Add :fixed_page_links option

When set to true, the number of links displayed (including gaps) will
always equal 3 (first, current_page, last) + 2 (to allow for gaps) +
2*inner_window + 2*outer_window.

This means that the width of the pagination bar does not substantially
change as a user browses through pages.

lib/will_paginate/view_helpers.rb

@@ -21,16 +21,18 @@ class << self
 
     # default view options
     self.pagination_options = Deprecation::Hash.new \
-      :class          => 'pagination',
-      :previous_label => nil,
-      :next_label     => nil,
-      :inner_window   => 4, # links around the current page
-      :outer_window   => 1, # links around beginning and end
-      :link_separator => ' ', # single space is friendly to spiders and non-graphic browsers
-      :param_name     => :page,
-      :params         => nil,
-      :page_links     => true,
-      :container      => true
+      :class            => 'pagination',
+      :previous_label   => nil,
+      :next_label       => nil,
+      :inner_window     => 4, # links around the current page
+      :outer_window     => 1, # links around beginning and end
+      :fixed_page_links => false, # if set to true, the number of link items
+                                  # displayed will be constant
+      :link_separator   => ' ', # single space is friendly to spiders and non-graphic browsers
+      :param_name       => :page,
+      :params           => nil,
+      :page_links       => true,
+      :container        => true
 
     label_deprecation = Proc.new { |key, value|
       "set the 'will_paginate.#{key}' key in your i18n locale instead of editing pagination_options" if defined? Rails

lib/will_paginate/view_helpers/link_renderer_base.rb

@@ -31,12 +31,29 @@ def windowed_page_numbers
         window_to = current_page + inner_window
 
         # adjust lower or upper limit if other is out of bounds
-        if window_to > total_pages
+
+        # if <tt>:fixed_page_links</tt> is set, out of bounds means 
+        # window_to + outer_window + 2 > total_pages or
+        # window_from - outer_window - 2 < 1
+        window_to_bound   = @options[:fixed_page_links] ? window_to + outer_window + 2 : window_to
+        window_from_bound = @options[:fixed_page_links] ? window_from - outer_window - 2 : window_from
+
+        if window_to_bound > total_pages
           window_from -= window_to - total_pages
+
+          # allow for the outer_window size, the first link and a possible gap
+          # to ensure items.length is always the same, regardless of whether
+          # current_page is near the middle or near an edge
+          window_from -= outer_window + 2 if @options[:fixed_page_links]
+
           window_to = total_pages
         end
-        if window_from < 1
+        if window_from_bound < 1
           window_to += 1 - window_from
+
+          # allow for the outer_window size, the last link and a possible gap
+          window_to += outer_window + 2 if @options[:fixed_page_links]
+
           window_from = 1
           window_to = total_pages if window_to > total_pages
         end

spec/view_helpers/link_renderer_base_spec.rb

@@ -63,6 +63,57 @@
       prepare({ :page => 10, :total_pages => 10 }, :inner_window => 2, :outer_window => 1)
       showing_pages 1, 2, :gap, 6, 7, 8, 9, 10
     end
+
+    context "with a fixed number of visible page links" do
+      it "should adjust upper limit if lower is out of bounds" do
+        prepare({ :page => 1, :total_pages => 10 }, :fixed_page_links => true, :inner_window => 2, :outer_window => 1)
+        showing_pages 1..10
+      end
+
+      it "should adjust lower limit if upper is out of bounds" do
+        prepare({ :page => 10, :total_pages => 10 }, :fixed_page_links => true, :inner_window => 2, :outer_window => 1)
+        showing_pages 1..10
+      end
+
+      context "with a large number of pages" do
+        it "should adjust upper limit if lower is out of bounds" do
+          prepare({ :page => 1, :total_pages => 21 }, :fixed_page_links => true, :inner_window => 4, :outer_window => 1)
+          showing_pages 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, :gap, 20, 21
+        end
+
+        it "should adjust upper limit if lower is out of bounds" do
+          prepare({ :page => 5, :total_pages => 21 }, :fixed_page_links => true, :inner_window => 4, :outer_window => 1)
+          showing_pages 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, :gap, 20, 21
+        end
+
+        it "should adjust upper limit if lower is out of bounds" do
+          prepare({ :page => 6, :total_pages => 21 }, :fixed_page_links => true, :inner_window => 4, :outer_window => 1)
+          showing_pages 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, :gap, 20, 21
+        end
+
+        it "should adjust upper limit if lower is out of bounds" do
+          prepare({ :page => 7, :total_pages => 21 }, :fixed_page_links => true, :inner_window => 4, :outer_window => 1)
+          showing_pages 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, :gap, 20, 21
+        end
+
+        it "should adjust lower limit if upper is out of bounds" do
+          prepare({ :page => 17, :total_pages => 21 }, :fixed_page_links => true, :inner_window => 4, :outer_window => 1)
+          showing_pages 1, 2, :gap, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21
+        end
+
+        it "should adjust lower limit if upper is out of bounds" do
+          prepare({ :page => 21, :total_pages => 21 }, :fixed_page_links => true, :inner_window => 4, :outer_window => 1)
+          showing_pages 1, 2, :gap, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21
+        end
+      end
+
+      context "with a small number of pages" do
+        it "should show all of the pages" do
+          prepare({ :page => 2, :total_pages => 5 }, :fixed_page_links => true, :inner_window => 4, :outer_window => 2)
+          showing_pages 1..5
+        end
+      end
+    end
 
     def showing_pages(*pages)
       pages = pages.first.to_a if Array === pages.first or Range === pages.first