Hi, Clojurists!
I'm writing some clojure in these days, and I stumbled upon an error that
I'm unable to easily understand.
My goal is to asynchronously process a 2D vector in order to solve a
parametric version of the 8 chess queens problem (where the parameters are:
the board's dimensions and the chess pieces to be put on the board).
My solution mainly consists in two functions: the worker and the boss. The
worker takes in input a board (2d array) and yields a collection of boards
that contain another chess piece placed in a certain way. This means that I:
> take all of the free squares from the board
> try to place the chesspiece on every one of them
> if I succeed (do not hit other pieces) - it goes to the output.
Until I use the version without the transients - it appears to be working.
But I wanted to optimize my code, because the last time I solved it using
recursive pmap - when I gave it an output that's a bit larger - it went
into a bats**t crazy heap overflow. So I tried to use a transient
collection.
And here my problems began. When I try to execute this code it dies after
just a couple of moves processed, giving me a nullpointer on the row where
conj! is situated:
NullPointerException clojure.core/conj! (core.clj:3257)
What a hellish issue may it be? Clojure is not helping - its error messages
are always cryptic, and really hard to understand.
Here is my code. It's not complete though - some functions are omitted, for
the sake of brevity. Please, help me to understand what am I missing here...
Thank you.
(def fw (agent {}))
(defn worker [subj]
(let [{brd :board pwns :pawns} subj
nxt (get-next-pawn pwns)]
(when (map? subj)
(when (and brd pwns)
(into #{}
(persistent!
(reduce #(when (not (keyword? %2))
(conj! %1
(if nxt
{:board %2 :pawns (update pwns nxt dec)}
brd)))
(transient [])
(map #(place nxt (first %) (last %) brd)
(get-free-squares brd)))))))))
(defn boss []
(send fw
#(into {} %2)
{:complete 0
:coll
#{{:board (blank-board 4 4)
:pawns
{:rook 1
:knight 3
:bishop 2
}}}})
(loop []
(await fw)
@fw
(let [{done :complete col :coll} @fw]
(if (= done (count col))
col
(do (send-off fw
#(identity {:complete (+ (:complete %1)
(count (filter
vector? %2)))
:coll (into (:coll %1) %2)})
(reduce #(into %1 %2)
#{}
(pmap #(worker %) col)))
(recur))))))
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