pagination: refactor to avoid counting pages in API.

Previously the page-based (numbered) paginator would always count the
total_pages, even in API calls when it wasn't needed. This could be very
slow in some cases. Refactor so that total_pages isn't calculated unless
it's called.

While we're at it, refactor to condense all the sequential vs. numbered
pagination logic into one module. This incidentally fixes a couple more
bugs:

* "page=b0" returned all pages rather than nothing.
* Bad parameters like "page=blaha123" and "page=a123blah" were accepted.

test/unit/paginator_test.rb

@@ -12,6 +12,11 @@ class PaginatorTest < ActiveSupport::TestCase
 
       assert_equal(expected_posts.map(&:id), posts.map(&:id))
     end
+
+    should "return nothing for b0" do
+      posts = Post.paginate("b0")
+      assert_empty(posts.map(&:id))
+    end
   end
 
   context "sequential pagination (after)" do
@@ -30,5 +35,30 @@ class PaginatorTest < ActiveSupport::TestCase
 
       assert_equal(expected_posts.map(&:id), posts.map(&:id))
     end
+
+    should "raise an error when exceeding the page limit" do
+      Danbooru.config.stubs(:max_numbered_pages).returns(5)
+      assert_raises(PaginationExtension::PaginationError) do
+        Post.paginate(10)
+      end
+    end
+
+    should "count pages correctly" do
+      assert_equal(5, Post.paginate(1, limit: 1).total_pages)
+      assert_equal(3, Post.paginate(1, limit: 2).total_pages)
+      assert_equal(2, Post.paginate(1, limit: 3).total_pages)
+      assert_equal(2, Post.paginate(1, limit: 4).total_pages)
+      assert_equal(1, Post.paginate(1, limit: 5).total_pages)
+    end
+
+    should "detect the first and last page correctly" do
+      assert(Post.paginate(0, limit: 1).is_first_page?)
+      assert(Post.paginate(1, limit: 1).is_first_page?)
+      refute(Post.paginate(1, limit: 1).is_last_page?)
+
+      refute(Post.paginate(5, limit: 1).is_first_page?)
+      assert(Post.paginate(5, limit: 1).is_last_page?)
+      assert(Post.paginate(6, limit: 1).is_last_page?)
+    end
   end
 end

test/unit/post_sets/intro_test.rb

@@ -1,5 +1,4 @@
 require 'test_helper'
-require "danbooru/paginator/pagination_error"
 
 module PostSets
   class IntroTest < ActiveSupport::TestCase

test/unit/post_sets/post_test.rb

@@ -1,5 +1,4 @@
 require 'test_helper'
-require "danbooru/paginator/pagination_error"
 
 module PostSets
   class PostTest < ActiveSupport::TestCase
@@ -86,7 +85,7 @@ module PostSets
         end
 
         should "fail" do
-          assert_raises(Danbooru::Paginator::PaginationError) do
+          assert_raises(PaginationExtension::PaginationError) do
             @set.posts
           end
         end