http://rosettacode.org/wiki/Perfect_shuffle#J
Using the "1 2 many" counting system, magic_cycle applied to the
required solution set runs for eternity allocating and copying memory
most of which is used only for the final tally. I've written an
explicit version that maintains the start and current deck as well as
the count which runs in a few minutes, not included to avoid clutter.
Until =: conjunction def 'u^:(0-:v)^:_'
perfect_shuffle =: [: , [: |: ([\~ (0 _1r2 p. #))
'perfect shuffling check' assert 'adbecf' -: perfect_shuffle 'abcdef'
Shuffles =: (@:{:)(,`)(`:6) NB. appends next state caused by a
shuffling verb
f =: perfect_shuffle Shuffles
magic_shuffle =: f Until ({. -: {:) @: f @: ,:
magic_shuffle 'abcdef'
abcdef
adbecf
aedcbf
acebdf
abcdef
CYCLE=: (,. ([: <: ([: # [: magic_shuffle i.)"0)) +:>:i.5000]50
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