2 hour sinatra

light.rb

require 'rack'
require 'json'
require 'erb'

class Application
  def self.application
    @application ||= new
  end

  def self.configure(&blk)
    application.instance_eval(&blk)
    application
  end

  def handle_request(env)
    request_method = env['REQUEST_METHOD'].downcase
    request_path = env['REQUEST_PATH']

    route_match = router.get_match_for(request_method, request_path)
    response = if route_match
      begin
        param_group = route_match.route.param_group

        query_params = env['QUERY_STRING'].split("&").map{|el| el.split("=")}.to_h
        scrubbed_params = param_group.validate_and_scrub_params!(route_match.params.merge(query_params))
        route_match.route.handler.call(scrubbed_params)
      rescue UserError => e
        Response.new(status_code: e.status_code, body: e.body)
      end
    elsif request_method == HTTPMethod.options
      allowable_methods = router.all_matching_route_methods(request_path).map(&:upcase).join(",")
      Response.new(status_code: 200, headers: {'Allow' => allowable_methods}, body: "")
    else
      Response.new(status_code: 404, body: "Not found.")
    end

    response.to_rack_format
  end

  private
  def router
    @router ||= Router.new
  end
end

class UserError < StandardError
  attr_reader :body, :status_code

  def initialize(body, status_code: 400)
    @body = body
    @status_code = status_code
  end
end

module HTTPMethod
  %w(get post put delete patch head options).each do |m|
    define_singleton_method(m) { m }
  end
end

class Router
  def initialize
    @routes = []
  end

  def self.compile_path(path)
    regexpified = path.gsub(/:([^\/]*)/, '(?<\1>[^\/]*)')
    Regexp.new("\\A#{regexpified}\\z")
  end

  def get(path, param_group, &blk)
    @routes << Route.new(HTTPMethod.get, Router.compile_path(path), param_group, &blk)
  end

  def post(path, param_group, &blk)
    @routes << Route.new(HTTPMethod.post, Router.compile_path(path), param_group, &blk)
  end

  def put(path, param_group, &blk)
    @routes << Route.new(HTTPMethod.put, Router.compile_path(path), param_group, &blk)
  end

  def delete(path, param_group, &blk)
    @routes << Route.new(HTTPMethod.delete, Router.compile_path(path), param_group, &blk)
  end

  def patch(path, param_group, &blk)
    @routes << Route.new(HTTPMethod.patch, Router.compile_path(path), param_group, &blk)
  end

  def head(path, param_group, &blk)
    @routes << Route.new(HTTPMethod.head, Router.compile_path(path), param_group, &blk)
  end

  def options(path, param_group, &blk)
    @routes << Route.new(HTTPMethod.options, Router.compile_path(path), param_group, &blk)
  end

  def get_match_for(http_method, path)
    @routes.each do |route|
      uri_params = route.match(http_method, path)

      return RouteMatch.new(route, uri_params) if uri_params
    end

    nil
  end

  def all_matching_route_methods(path)
    matching_routes = @routes.select do |route|
      route.path_match?(path)
    end

    matching_routes.map(&:method)
  end
end

class RouteMatch
  attr_reader :route, :params

  def initialize(route, params)
    @route = route
    @params = params
  end
end

class Route
  attr_reader :method, :matcher, :param_group, :handler

  def initialize(method, matcher, param_group, &handler)
    @method = method
    @matcher = matcher
    @param_group = param_group
    @handler = handler
  end

  def path_match?(uri)
    !!@matcher.match(uri)
  end

  def match(method, uri)
    if method == self.method
      match = @matcher.match(uri)

      if match
        return match.names.zip(match.captures).to_h
      end
    end

    nil
  end
end

class Response
  attr_reader :status_code, :headers, :body

  def initialize(body:, status_code: 200, headers:{})
    @status_code = status_code
    @headers = headers
    @body = body
  end

  def to_rack_format
    [status_code, headers, [body]]
  end
end

class AbstractPresenter
  attr_reader :target

  def initialize(target)
    @target = target
  end

  def representation; raise NotImplementedError; end
end

class AbstractJSONPresenter < AbstractPresenter
  class Error < StandardError; end

  class Node
    attr_reader :name, :handler

    def initialize(name, &blk)
      @name = name
      @handler = blk
    end
  end

  def self.node(name, &blk)
    nodes << Node.new(name, &blk)
  end

  def self.nodes
    @nodes ||= []
  end

  # @Override
  def representation
    kv_pairs = self.class.nodes.map do |node|
      name = node.name

      if node.handler
        [name, node.handler.call(target)]
      elsif target.respond_to?(name)
        [name, target.send(name)]
      else
        raise ::AbstractJSONPresenter::Error.new("Unable to determine representation of field '#{name}' for target #{target.class.name}")
      end
    end

