require 'forwardable'

module Merb
  class Paginator
    
    VERSION = '1.0.6'
  
    class ArgumentError < ::ArgumentError; end
    class MissingCountError < ArgumentError; end
    class MissingSelectError < ArgumentError; end  
    
    attr_reader :per_page  
    
    # Instantiate a new Paginator object
    #
    # Provide:
    # * A total count of the number of objects to paginate
    # * The number of objects in each page
    # * A block that returns the array of items
    #   * The block is passed the item offset 
    #     (and the number of items to show per page, for
    #     convenience, if the arity is 2)
    def initialize(count, per_page, &select)
      @count, @per_page = count, per_page
      unless select
        raise MissingSelectError, "Must provide block to select data for each page"
      end
      @select = select
    end
    
    # Total number of pages
    def number_of_pages
      (@count / @per_page) + (@count % @per_page > 0 ? 1 : 0)
    end
    
    # First page object
    def first
      page 1
    end
    
    # Last page object
    def last
      page number_of_pages
    end
    
    # Iterate through pages
    def each
      each_with_index do |item, index|
        yield item
      end
    end
    
    # Iterate through pages with indices
    def each_with_index
      1.upto(number_of_pages) do |number|
        yield(page(number), number - 1)
      end
    end
    
    # Retrieve page object by number
    def page(number)
      number = (n = number.to_i) > 0 ? n : 1
      Page.new(self, number, lambda { 
        offset = (number - 1) * @per_page
        args = [offset]
        args << @per_page if @select.arity == 2
        @select.call(*args)
      })
    end
    
    # Page object
    #
    # Retrieves items for a page and provides metadata about the position
    # of the page in the paginator
    class Page
      
      extend Forwardable
      def_delegator :@pager, :first, :first
      def_delegator :@pager, :last, :last
      def_delegator :items, :each
      def_delegator :items, :each_with_index    
          
      attr_reader :number, :pager
      
      def initialize(pager, number, select) #:nodoc:
        @pager, @number = pager, number
        @select = select
      end
      
      # Retrieve the items for this page
      # * Caches
      def items
        @items ||= @select.call
      end
      
      # Checks to see if there's a page before this one
      def prev?
        @number > 1
      end
      
      # Get previous page (if possible)
      def prev
        @pager.page(@number - 1) if prev?
      end
      
      # Checks to see if there's a page after this one
      def next?
        @number < @pager.number_of_pages
      end
      
      # Get next page (if possible)
      def next
        @pager.page(@number + 1) if next?
      end
      
      def ==(other) #:nodoc:
        @pager == other.pager && self.number == other.number
      end
      
    end
    
  end
  
end