Comments on: Google Treasure Hunt, Question #4

By: Christophe Grand

Christophe Grand — Tue, 17 Jun 2008 06:38:00 +0000

@david: ah, it fits better in the natural order of all things :-)

By: David

David — Tue, 17 Jun 2008 06:13:00 +0000

Ok, saw a canonical solution in Haskell by a more proficient author :) — in defense of my language du jour, the canonical solution terminated in a couple seconds with no compiler optimizations as far as I know. I’m fairly certain that my bottleneck was my sums function, which potentially recalculated the first n primes to create a sum containing the n+1th prime.

By: que

que — Wed, 11 Jun 2008 12:15:00 +0000

Well, here is result of mine ruby implementation:
Solving…
Found: 9174901. Testing…solved.
Time elapsed: 3.343
:)

By: Christophe Grand

Christophe Grand — Wed, 11 Jun 2008 05:41:00 +0000

@david: my implementation of primes can be found here: http://clj-me.blogspot.com/2008/06/primes.html

To be fair, I retimed:
user=> (time (find= (cons primes (map sum-primes [3, 5, 11, 1493]))))
“Elapsed time: 88650.283232 msecs”
9174901
user=> (time (find= (cons primes (map sum-primes [3, 5, 11, 1493]))))
“Elapsed time: 5894.601122 msecs”
9174901

I guess that I got 20s by having some primes computed from a prior invocation. So nearly 90s with no primes precomputed and 6s when all required primes are already computed.

By: David

David — Tue, 10 Jun 2008 20:35:00 +0000

Very impressive. What's your implementation of primes? My Haskell implementation, with all compiler optimizations turned on, took about 16 minutes. (I'm a Haskell novice, but still, 20 seconds on JVM vs. 16 minutes for native code says something).

import Primes
import Data.List hiding (find)

sums n = map (\m -> sum . take n $ drop m primes) [0..]

find ss@((x:_):_) =
if all (== x) (map head ss) then x
else let ss' = map snd $ sort $ zip (map head ss) ss in
find $ tail (head ss') : tail ss'

main = print $ find (primes : map sums [3, 5, 11, 1493])