Implementing MS-Paint's “paint bucket”

paint_bucket.rb

def flood_fill(image, x, y, new_color)
  target_color = image[x][y]
  fill(image, x, y, target_color, new_color)
end

def fill(image, x, y, target_color, new_color)
  return if x < 0 || x >= image.length || y < 0 || y >= image[0].length
  return if image[x][y] != target_color
  return if image[x][y] == new_color

  image[x][y] = new_color
  fill(image, x - 1, y, target_color, new_color)
  fill(image, x + 1, y, target_color, new_color)
  fill(image, x, y - 1, target_color, new_color)
  fill(image, x, y + 1, target_color, new_color)
end

# Manual Tests
image = [
  ['.','#','#','#','.','.'],
  ['.','#','.','.','#','.'],
  ['.','#','#','#','.','.'],
  ['.','#','.','.','.','.']
]

flood_fill(image, 0, 1, 'O')
image.each { |row| puts row.join }

puts "------"

image = [
  ['.','#','#','#','.','.'],
  ['.','#','.','.','#','.'],
  ['.','#','#','#','.','.'],
  ['.','#','.','.','.','.']
]

flood_fill(image, 1, 3, 'o')
image.each { |row| puts row.join }

puts "------"

image = [
  ['.','#','#','#','.','.'],
  ['.','#','.','.','#','.'],
  ['.','#','#','#','.','.'],
  ['.','#','.','.','.','.']
]

flood_fill(image, 1, 3, '#')
image.each { |row| puts row.join }

# Tests

require 'rspec/autorun'

describe 'flood_fill' do
  it 'correctly fills an area with the new color' do
    image = [
      ['.','#','#','#','.','.'],
      ['.','#','.','.','#','.'],
      ['.','#','#','#','.','.'],
      ['.','#','.','.','.','.']
    ]

    expected_image = [
      ['.','O','O','O','.','.'],
      ['.','O','.','.','#','.'],
      ['.','O','O','O','.','.'],
      ['.','O','.','.','.','.']
    ]

    flood_fill(image, 0, 1, 'O')
    expect(image).to eq(expected_image)
  end
end