I used brute force. This article is relevant, though I didn't use it.
http://www.jsoftware.com/papers/play132.htm
MY_INPUT=: 368078
Dyad=: [: :
While=: 2 :'u^:(0-.@:-:v)^:_'
rotate=: |:@:|.
NB. f rotates the current matrix and then prepends the next group of numbers
f=: (,~ (([: i. [: - _1 { $)+[:>:(<0 _1)&{))@:rotate
NB. example use of f f^:14 ,:,1
NB. f While(MY_INPUT > (<0 0)&{),:,1
NB. computes the multi-dimensional index vector of x in y
find_index=: ([: , $@:] #: I.@:(= ,)) Dyad
A=:f While(MY_INPUT > (<0 0)&{),:,1
1 find_index A
303 303
MY_INPUT find_index A
0 371
+/ | 0 371 - 303 303
371
NB. part2 looks up the index of the next position
NB. using the spiral matrix from the first part.
NB. Then it sums up the neighborhood in a matrix
NB. of same size with 1 in the center and all other
NB. zeros, and modifies that matrix.
part2=: 4 :0
a=. 1 = y
i=. 1
whilst. s < x do.
k =. < j =. i find_index y
s =. j (+/@:,@:([;.0~ (3 ,:~ _1 + [))~) a
a =. j (s"_)`(k"_)`]} a
i =. >: i
end.
a
)
C=:MY_INPUT part2 A
>./,C
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