1 ! Copyright (c) 2009 Jon Harper.
2 ! See http://factorcode.org/license.txt for BSD license.
3 USING: arrays io kernel locals math math.functions math.parser math.ranges
4 namespaces prettyprint project-euler.common sequences threads ;
7 ! http://projecteuler.net/index.php?section=problems&id=255
12 ! We define the rounded-square-root of a positive integer n as the square root
13 ! of n rounded to the nearest integer.
15 ! The following procedure (essentially Heron's method adapted to integer
16 ! arithmetic) finds the rounded-square-root of n:
18 ! Let d be the number of digits of the number n.
19 ! If d is odd, set x_(0) = 2×10^((d-1)⁄2).
20 ! If d is even, set x_(0) = 7×10^((d-2)⁄2).
22 ! Repeat: [see URL for figure ]
24 ! until x_(k+1) = x_(k).
26 ! As an example, let us find the rounded-square-root of n = 4321.
27 ! n has 4 digits, so x_(0) = 7×10^((4-2)⁄2) = 70.
29 ! [ see URL for figure ]
31 ! Since x_(2) = x_(1), we stop here.
33 ! So, after just two iterations, we have found that the rounded-square-root of
34 ! 4321 is 66 (the actual square root is 65.7343137…).
36 ! The number of iterations required when using this method is surprisingly low.
37 ! For example, we can find the rounded-square-root of a 5-digit integer
38 ! (10,000 ≤ n ≤ 99,999) with an average of 3.2102888889 iterations (the average
39 ! value was rounded to 10 decimal places).
41 ! Using the procedure described above, what is the average number of iterations
42 ! required to find the rounded-square-root of a 14-digit number
43 ! (10^(13) ≤ n < 10^(14))? Give your answer rounded to 10 decimal places.
45 ! Note: The symbols ⌊x⌋ and ⌈x⌉ represent the floor function and ceiling
46 ! function respectively.
53 ! same as produce, but outputs the sum instead of the sequence of results
54 : produce-sum ( id pred quot -- sum )
55 [ 0 ] 2dip [ [ dip swap ] curry ] [ [ dip + ] curry ] bi* while ; inline
58 number-length dup even?
59 [ 2 - 2 / 10 swap ^ 7 * ]
60 [ 1 - 2 / 10 swap ^ 2 * ] if ;
62 : ⌈a/b⌉ ( a b -- ⌈a/b⌉ )
65 : xk+1 ( n xk -- xk+1 )
66 [ ⌈a/b⌉ ] keep + 2 /i ;
68 : next-multiple ( a multiple -- next )
69 [ [ 1 - ] dip /i 1 + ] keep * ;
72 ! Gives the number of iterations when xk+1 has the same value for all a<=i<=n
73 :: (iteration#) ( i xi a b -- # )
75 [ drop i b a - 1 + * ]
76 [ i 1 + swap a b iteration# ] if ;
78 ! Gives the number of iterations in the general case by breaking into intervals
79 ! in which xk+1 is the same.
80 :: iteration# ( i xi a b -- # )
85 ! set up the values for the next iteration
86 [ nip [ 1 + ] [ xi + ] bi ] 2keep
87 ! set up the arguments for (iteration#)
88 [ i xi ] 2dip (iteration#)
90 ! deal with the last numbers
91 [ drop b [ i xi ] 2dip (iteration#) ] dip
94 : (euler255) ( a b -- answer )
96 [ [ drop x0 1 swap ] 2keep iteration# ] 2keep
101 : euler255 ( -- answer )
102 13 14 (euler255) 10 nth-place ;
104 ! [ euler255 ] gc time
105 ! Running time: 37.468911341 seconds