if (= i c)
out
(recur (conj! out (fun (nth vect i))) (inc i))))))
(defn vmap [fun vect]
(persistent! (vmap* fun vect)))
Vector in, vector out, conj'd up using a transient.
And, of course:
(defn vpmap [fun vect]
(loop [out (vmap* #(future (fun %)) vect) i (dec (count vect))]
(let [o2 (assoc! out i @(nth out i))]
(if (zero? i)
(persistent! o2)
(recur o2 (dec i)))))
Note that this last manipulates a transient vector in a single thread,
though other threads (from the agent pool) calculate a bunch of futures that
are stored in it. The transient vector of futures is generated using the
vmap* from above, and then the futures are replaced with their values
in-place by the loop in vpmap, before this persistentizes and returns that
vector. The future-dereferencing loop works backwards from the end of the
vector in case zero? is a quicker test than a general equality test. This is
likely on hardware that implements an equality test as a subtraction
followed by a zero test, because it eliminates the subtraction. On the other
hand, hardware with a 1-cycle equality test of 32-bit ints is plausible, as
is hardware that optimizes forward traversal of vectors, so I can't vouch
for that being an optimization. The first loop has to go forward to conj up
the output vector without reversing it, though.
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