module Regexp::Syntax

def comparable_version(name)
  # add .99 to treat versions without a patch value as latest patch version
  Gem::Version.new((name.to_s.scan(/\d+/) << 99).join('.'))
end

def const_missing(const_name)
  if const_name =~ VERSION_CONST_REGEXP
    return fallback_version_class(const_name)
  end
  super
end

def fallback_version_class(version)
  sorted_versions = (specified_versions + [version])
                    .sort_by { |name| comparable_version(name) }
  return if (version_index = sorted_versions.index(version)) < 1
  next_lower_version = sorted_versions[version_index - 1]
  inherit_from_version(next_lower_version, version)
end

def inherit_from_version(parent_version, new_version)
  new_const = version_const_name(new_version)
  parent = const_get(version_const_name(parent_version))
  const_defined?(new_const) || const_set(new_const, Class.new(parent))
  warn_if_future_version(new_const)
  const_get(new_const)
end

def new(name)
  return Regexp::Syntax::Any.new if ['*', 'any'].include?(name.to_s)
  version_class(name).new
end

def specified_versions
  constants.select { |const_name| const_name =~ VERSION_CONST_REGEXP }
end

def supported?(name)
  name =~ VERSION_REGEXP &&
    comparable_version(name) >= comparable_version('1.8.6')
end

def version_class(version)
  version =~ VERSION_REGEXP || raise(InvalidVersionNameError, version)
  version_const_name = version_const_name(version)
  const_get(version_const_name) || raise(UnknownSyntaxNameError, version)
end

def version_const_name(version_string)
  "V#{version_string.to_s.scan(/\d+/).join('_')}"
end

def warn_if_future_version(const_name)
  return if comparable_version(const_name) < comparable_version('4.0.0')
  warn('This library has only been tested up to Ruby 3.x, '\
       "but you are running with #{const_get(const_name).inspect}")
end