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Published at Mar 18 2021
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Instructions

Test suite

Solution

Given the size, return a square matrix of numbers in spiral order.

The matrix should be filled with natural numbers, starting from 1 in the top-left corner, increasing in an inward, clockwise spiral order, like these examples:

```
1 2 3
8 9 4
7 6 5
```

```
1 2 3 4
12 13 14 5
11 16 15 6
10 9 8 7
```

Reddit r/dailyprogrammer challenge #320 [Easy] Spiral Ascension. https://www.reddit.com/r/dailyprogrammer/comments/6i60lr/20170619_challenge_320_easy_spiral_ascension/

This exercise has been tested on Julia versions >=1.0.

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

```
using Test
include("spiral-matrix.jl")
@testset "Different valid values" begin
@testset "Empty spiral" begin
@test spiral_matrix(0) == Matrix{Int}(undef,0,0)
end
@testset "Trivial spiral" begin
@test spiral_matrix(1) == reshape([1],(1,1))
end
@testset "Spiral of size 2" begin
@test spiral_matrix(2) == [1 2; 4 3]
end
@testset "Spiral of size 3" begin
@test spiral_matrix(3) == [1 2 3; 8 9 4; 7 6 5]
end
@testset "Spiral of size 4" begin
@test spiral_matrix(4) == [1 2 3 4; 12 13 14 5; 11 16 15 6; 10 9 8 7]
end
@testset "Spiral of size 5" begin
@test spiral_matrix(5) == [1 2 3 4 5; 16 17 18 19 6; 15 24 25 20 7; 14 23 22 21 8; 13 12 11 10 9]
end
end
```

```
function spiral_matrix(T::Type{<:Integer}, n::Integer)::Matrix{T}
result = Matrix{T}(undef, n, n)
fillmatrix!(result, Iterators.Stateful(Iterators.countfrom(oneunit(T))))
result
end
spiral_matrix(n::Integer) = spiral_matrix(typeof(n), n)
function fillmatrix!(result::AbstractMatrix, iter::Iterators.Stateful)
if iszero(size(result, 1))
return
end
fix = firstindex(result, 1)
fiy = firstindex(result, 2)
lix = lastindex(result, 1)
liy = lastindex(result, 2)
for i in fix:lix
result[fiy, i] = popfirst!(iter)
end
for j in fiy+1:liy
result[j, lix] = popfirst!(iter)
end
for i in lix-1:-1:fix
result[liy, i] = popfirst!(iter)
end
for j in liy-1:-1:fiy+1
result[j, fix] = popfirst!(iter)
end
fillmatrix!(view(result, fix+1:lix-1, fiy+1:liy-1), iter)
end
```

Generic solution which can fill any AbstractMatrix with any Integer series.

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