11 February 2014

Solving Project Euler problem 204 turned out to be rather simple for a problem in the 200~ series.
Counting generilized Hamming Numbers is similar to counting coin combinations in problem 31, but with products instead of sums.
Below is the original sum counting algorithm:

sumCombinationCount 0 _ = 1
sumCombinationCount _ [] = 0
sumCombinationCount r (c:cs) = if r < 0
    then 0
    else sumCombinationCount (r - c) (c:cs) + sumCombinationCount r cs

Adapting the recursive algorithm from sum counting to product counting is quite straightforward:

prodCombinationCount limit = prodCombinationCount' 1 1
    where prodCombinationCount' count _ [] = count
          prodCombinationCount' count p (m:ms) = if p > limit
              then count - 1
              else prodCombinationCount' (count + 1) (p * m) (m:ms)
                 + prodCombinationCount' 0 p ms

Within the given conditions of the problem, this algorithm runs in roughly half a second on my laptop.

Below is the code for a standalone solution:

The full solution is also available on my Project Euler Haskell solutions repository on GitHub.

To go one step further, the problem can also be solved by using logarithms to transform the product counting problem into a sum counting problem. Kudos to those who came up with this approach!

After earning the “Daring Dozen” award on this one – 12 problems of 3-digit IDs – I’m now only 13 solutions away from reaching level 4. There’s no time to rest!

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