class Pagy

def initialize(vars)
  @vars = VARS.merge(vars.delete_if{|_,v| v.nil? || v == '' })          # default vars + cleaned instance vars
  { count:0, items:1, outset:0, page:1 }.each do |k,min|                # validate core variables
    (@vars[k] && instance_variable_set(:"@#{k}", @vars.delete(k).to_i) >= min) \
       or raise(ArgumentError, "expected :#{k} >= #{min}; got #{instance_variable_get(:"@#{k}").inspect}")
  end
  @pages  = @last = [(@count.to_f / @items).ceil, 1].max                # cardinal and ordinal meanings
  @page >= 1 && @page <= @last or raise(OutOfRangeError, "expected :page in 1..#{@last}; got #{@page.inspect}")
  @offset = @items * (@page - 1) + @outset                              # pagination offset + outset (initial offset)
  @items  = @count - ((@pages-1) * @items) if @page == @last            # adjust items for last page
  @from   = @count == 0 ? 0 : @offset+1 - @outset                       # page begins from item
  @to     = @offset + @items - @outset                                  # page ends to item
  @prev   = (@page-1 unless @page == 1)                                 # nil if no prev page
  @next   = (@page+1 unless @page == @last)                             # nil if no next page
end