require 'imagestring'

def better_difference?(current, new)
  if current.abs < new.abs
    return false
  elsif current.abs > new.abs
    return true
  else
    return (current < new)
  end
end

filename = ARGV[0]
i = ImageString.new(filename)
path = []

# find lowest position in first column
(current_row, current_col) = 0, 0
min = i.pixel_at(current_row, current_col)

for row in 1..(i.rows - 1)
  if i.pixel_at(row, 0) < min
    current_row = row
    min = i.pixel_at(row, 0)
  end
end

path.push([current_row, current_col])


while path.size <= i.columns
  current_pixel = i.pixel_at(current_row, current_col)
  (next_row, next_col) = current_row, current_col + 1
  # How much difference is safe?
  min_diff = current_pixel - i.pixel_at(next_row, next_col)

  if current_row != 0
    up_right = current_pixel - i.pixel_at(current_row - 1, next_col)
    if better_difference?(min_diff, up_right)
      next_row = current_row - 1
    end
  end

  if current_row != i.rows - 1
    down_right = current_pixel - i.pixel_at(current_row + 1, next_col)
    if better_difference?(min_diff, down_right)
      next_row = current_row + 1
    end
  end

  path.push([next_row, next_col])
  (current_row, current_col) = next_row, next_col
end


# display path on image
path.each do |point|
  i.set_pixel(point[0], point[1], 255)
end


pathed_filename = filename.sub(/\./, '.pathed.')
i.dump_to_file(pathed_filename)