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# lvlte's solution

## to Spiral Matrix in the Julia Track

Published at May 01 2021 · 0 comments
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:

###### Spiral matrix of size 3
``````1 2 3
8 9 4
7 6 5
``````
###### Spiral matrix of size 4
`````` 1  2  3 4
12 13 14 5
11 16 15 6
10  9  8 7
``````

## Source

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

## Version compatibility

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

## Submitting Incomplete Solutions

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

### runtests.jl

``````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(mn::Int)::Matrix{Int}
mn > 0 || return Matrix{Int}(undef, 0, 0)

m = vcat(reshape(1:mn, 1, :), zeros(Int, mn-1, mn))
n, l = mn + 1, mn - 1
i, j = 1, 2:mn

function fill_row(idx_inc)
m = rotl90(m)
if m[i, j.:start] != 0
i, j = idx_inc(i, j)
end
m[i, j] = reshape(n:n+l-1, 1, l)
n += l
end

while l > 0
fill_row((i,j) -> (i, j.:start+1:j.:stop+1))
fill_row((i,j) -> (i + 1, j))
j = j.:start:j.:stop - 1
l -= 1
end

isodd(mn) ? m : rot180(m)
end``````