memo3

gistfile1.rb

require 'mathn'
MAX = 30
AMAX = 10
BMAX = 20

class Pages
  attr_accessor :name, :line, :line_of_page
  def initialize; yield self; end
  def last; (@line / @line_of_page).ceil; end
  def line_at_last; (mod_line == 0) ? @line_of_page : mod_line ; end
  def line_offset(page); @line_of_page * (page-1); end
  def mod_line; @line % @line_of_page; end
  def offset_key; "offset_#{name}".to_sym; end
  def limit_key; "limit_#{name}".to_sym; end
end

def conditions(page, count_a, count_b)
  @all_line = count_a + count_b
  a = Pages.new {|p| p.name = :a; p.line = count_a; p.line_of_page = AMAX}
  b = Pages.new {|p| p.name = :b; p.line = count_b; p.line_of_page = BMAX}
  @boundary_page = [a.last, b.last].min
  @all_page = (@all_line / MAX).ceil

  a, b = b, a if a.last > b.last
  case
  when page > @all_page then nil
  when page < @boundary_page
    return before_boundary_offsets(page, a, b)
  when page == @boundary_page
    return on_boundary_offsets(page, a, b)
  when page > @boundary_page
    return over_boundary_offsets(page, a, b)
  end
end

def before_boundary_offsets(page, a, b)
  {}.tap do |h|
    h[a.offset_key] = a.line_offset(page)
    h[a.limit_key] = (page == a.last) ? a.line_at_last : a.line_of_page
    h[b.offset_key] = b.line_offset(page)
    h[b.limit_key] = (page == b.last) ? b.line_at_last : b.line_of_page
  end
end

def on_boundary_offsets(page, a, b)
  {}.tap do |h|
    h[a.offset_key] = a.line_offset(page)
    h[a.limit_key] = a.line_at_last
    h[b.offset_key] = b.line_offset(page)
    h[b.limit_key] = b.line_of_page + (a.line_of_page - a.line_at_last)
  end
end

def over_boundary_offsets(page, a, b)
  line_at_last_of_boundary = b.line_offset(@boundary_page + 1)
  line_at_last_of_boundary += (a.mod_line == 0 ? 0 : a.line_at_last)
  line_of_over_boundary = (page - 1 - @boundary_page) * MAX

  {}.tap do |h|
    h[a.offset_key] = nil
    h[a.limit_key] = nil
    h[b.offset_key] = line_at_last_of_boundary + line_of_over_boundary
    h[b.limit_key] = page == @all_page ? @all_line % MAX : MAX
  end
end

count_a = 105
count_b = 2005
p conditions(1, count_a, count_b)
p conditions(9, count_a, count_b)
p conditions(10, count_a, count_b)
p conditions(11, count_a, count_b)
p conditions(12, count_a, count_b)
p conditions(13, count_a, count_b)
p conditions(69, count_a, count_b)
p conditions(70, count_a, count_b)
p conditions(71, count_a, count_b)
p conditions(72, count_a, count_b)

p '-----'

count_a = 100
count_b = 2000
p conditions(1, count_a, count_b)
p conditions(9, count_a, count_b)
p conditions(10, count_a, count_b)
p conditions(11, count_a, count_b)
p conditions(12, count_a, count_b)
p conditions(13, count_a, count_b)
p conditions(69, count_a, count_b)
p conditions(70, count_a, count_b)
p conditions(71, count_a, count_b)
p conditions(72, count_a, count_b)