Outward spiral matrix builder

swirl.rb

#!/usr/bin/env ruby
#
# Outward spiral matrix builder
# 2012, Paul Philippov <themactep@gmail.com>
#

def show_usage
  puts 'Usage: $0 <number>'
  exit
end

def print_matrix(matrix)
  matrix.each do |row|
    puts row.join(' ')
  end
end

def spiral_matrix(n)
  movement = [[1,0],[0,1],[-1,0],[0,-1]]
  direction = nil

  max_length = n.to_s.length
  output_format = "%-#{max_length}d"

  max_x = max_y = Math.sqrt(n).floor
  if max_x * max_y < n
    max_x += 1
    max_y += 1 if max_x * max_y < n
  end

  matrix = Array.new(max_x) { Array.new(max_y) { ' ' * max_length } }
  x, y, side, side_steps, steps_to_go = -1, 0, 1, max_x, 0

  n.downto(1) do |i|
    if steps_to_go == 0
      movement.push(direction = movement.shift)

      side += 1
      if side > 2
        side = 1
        side_steps -= 1
      end
      steps_to_go = side_steps
    end

    x += direction[0]
    y += direction[1]
    matrix[y][x] = output_format % i

    steps_to_go -= 1
  end

  print_matrix matrix
end

if ARGV.empty?
  show_usage
end

n = ARGV[0].to_i
if n == 0
  puts 'Error: Argument is not a number!'
  show_usage
end

spiral_matrix(n)

exit 0