    kv_pairs.to_h.to_json
  end
end

class AbstractERBPresenter < AbstractPresenter
  def template; raise NotImplementedError; end

  def representation
    target_binding = @target.instance_eval { binding }
    ERB.new(template).result(target_binding)
  end
end

class EntityJSONPresenter < AbstractJSONPresenter
  node :id
  node :amount
  node :synthetic do
    "derived property"
  end
end

class Entity < Struct.new(:id, :amount); end

class EntityERBPresenter < AbstractERBPresenter
  def template
    "<div class='lg'>entity <%= id %> with amount <%= amount %></div>"
  end
end

class ParameterDefinition
  attr_reader :name, :type, :validators

  class Type
    def self.scrub
      value
    end
  end

  class Integer < Type
    def self.scrub(value)
      value && value.match(/\A[-]*\d+\z/) && value.to_i
    end
  end

  class Float < Type
    def self.scrub(value)
      value && value.match(/\A[-]*\d+\.\d+\z/) && value.to_f
    end
  end

  class Boolean < Type
    def self.scrub(value)
      value == "true"
    end
  end

  class String < Type; end

  def initialize(name, type, required, validators)
    @name = name
    @type = type
    @validators = validators
    @validators.push(PresentValidator) if required
    @validators.freeze
  end

  def validate(scrubbed_value)
    validation_results = validators.map do |validator|
      validator.validate(scrubbed_value)
    end

    validation_results.compact
  end

  def scrub(value)
    @type.scrub(value)
  end
end

class AbstractParameterGroup
  def self.param(name, type, validators = [], required: true)
    self.params << ParameterDefinition.new(name, type, required, validators)
  end

  def self.params
    @params ||= []
  end

  def self.validate_and_scrub_params!(params)
    extraneous_params = params.keys - self.params.map(&:name)

    if reject_extraneous_params && extraneous_params.any?
      raise ::UserError.new("Extraneous parameters provided: #{extraneous_params.join(', ')}")
    end

    scrubbed_params = scrub_params(params)

    self.params.each do |param_def|
      validation_results = param_def.validate(scrubbed_params[param_def.name])
      unless validation_results.all?(&:valid)
        validation_result = validation_results.first
        validation_error = validation_result.error_message.gsub('{}', param_def.name)
        raise ::UserError.new(validation_error)
      end
    end

    scrubbed_params
  end

  def self.scrub_params(params)
    results = self.params.map do |param_def|
      [param_def.name, param_def.scrub(params[param_def.name])]
    end

    results.to_h
  end

  def self.reject_extraneous_params
    true
  end
end

class AbstractValidator
  class ValidationResult < Struct.new(:valid, :error_message); end

  def self.validate(value); raise NotImplementedError; end
end

class PresentValidator < AbstractValidator
  def self.validate(value)
    ValidationResult.new(!!value, "{} must be provided.")
  end
end

class NonNegativeIntegerValidator < AbstractValidator
  def self.validate(value)
    ValidationResult.new(value && value >= 0 && value.is_a?(Fixnum), "{} is not a non-negative integer.")
  end
end

class EntityIdInput < AbstractParameterGroup
  param('id', ParameterDefinition::Integer, [NonNegativeIntegerValidator])
end

class AnyInput < AbstractParameterGroup
  def self.reject_extraneous_params; false; end
end

class EntityIdRefudIdInput < AbstractParameterGroup
  param('id', ParameterDefinition::Integer)
  param('subentity_id', ParameterDefinition::Integer)
end

Application.configure do
  router.get "/v1/entities", AnyInput do |params|
    Response.new(body: "This is /v1/entities with #{params}")
  end

  router.get "/v1/entities/:id", EntityIdInput do |params|
    entity = Entity.new(params['id'], 15000)
    Response.new(body: EntityJSONPresenter.new(entity).representation)
  end

  router.get "/entities/:id", EntityIdInput do |params|
    entity = Entity.new(params['id'], 15000)
    Response.new(body: EntityERBPresenter.new(entity).representation)
  end

  router.get "/v1/entities/:id/subentities/:subentity_id", AnyInput do |params|
    Response.new(body: "This is /v1/entities/:id/subentities/:subentity_id with #{params}")
  end
end

Rack::Handler::WEBrick.run -> env { Application.application.handle_request(env) }