LED Clock in Ruby

March 6, 2016

A few weeks ago I built a program to display an LED clock. Here’s the problem, and my solution.

LED Clock: You are (voluntarily) in a room that is completely dark except for the light coming from an old LED digital alarm clock. This is one of those clocks with 4 seven segment displays using an HH:MM time format. The clock is configured to display time in a 24 hour format and the leading digit will be blank if not used. What is the period of time between when the room is at its darkest to when it is at its lightest?

My solution is below, and also here:

https://gist.github.com/jwworth/65dbc7023dede6ff0662

I like a few things about my solution. I like that it utilizes a functional style. I like that it is readable from top to bottom as a single idea. I like that it solves in a repeatable way a problem that would be trivial to solve with a pencil and paper.

I don’t like a few things about my solution. I don’t like that it eschews Ruby’s object-oriented nature. I don’t like that I had to use the Time library to do the time math. I don’t like that it is untested. All of these were tradeoffs I accepted to get to a solution quickly. With some tests the code could easily be refactored.

My solution:

# led.rb

def compute_brightness(units)
  cells_per_number = [ 6, 2, 5, 5, 4, 5, 6, 3, 7, 6 ]

  units.each_with_object({}) do |t, hash|
    digits = t.split('')
    hash[t] = digits.map { |d| cells_per_number[d.to_i] }.reduce(:+)
  end
end

def compute_brightest_time(hours_hash, minutes_hash)
  Time.new(2016, 1, 1, hours_hash.key(hours_hash.values.max), minutes_hash.key(minutes_hash.values.max))
end

def compute_darkest_time(hours_hash, minutes_hash)
  Time.new(2016, 1, 1, hours_hash.key(hours_hash.values.min), minutes_hash.key(minutes_hash.values.min))
end

hours_hash = compute_brightness('0'..'23')
minutes_hash = compute_brightness('00'..'59')

brightest_time = compute_brightest_time(hours_hash, minutes_hash)
darkest_time = compute_darkest_time(hours_hash, minutes_hash)

puts "With no leading zeroes:"
puts "Brightest: #{brightest_time.strftime('%H:%M')}"
puts "Darkest: #{darkest_time.strftime('%l:%M')}"
puts "Diff: #{(brightest_time - darkest_time).abs}s"

hours_hash = compute_brightness('00'..'23')

brightest_time = compute_brightest_time(hours_hash, minutes_hash)
darkest_time = compute_darkest_time(hours_hash, minutes_hash)

puts "\nWith leading digit always included:"
puts "Brightest: #{brightest_time.strftime('%H:%M')}"
puts "Darkest: #{darkest_time.strftime('%H:%M')}"
puts "Diff: #{(brightest_time - darkest_time).abs}s"

# With no leading zeroes:
# Brightest: 20:08
# Darkest:  1:11
# Diff: 68220.0s

# With leading digit always included:
# Brightest: 08:08
# Darkest: 11:11
# Diff: 10980.0s

Thanks to Josh Branchaud for keeping us all on our toes with these challenges.

Ruby