The 'palindrome?' function can be made much faster. Your version --
which is idiomatic and fine when perf isn't a factor -- turns the test
string into a sequence, reverses it, turns it back into a string, then
checks for full equality with the original. There are faster (if
uglier) ways to check for palindromes. This version finds the Level 1
answer in about 150ms on my machine:
(defn palindrome?
[^String s]
(let [len (.length s)
mid (quot len 2)]
(loop [i 0
j (dec len)]
(if (= i mid)
true
(when (= (.charAt s i) (.charAt s j))
(recur (inc i) (dec j)))))))
Also, the built-in function max-key will do what your sort/max-comp
code is doing more concisely:
(apply max-key count (filter palindrome? (all-combs input)))
HTH,
Justin
On Oct 12, 3:02 pm, tonyl <celtich...@gmail.com> wrote:
> Hi, I just started to learn clojure in a more serious way and I am
> doing the first level of the greplin challenge.
>
> I made it to work with a short palindrome like the example they give
> me, but when it comes to work with the input file, it takes for ever
> and I have to stop it.
>
> $ time clj level1.clj
> ^C
> real 11m35.477s
> user 1m44.431s
> sys 9m3.878s
>
> This is my code:
>
> (defn palindrome? [s]
> (= s (reduce str (reverse s))))
>
> (defn all-combs [in]
> (let [len (count in)]
> (for [i (range 0 (- len 2)), j (range (+ i 1) len)]
> (subs in i j))))
>
> (defn max-comp [x y]
> (let [lenx (count x), leny (count y)]
> (cond (< lenx leny) 1
> (= lenx leny) 0
> (> lenx leny) -1)))
>
> ;;(let [input "I like racecars that go fast"]
> (let [input (slurp "../_input/level1.in")]
> (println (nth (sort max-comp (filter palindrome? (all-combs
> input))) 0)))
>
> The input file is thishttp://challenge.greplin.com/static/gettysburg.txt
>
> It looks a bit procedural. It is long, but I don't think is the
> biggest bottleneck, I think it is my approach to create all the
> combinations possible for substrings. Maybe I should be using a lazy
> seq? How would I go to do that?
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