Colocate Traversal classes with classes they traverse

This puts all knowledge of the traversal in the same file, and removes
the need for a CASES registry.

lib/liquid.rb

@@ -45,6 +45,7 @@ module Liquid
 end
 
 require "liquid/version"
+require 'liquid/traversal'
 require 'liquid/lexer'
 require 'liquid/parser'
 require 'liquid/i18n'

lib/liquid/condition.rb

@@ -128,6 +128,15 @@ module Liquid
         end
       end
     end
+
+    class Traversal < Liquid::Traversal
+      def children
+        [
+          @node.left, @node.right,
+          @node.child_condition, @node.attachment
+        ].compact
+      end
+    end
   end
 
   class ElseCondition < Condition

lib/liquid/tags/assign.rb

@@ -47,6 +47,12 @@ module Liquid
         1
       end
     end
+
+    class Traversal < Liquid::Traversal
+      def children
+        [@node.from]
+      end
+    end
   end
 
   Template.register_tag('assign'.freeze, Assign)

lib/liquid/tags/case.rb

@@ -82,6 +82,12 @@ module Liquid
       block.attach(BlockBody.new)
       @blocks << block
     end
+
+    class Traversal < Liquid::Traversal
+      def children
+        [@node.left] + @node.blocks
+      end
+    end
   end
 
   Template.register_tag('case'.freeze, Case)

lib/liquid/tags/cycle.rb

@@ -53,6 +53,12 @@ module Liquid
         $1 ? Expression.parse($1) : nil
       end.compact
     end
+
+    class Traversal < Liquid::Traversal
+      def children
+        Array(@node.variables)
+      end
+    end
   end
 
   Template.register_tag('cycle', Cycle)

lib/liquid/tags/for.rb

@@ -191,6 +191,12 @@ module Liquid
     def render_else(context)
       @else_block ? @else_block.render(context) : ''.freeze
     end
+
+    class Traversal < Liquid::Traversal
+      def children
+        (super + [@node.limit, @node.from, @node.collection_name]).compact
+      end
+    end
   end
 
   Template.register_tag('for'.freeze, For)

lib/liquid/tags/if.rb

@@ -110,6 +110,12 @@ module Liquid
         Condition.new(a)
       end
     end
+
+    class Traversal < Liquid::Traversal
+      def children
+        @node.blocks
+      end
+    end
   end
 
   Template.register_tag('if'.freeze, If)

lib/liquid/tags/include.rb

@@ -109,6 +109,15 @@ module Liquid
 
       file_system.read_template_file(context.evaluate(@template_name_expr))
     end
+
+    class Traversal < Liquid::Traversal
+      def children
+        [
+          @node.template_name_expr,
+          @node.variable_name_expr
+        ] + @node.attributes.values
+      end
+    end
   end
 
   Template.register_tag('include'.freeze, Include)

lib/liquid/tags/table_row.rb

@@ -50,6 +50,12 @@ module Liquid
       result << "</tr>\n"
       result
     end
+
+    class Traversal < Liquid::Traversal
+      def children
+        super + @node.attributes.values + [@node.collection_name]
+      end
+    end
   end
 
   Template.register_tag('tablerow'.freeze, TableRow)

lib/liquid/traversal.rb

@@ -3,8 +3,11 @@
 module Liquid
   class Traversal
     def self.for(node, callbacks = Hash.new(proc {}))
-      kase = CASES.find { |(klass, _)| node.is_a?(klass) }&.last
-      (kase || self).new(node, callbacks)
+      if defined?(node.class::Traversal)
+        node.class::Traversal
+      else
+        self
+      end.new(node, callbacks)
     end
 
     def initialize(node, callbacks)
@@ -35,84 +38,5 @@ module Liquid
     def children
       @node.respond_to?(:nodelist) ? Array(@node.nodelist) : []
     end
-
-    class Assign < Traversal
-      def children
-        [@node.from]
-      end
-    end
-
-    class Case < Traversal
-      def children
-        [@node.left] + @node.blocks
-      end
-    end
-
-    class Condition < Traversal
-      def children
-        [
-          @node.left, @node.right,
-          @node.child_condition, @node.attachment
-        ].compact
-      end
-    end
-
-    class Cycle < Traversal
-      def children
-        Array(@node.variables)
-      end
-    end
-
-    class For < Traversal
-      def children
-        (super + [@node.limit, @node.from, @node.collection_name]).compact
-      end
-    end
-
-    class If < Traversal
-      def children
-        @node.blocks
-      end
-    end
-
-    class Include < Traversal
-      def children
-        [
-          @node.template_name_expr,
-          @node.variable_name_expr
-        ] + @node.attributes.values
-      end
-    end
-
-    class TableRow < Traversal
-      def children
-        super + @node.attributes.values + [@node.collection_name]
-      end
-    end
-
-    class Variable < Traversal
-      def children
-        [@node.name] + @node.filters.flatten
-      end
-    end
-
-    class VariableLookup < Traversal
-      def children
-        @node.lookups
-      end
-    end
-
-    CASES = {
-      Liquid::Assign => Assign,
-      Liquid::Case => Case,
-      Liquid::Condition => Condition,
-      Liquid::Cycle => Cycle,
-      Liquid::For => For,
-      Liquid::If => If,
-      Liquid::Include => Include,
-      Liquid::TableRow => TableRow,
-      Liquid::Variable => Variable,
-      Liquid::VariableLookup => VariableLookup
-    }.freeze
   end
 end

lib/liquid/variable.rb

@@ -138,5 +138,11 @@ module Liquid
         raise error
       end
     end
+
+    class Traversal < Liquid::Traversal
+      def children
+        [@node.name] + @node.filters.flatten
+      end
+    end
   end
 end

lib/liquid/variable_lookup.rb

@@ -78,5 +78,11 @@ module Liquid
     def state
       [@name, @lookups, @command_flags]
     end
+
+    class Traversal < Liquid::Traversal
+      def children
+        @node.lookups
+      end
+    end
   end
 end

test/integration/traversal_test.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require 'test_helper'
-require 'liquid/traversal'
 
 class TraversalTest < Minitest::Test
   include Liquid