ls.rb

#!/Users/aaron/.rubies/arm64/ruby-trunk/bin/ruby

# Configure Vim (using vim-lsp) like this where `ls.rb` is this script:
#
# if executable('./ls.rb')
#   au User lsp_setup call lsp#register_server({
#         \ 'name': 'ls.rb',
#         \ 'cmd': ['./ls.rb'],
#         \ 'allowlist': ['ruby'],
#         \ })
# endif

require "logger"
require "json"
require "uri"

module LSP
  class Writer
    def initialize
      @io = $stdout.binmode
    end

    def write response
      str = JSON.dump(response.merge("jsonrpc" => "2.0"))
      @io.write "Content-Length: #{str.bytesize}\r\n"
      @io.write "\r\n"
      @io.write str
      @io.flush
    end
  end

  class Reader
    def initialize
      @io = $stdin.binmode
    end

    def read
      buffer = @io.gets("\r\n\r\n")
      content_length = buffer.match(/Content-Length: (\d+)/i)[1].to_i
      message = @io.read(content_length)
      JSON.parse message, symbolize_names: true
    end
  end

  $logger = Logger.new("#{File.dirname(File.expand_path(__FILE__))}/out.log")

  class Events
    DISPATCH = {
      "initialize" => :on_initialize,
      "textDocument/didSave" => :did_save,
      "textDocument/hover" => :did_hover,
      "textDocument/definition" => :on_definition,
    }

    def handle method, message, write
      send DISPATCH.fetch(method) { :unknown }, message, write
    end

    def on_definition message, writer
      uri = message.dig(:params, :textDocument, :uri)
      file = URI.parse(message.dig(:params, :textDocument, :uri)).path
      text = File.binread file
      line = text.lines[message.dig(:params, :position, :line)]
      idx  = message.dig(:params, :position, :character)
      token = line[/^.{#{idx}}\w+/][/\w+$/]
      $logger.debug "Want definition for #{token}"
    end

    def on_initialize message, writer
      result = {
        "capabilities" => {
          "textDocumentSync" => {
            "openClose" => true,
            "change" => 1,
            "save" => true
          },
          "diagnosticProvider" => {
            "interFileDependencies" => true,
          },
          "definitionProvider" => true,
          "hoverProvider" => true,
        }
      }

      writer.write(id: message[:id], result: result)
    end

    def did_save message, writer
      saved_file = URI.parse(message.dig(:params, :textDocument, :uri)).path
      # If we got a "did_save" event for this file, then `exec` ourselves
      # so that we can get the updated code.
      if saved_file == File.expand_path(__FILE__)
        $logger.debug "Refreshing LSP"
        exec $0
      end
    end

    def did_hover message, writer
      result = {
        contents: {
          kind: "markdown",
          value: "# Hi YouTubers!"
        },
      }
      writer.write(id: message[:id], result: result)
    end

    def unknown message, writer
      $logger.debug "GOT MESSAGE: #{message}"
    end
  end

  def self.run
    reader = Reader.new
    writer = Writer.new

    subscriber = LSP::Events.new

    loop do
      message = reader.read
      $logger.debug message.inspect
      subscriber.handle message[:method], message, writer
    end
  end
end

if $0 == __FILE__
  LSP.run
end