Create Paginator module

-Paginator module is responsible for calculating and rendering a
pagination bar based on the passed parameters.
-Added rspec for Paginator.

lib/utilities/paginator.rb

@@ -0,0 +1,111 @@
+module Utilities
+  module Paginator
+    ELLIPSES = '...'.freeze
+    OFFSET = 1
+    FIRST_PAGE = 1
+    DIVIDER = 2
+
+    def items_to_display(model:, items_per_page: 5, current_page: 1)
+      @items_per_page = items_per_page
+      @current_page = current_page
+      @number_of_items = model.count
+      @last_page = (@number_of_items.to_f / @items_per_page).ceil
+      model.offset(items_per_page * (@current_page - OFFSET))
+           .limit(items_per_page)
+    end
+
+    def paginator(max_page_blocks: 7)
+      render partial: 'shared/paginator',
+             locals: { blocks: pages_to_display(@current_page,
+                                                max_page_blocks,
+                                                @last_page),
+                       current_page: @current_page,
+                       previous_link_css_class: previous_link_css_class,
+                       next_link_css_class: next_link_css_class,
+                       last_page: @last_page }
+    end
+
+    def page_item_css_class(page, current_page)
+      if page == current_page
+        'page-item active'
+      elsif page.to_s == ELLIPSES
+        'page-item disabled'
+      end
+    end
+
+    private
+
+    def previous_link_css_class
+      @current_page == FIRST_PAGE ? 'page-item disabled' : 'page-item'
+    end
+
+    def next_link_css_class
+      @current_page == @last_page ? 'page-item disabled' : 'page-item'
+    end
+
+    def pages_to_display(current_page, max_page_blocks, number_of_pages)
+      if all_pages_fit?(number_of_pages, max_page_blocks)
+        complete_pagination(number_of_pages)
+      elsif current_page_at_the_beginning?(current_page, max_page_blocks)
+        beginning_pagination(max_page_blocks)
+      elsif current_page_at_the_end?(current_page, number_of_pages,
+                                     max_page_blocks)
+        end_pagination(max_page_blocks, number_of_pages)
+      else
+        general_pagination(current_page, max_page_blocks)
+      end
+    end
+
+    def all_pages_fit?(number_of_pages, max_page_blocks)
+      number_of_pages < max_page_blocks
+    end
+
+    def current_page_at_the_beginning?(current_page, max_page_blocks)
+      current_page <= (max_page_blocks / DIVIDER)
+    end
+
+    def current_page_at_the_end?(current_page, number_of_pages, max_page_blocks)
+      current_page > (number_of_pages - (max_page_blocks / DIVIDER))
+    end
+
+    def general_pagination(current_page, max_page_blocks)
+      if max_page_blocks == 1
+        [current_page]
+      elsif max_page_blocks == 2
+        [current_page, ELLIPSES]
+      elsif max_page_blocks.odd?
+        odd_pages(current_page, max_page_blocks)
+      else
+        even_pages(current_page, max_page_blocks)
+      end
+    end
+
+    def beginning_pagination(max_page_blocks)
+      start = FIRST_PAGE
+      finish = max_page_blocks - OFFSET
+      [*(start..finish), ELLIPSES]
+    end
+
+    def end_pagination(max_page_blocks, number_of_pages)
+      start = number_of_pages - max_page_blocks + OFFSET * 2
+      finish = number_of_pages
+      [ELLIPSES, *(start..finish)]
+    end
+
+    def complete_pagination(last_page)
+      [*FIRST_PAGE..last_page]
+    end
+
+    def odd_pages(current_page, max_page_blocks)
+      start = (current_page - (max_page_blocks - OFFSET) / DIVIDER) + OFFSET
+      finish = (current_page + (max_page_blocks - OFFSET) / DIVIDER) - OFFSET
+      [ELLIPSES, *(start..finish), ELLIPSES]
+    end
+
+    def even_pages(current_page, max_page_blocks)
+      start = (current_page - (max_page_blocks - OFFSET) / DIVIDER) + OFFSET
+      finish = (current_page + (max_page_blocks / DIVIDER)) - OFFSET
+      [ELLIPSES, *(start..finish), ELLIPSES]
+    end
+  end
+end

spec/modules/paginator_spec.rb

@@ -0,0 +1,93 @@
+require 'rails_helper'
+
+RSpec.describe Utilities::Paginator, type: :module do
+  let!(:paginator) { Class.new { extend Utilities::Paginator } }
+
+  describe('#pages_to_display') do
+    subject(:pages_to_display) do
+      paginator.send(:pages_to_display,
+                     current_page,
+                     maximum_page_blocks,
+                     number_of_pages)
+    end
+
+    context 'All pages fit within the pagination list' do
+      let(:maximum_page_blocks) { 7 }
+      let(:current_page) { 1 }
+      let(:number_of_pages) { 5 }
+      let(:expected_range_of_pages) { [1, 2, 3, 4, 5] }
+
+      it 'displays correct range of pages' do
+        expect(pages_to_display).to eq expected_range_of_pages
+      end
+    end
+
+    context 'Current page is in the beginning and there are more pages towards the end of the pagination list' do
+      let(:maximum_page_blocks) { 7 }
+      let(:current_page) { 3 }
+      let(:number_of_pages) { 11 }
+      let(:expected_range_of_pages) { [1, 2, 3, 4, 5, 6, '...'] }
+
+      it 'displays correct range of pages' do
+        expect(pages_to_display).to eq expected_range_of_pages
+      end
+    end
+
+    context 'Current page is at the end and there are more pages towards the beginning of the pagination list' do
+      let(:maximum_page_blocks) { 7 }
+      let(:current_page) { 9 }
+      let(:number_of_pages) { 11 }
+      let(:expected_range_of_pages) { ['...', 6, 7, 8, 9, 10, 11] }
+
+      it 'displays correct range of pages' do
+        expect(pages_to_display).to eq expected_range_of_pages
+      end
+    end
+
+    context 'Current page is in the middle and there are more pages towards the beginning and the end of the pagination list' do
+      context 'Maximum number of page blocks is 1' do
+        let(:maximum_page_blocks) { 1 }
+        let(:current_page) { 5 }
+        let(:number_of_pages) { 11 }
+        let(:expected_range_of_pages) { [5] }
+
+        it 'displays correct range of pages' do
+          expect(pages_to_display).to eq expected_range_of_pages
+        end
+      end
+
+      context 'Maximum number of page blocks is 2' do
+        let(:maximum_page_blocks) { 2 }
+        let(:current_page) { 6 }
+        let(:number_of_pages) { 11 }
+        let(:expected_range_of_pages) { [6, '...'] }
+
+        it 'displays correct range of pages' do
+          expect(pages_to_display).to eq expected_range_of_pages
+        end
+      end
+
+      context 'Maximum number of page blocks is odd' do
+        let(:maximum_page_blocks) { 7 }
+        let(:current_page) { 5 }
+        let(:number_of_pages) { 11 }
+        let(:expected_range_of_pages) { ['...', 3, 4, 5, 6, 7, '...'] }
+
+        it 'displays correct range of pages' do
+          expect(pages_to_display).to eq expected_range_of_pages
+        end
+      end
+
+      context 'Maximum number of page blocks is even' do
+        let(:maximum_page_blocks) { 6 }
+        let(:current_page) { 5 }
+        let(:number_of_pages) { 11 }
+        let(:expected_range_of_pages) { ['...', 4, 5, 6, 7, '...'] }
+
+        it 'displays correct range of pages' do
+          expect(pages_to_display).to eq expected_range_of_pages
+        end
+      end
+    end
+  end
+end