RosettaCodeData/Task/Fibonacci-sequence/Icon/fibonacci-sequence-2.icon

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procedure main(args)
write(fib(integer(!args) | 1000000))
end
procedure fib(n)
return fibMat(n)[1]
end
procedure fibMat(n)
if n <= 0 then return [0,0]
if n = 1 then return [1,0]
fp := fibMat(n/2)
c := fp[1]*fp[1] + fp[2]*fp[2]
d := fp[1]*(fp[1]+2*fp[2])
if n%2 = 1 then return [c+d, d]
else return [d, c]
end