Write a function that accepts four arguments. The first two arguments are the size of the grid (h x w), filled with ascending integers from left to right, top to bottom, starting from 1. The next two arguments are is the starting positions, the row (r) and column (c). Return an array of integers obtained by spiraling outward anti-clockwise from …

spiral.rb

require 'matrix'
def spiral(height, width, start_row, start_col) 
  @results = []
  i = 0
  m = Matrix.build(height, width) do |row,  col|
    i+=1
  end
  start_row = x = start_row-1 
  start_col = y = start_col -1 
  distance = 1
  @max_length = m.row_size * m.column_size
  @results << m[x, y]

  def store_result(m, x, y)
    if @results.length < @max_length and m[x, y] and x >= 0 and y >= 0
      @results << m[x, y]
    end
  end

  until(@results.length == @max_length) do 
    # go up
    until(x==(start_row - distance)) 
      x -= 1
      store_result(m, x, y)
    end

    # go left
    until(y==(start_col - distance)) 
      y -= 1
      store_result(m, x, y)
    end

    # go down
    until(x==(start_row + distance)) 
      x += 1
      store_result(m, x, y)
    end

    # go right"
    until(y==(start_col + distance)) 
      y += 1
      store_result(m, x, y)
    end
    distance+=1
  end
  @results
end

spiral_test.rb

require 'minitest/autorun'
require './spiral'

describe "Spiral" do

  it "spirals in a square" do
    assert_equal(spiral(5, 5, 3, 3), [13, 8, 7, 12, 17, 18, 19, 14, 9, 4, 3, 2, 1, 6, 11, 16, 21, 22, 23, 24, 25, 20, 15, 10, 5])
  end

  it "spirals in a non-square rectangle" do
    assert_equal(spiral(2, 4, 1, 2), [ 2, 1, 5, 6, 7, 3, 8, 4 ])
  end
end

really like this answer.