gistfile1.rb

PAGE_SIZE = 5
EVENTS = %w(a a b c b c a a a c b c b b b c a a c a a a c b b b c b c a a a c)
EVENTS_WITH_TIMES = EVENTS.map.with_index { |id, i| [id, i] }

PARTS_A = EVENTS_WITH_TIMES.select { |entry| entry.first == 'a' }
PARTS_B = EVENTS_WITH_TIMES.select { |entry| entry.first == 'b' }
PARTS_C = EVENTS_WITH_TIMES.select { |entry| entry.first == 'c' }


def get_page(start_at)
  parts_a_old_enough = PARTS_A.select { |entry| entry.last > start_at }
  parts_b_old_enough = PARTS_B.select { |entry| entry.last > start_at }
  parts_c_old_enough = PARTS_C.select { |entry| entry.last > start_at }

  combined = parts_a_old_enough + parts_b_old_enough + parts_c_old_enough
  sorted = combined.sort_by(&:last)
  limited = sorted[0...PAGE_SIZE]

  limited
end

def check_page(page, expected)
  puts expected == page.map(&:first).join(' ')
end


page1 = get_page(-1)
page2 = get_page(page1.last.last)
page3 = get_page(page2.last.last)

check_page(page1, 'a a b c b')
check_page(page2, 'c a a a c')
check_page(page3, 'b c b b b')