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  1. {-# OPTIONS_GHC -O2 #-}
  2. {-# LANGUAGE BangPatterns #-}
  3.  
  4. -- http://rosettacode.org/wiki/Hamming_numbers#Haskell
  5.  
  6. module Main where
  7. import Data.List
  8. import Data.Function
  9.  
  10. hamming = 1 : map (2*) hamming `union` map (3*) hamming `union` map (5*) hamming
  11. where
  12. union a@(x:xs) b@(y:ys) = case compare x y of
  13. LT -> x : union xs b
  14. EQ -> x : union xs ys
  15. GT -> y : union a ys
  16.  
  17. main = do
  18. s <- getLine
  19. case s of
  20. "a" -> do print $ take 20 hamming
  21. print (hamming !! 1690, hamming !! 1691)
  22. print $ hamming !! (1000000-1) -- 9 MB: releases the prefix of the list
  23. "b" -> do
  24. print $ hamming !! (1000000-1)
  25. print $ hamming !! 1000000 -- 77 MB: does NOT release the prefix (is needed twice)
  26. "c" -> do
  27. mapM_ print $ take 2 $ drop 999999 hamming -- 9 MB: used once, prefix gc-d
  28. "d" -> do
  29. let (r,t) = nthHam (1000000) -- 4 MB: stores upper band only
  30. (r2,t2) = nthHam (1000001)
  31. print (t,trival t)
  32. print (t2,trival t2)
  33. _ -> do -- 10^8: 4 MB 0.27 sec
  34. let (r,t) = nthHam (read s) -- 10^9: 6 MB (-4=2) 1.42 sec O(n^0.7)
  35. print t -- 10^10: 14 MB (-4=10) 7.20 sec O(n^0.7)
  36.  
  37. -- directly find n-th Hamming number (base 1, from 2), in ~ O(n^{2/3}) time
  38. -- by Will Ness, based on "top band" idea by Louis Klauder from DDJ discussion
  39. -- http://drdobbs.com/blogs/architecture-and-design/228700538
  40.  
  41. ln2 = log 2; ln3 = log 3; ln5 = log 5
  42. logval (i,j,k) = fromIntegral i*ln2 + fromIntegral j*ln3 + fromIntegral k*ln5
  43. trival (i,j,k) = 2^i * 3^j * 5^k
  44. estval n = (6*ln2*ln3*ln5* fromIntegral n)**(1/3) -- estimated logval
  45. rngval n
  46. | n > 500000 = (1.698 , 0.0050) -- empirical
  47. | n > 50000 = (1.693 , 0.0100) -- estimation
  48. | n > 500 = (1.66 , 0.0500) -- correction
  49. | n > 1 = (1.56 , 0.2000) -- (dist,width)
  50. | otherwise = (1.56 , 0.4000)
  51.  
  52. -- estval(n) = log (M[n]) = ln2 * logBase 2 (M[n])
  53.  
  54. lb3 = logBase 2 3; lb5 = logBase 2 5 -- lb3 == log 3/log 2, lb5 == log 5/log 2
  55. logval2 (i,j,k) = fromIntegral i + fromIntegral j*lb3 + fromIntegral k*lb5
  56. estval2 n = (6*lb3*lb5* fromIntegral n)**(1/3) -- estimated logval **Base 2**
  57. rngval2 n
  58. | n > 500000 = (2.4496 , 0.0076 ) -- empirical
  59. | n > 50000 = (2.4424 , 0.0146 ) -- estimation
  60. | n > 500 = (2.3948 , 0.0723 ) -- correction - base 2
  61. | n > 1 = (2.2506 , 0.2887 ) -- (dist,width)
  62. | otherwise = (2.2506 , 0.5771 )
  63.  
  64. nthHam n -- n: 1-based 1,2,3...
  65. | w >= 1 = error $ "Breach of contract: (w < 1): " ++ show (w)
  66. | m < 0 = error $ "Not enough triples generated: " ++ show (c,n)
  67. | m >= nb = error $ "Generated band is too narrow: " ++ show (m,nb)
  68. | True = res
  69. where
  70. (d,w) = rngval2 n -- correction dist, width
  71. hi = estval2 n - d -- hi > logval2 > hi-w
  72. (c,b) = foldl_ -- total count, the band
  73. (\(!c,!b) (i,t) -> case t of [] -> (i+c,b)
  74. [x] -> (i+c,x:b))
  75. (0,[])
  76. [ ( i+1, -- total triples w/ this (j,k)
  77. [ (r,(i,j,k)) | frac < w ] ) -- store it, if inside band
  78. | k <- [ 0 .. floor ( hi /lb5) ], let p = fromIntegral k*lb5,
  79. j <- [ 0 .. floor ((hi-p)/lb3) ], let q = fromIntegral j*lb3 + p,
  80. let (i,frac) = pr (hi-q) ; r = hi - frac -- r = i + q
  81. ] where pr = properFraction
  82. (m,nb) = ( fromIntegral $ c - n, length b ) -- m 0-based from top, |band|
  83. (s,res) = ( sortBy (flip compare `on` fst) b, s!!m ) -- sorted decreasing, result
  84. foldl_ = foldl'
Success #stdin #stdout 8.33s 9480KB
stdin
10200300400
stdout
(942,2276,660)