factor/extra/project-euler/055/055.factor

! Copyright (c) 2008 Aaron Schaefer.
! See http://factorcode.org/license.txt for BSD license.
USING: kernel math math.parser project-euler.common sequences sequences.lib ;
IN: project-euler.055

! http://projecteuler.net/index.php?section=problems&id=55

! DESCRIPTION
! -----------

! If we take 47, reverse and add, 47 + 74 = 121, which is palindromic.

! Not all numbers produce palindromes so quickly. For example,

!    349 + 943 = 1292,
!    1292 + 2921 = 4213
!    4213 + 3124 = 7337

! That is, 349 took three iterations to arrive at a palindrome.

! Although no one has proved it yet, it is thought that some numbers, like 196,
! never produce a palindrome. A number that never forms a palindrome through
! the reverse and add process is called a Lychrel number. Due to the
! theoretical nature of these numbers, and for the purpose of this problem, we
! shall assume that a number is Lychrel until proven otherwise. In addition you
! are given that for every number below ten-thousand, it will either (i) become a
! palindrome in less than fifty iterations, or, (ii) no one, with all the
! computing power that exists, has managed so far to map it to a palindrome. In
! fact, 10677 is the first number to be shown to require over fifty iterations
! before producing a palindrome: 4668731596684224866951378664 (53 iterations,
! 28-digits).

! Surprisingly, there are palindromic numbers that are themselves Lychrel
! numbers; the first example is 4994.

! How many Lychrel numbers are there below ten-thousand?

! NOTE: Wording was modified slightly on 24 April 2007 to emphasise the
! theoretical nature of Lychrel numbers.


! SOLUTION
! --------

<PRIVATE

: add-reverse ( n -- m )
    dup number>digits reverse 10 digits>integer + ;

: (lychrel?) ( n iteration -- ? )
    dup 50 < [
        >r add-reverse dup palindrome?
        [ r> 2drop f ] [ r> 1+ (lychrel?) ] if
    ] [
        2drop t
    ] if ;

: lychrel? ( n -- ? )
    1 (lychrel?) ;

PRIVATE>

: euler055 ( -- answer )
    10000 [ lychrel? ] count ;

! [ euler055 ] 100 ave-time
! 1370 ms run / 31 ms GC ave time - 100 trials

MAIN: euler055