diff --git a/Gemfile b/Gemfile
index 42e51348d..29f504f84 100644
--- a/Gemfile
+++ b/Gemfile
@@ -19,7 +19,6 @@ gem 'tempfile'
 gem "rdoc"
 gem "fileutils"
 gem "raap"
-gem "activesupport"
 
 group :libs do
   # Libraries required for stdlib test
diff --git a/Gemfile.lock b/Gemfile.lock
index 504ed20c0..f9175f9e7 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -158,7 +158,6 @@ PLATFORMS
 
 DEPENDENCIES
   abbrev
-  activesupport
   base64
   benchmark-ips
   bigdecimal
diff --git a/templates/template.rb b/templates/template.rb
index 8f6491b5f..270b0f799 100644
--- a/templates/template.rb
+++ b/templates/template.rb
@@ -4,9 +4,6 @@
 require "fileutils"
 require "yaml"
 
-require "active_support"
-require "active_support/core_ext/string/inflections"
-
 module RBS
   class Template
     class Field
@@ -48,14 +45,11 @@ class Type
 
       def initialize(yaml)
         @ruby_full_name = yaml["name"]
-        @ruby_class_name = @ruby_full_name.demodulize
-        @c_function_name = if @ruby_full_name =~ /^RBS::Types/
-          name = @ruby_full_name.gsub("::", "_").underscore
-          name.gsub("_types_", "_")
-        else
-          name = @ruby_full_name.gsub("::", "_").underscore
-          name.gsub("_declarations_", "_decl_")
-        end
+        @ruby_class_name = @ruby_full_name[/[^:]+\z/] # demodulize-like
+        name = @ruby_full_name.gsub("::", "_")
+        @c_function_name = name.gsub(/(^)?(_)?([A-Z](?:[A-Z]*(?=[A-Z_])|[a-z0-9]*))/) { ($1 || $2 || "_") + $3.downcase } # underscore-like
+        @c_function_name.gsub!(/^rbs_types_/, 'rbs_')
+        @c_function_name.gsub!(/^rbs_ast_declarations_/, 'rbs_ast_decl_')
         @c_constant_name = @ruby_full_name.gsub("::", "_")
         @c_parent_constant_name = @ruby_full_name.split("::")[0..-2].join("::").gsub("::", "_")