Given an array of fireworks representing a series going off, write a function to find the "grand finale" of the show!

grand_finale.rb

def grand_finale_start(fireworks)
  best = { start: nil, length: 0 }

  (0...fireworks.size).each do |start_idx|
    fireworks[start_idx..].each_with_index do |_, offset|
      window = fireworks[start_idx..start_idx + offset]
      next unless valid_window?(window)

      if window.size > best[:length]
        best[:start] = start_idx
        best[:length] = window.size
      end
    end
  end

  best[:start]
end

private

def valid_window?(window)
  average_size(window) >= 5 &&
    window.all? { |f| f[:velocity] >= 3 } &&
    height_range(window) <= 10
end

def average_size(window)
  window.sum { |f| f[:size] }.to_f / window.size
end

def height_range(window)
  heights = window.map { |f| f[:height] }
  heights.max - heights.min
end