work_hour.rb

def permutations(day_hours, missing_days, target)
  results = []
  values = (0..day_hours).to_a
  size = missing_days
  puts target
  permuts = values.repeated_permutation(size).to_a
  permuts.each do |permut|
    if permut.inject(:+) == target.to_i
      results.push(permut.join(','))
    end
  end
  results
end

def find_schedules(work_hours, day_hours, pattern)
  result = []
  days = pattern.split("")
  missing_days = days.find_all { |i| i.eql? '?' }.count 
  missing_days_indexes = (days.map.with_index do |day, i|
    (day == '?') ? i : nil
  end).compact
  hours_worked = days.select{ |i| i != '?'}.map(&:to_i).inject(:+) || 0
  pending_work_hours = work_hours.to_i - hours_worked.to_i
  periods = permutations(day_hours, missing_days, pending_work_hours)
  periods.each.with_index do |period, index|
    new_periods = period.split(',')
    new_pattern = Hash[missing_days_indexes.zip(new_periods)]
    new_pattern.keys.each do |key|
      days[key] = new_pattern[key]
    end
    result.push(days.join(''))
  end
  result
end

#find_schedules(56, 8, '???8???');
find_schedules(24, 4, '08??840');

#https://robots.thoughtbot.com/better-date-manipulation-in-postgres-queries
#https://www.mathplanet.com/education/algebra-2/discrete-mathematics-and-probability/permutations-and-combinations
https://codepen.io/jopico/pen/EGpim?editors=1000