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joaofcosta's solution

to Spiral Matrix in the Elixir Track

Published at Apr 16 2019 · 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

Running tests

Execute the tests with:

$ elixir spiral_matrix_test.exs

Pending tests

In the test suites, all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by commenting out the relevant @tag :pending with a # symbol.

For example:

# @tag :pending
test "shouting" do
  assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"
end

Or, you can enable all the tests by commenting out the ExUnit.configure line in the test suite.

# ExUnit.configure exclude: :pending, trace: true

If you're stuck on something, it may help to look at some of the available resources out there where answers might be found.

Source

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

Submitting Incomplete Solutions

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

spiral_test.exs

if !System.get_env("EXERCISM_TEST_EXAMPLES") do
  Code.load_file("spiral.exs", __DIR__)
end

ExUnit.start()
ExUnit.configure(trace: true, exclude: :pending)

defmodule SpiralTest do
  use ExUnit.Case

  # @tag :pending
  test "empty spiral" do
    assert Spiral.matrix(0) == []
  end

  @tag :pending
  test "trivial spiral" do
    assert Spiral.matrix(1) == [[1]]
  end

  @tag :pending
  test "spiral of side length 2" do
    assert Spiral.matrix(2) == [
             [1, 2],
             [4, 3]
           ]
  end

  @tag :pending
  test "spiral of side length 3" do
    assert Spiral.matrix(3) == [
             [1, 2, 3],
             [8, 9, 4],
             [7, 6, 5]
           ]
  end

  @tag :pending
  test "spiral of side length 4" do
    assert Spiral.matrix(4) == [
             [1, 2, 3, 4],
             [12, 13, 14, 5],
             [11, 16, 15, 6],
             [10, 9, 8, 7]
           ]
  end

  @tag :pending
  test "spiral of size 5" do
    assert 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
defmodule Spiral do
  @doc """
  Given the dimension, return a square matrix of numbers in clockwise spiral order.
  """
  @spec matrix(dimension :: integer) :: list(list(integer))
  def matrix(0), do: []
  def matrix(size), do: matrix(size, size)

  def matrix(row, col, start \\ 1)
  def matrix(row, 0, start), do: [[]]
  def matrix(row, col, start) do
    tail = col
      |> matrix(row - 1, start + col)
      |> rotate_right()

    [Enum.to_list(start..start + col - 1) | tail]
  end


  @doc """
  Transpose a matrix.
  """
  def transpose(matrix), do: matrix |> List.zip() |> Enum.map(&Tuple.to_list/1)

  @doc """
  Right rotation of a matrix.
  """
  def rotate_right(matrix), do: matrix |> transpose() |> Enum.map(&Enum.reverse/1)
end

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