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13 lines
358 B
13 lines
358 B
3 years ago
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module Matchings where
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import Data.MemoTrie
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import Binomial
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-- number of ways to match up k elements with n elements
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-- matchings may be partial
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matchings :: Int -> Int -> Integer
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matchings = memo2 m where
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m n k = if n > k
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then matchings k n
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else sum [ binomial n i * product [fromIntegral (k-i+1) .. fromIntegral k] | i <- [0 .. n]]
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