Use #children_tree instead of #children when possible

Add `#first_child` method to achieve this

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## 1.0.3 [compare][compare_v1_0_2_and_master]
 
+- Use `#children_tree` instead of `#children` when possible [@gonzedge][github_user_gonzedge]
 - Remove unnecessary assignment in `#letter=` [@gonzedge][github_user_gonzedge]
 - Use `#each_value` instead of `#values`.`#each` in `Enumerable#each` [@gonzedge][github_user_gonzedge]
 - Use `#each_key` instead of `#keys`.`#each` in `CompressedNode#current_key` [@gonzedge][github_user_gonzedge]

lib/rambling/trie/compressor.rb

@@ -16,7 +16,7 @@ def compress node
       private
 
       def merge_with_child_and_compress node
-        child = node.children.first
+        child = node.first_child
 
         letter = node.letter.to_s << child.letter.to_s
         new_node = new_compressed_node node, letter, child.terminal?
@@ -28,7 +28,7 @@ def merge_with_child_and_compress node
       def copy_node_and_compress_children node
         new_node = new_compressed_node node, node.letter, node.terminal?
 
-        node.children.each do |child|
+        node.children_tree.each_value do |child|
           compressed_child = compress child
 
           compressed_child.parent = new_node

lib/rambling/trie/node.rb

@@ -48,6 +48,16 @@ def children
         children_tree.values
       end
 
+      # First child node.
+      # @return [Node, nil] the first child contained in the current node.
+      def first_child
+        return if children_tree.empty?
+
+        children_tree.each_value do |child|
+          return child
+        end
+      end
+
       # Indicates if the current node is the root node.
       # @return [Boolean] `true` if the node does not have a parent, `false`
       #   otherwise.

spec/lib/rambling/trie/raw_node_spec.rb

@@ -68,7 +68,7 @@
         end
 
         it 'includes a child with the expected letter' do
-          expect(node.children.first.letter).to eq :a
+          expect(node.first_child.letter).to eq :a
         end
 
         it 'has the expected letter as a key' do