class ReactOnRails::Dev::PortSelector

def explicit_rails_port 

def explicit_rails_port
  ENV["PORT"]&.to_i&.then { |p| p.between?(1, 65_535) ? p : nil }
end

def explicit_webpack_port 

def explicit_webpack_port
  ENV["SHAKAPACKER_DEV_SERVER_PORT"]&.to_i&.then { |p| p.between?(1, 65_535) ? p : nil }
end

def find_available_port(start_port, exclude: nil) 

def find_available_port(start_port, exclude: nil)
  MAX_ATTEMPTS.times do |i|
    port = start_port + i
    next if port == exclude
    return port if port_available?(port)
  end
  raise NoPortAvailable, "No available port found starting at #{start_port}."
end

def port_available?(port)

probe-then-use pattern and acceptable for the worktree port-selection use case.
server.close and the caller binding to it. This is unavoidable with the
NOTE: Inherent TOCTOU race — another process can claim the port between
Public so it can be stubbed in tests. 
def port_available?(port)
  # Check both IPv4 and IPv6 loopback. Node 22+ resolves "localhost"
  # to ::1 first, so webpack-dev-server often binds only to IPv6.
  # A pure-IPv4 probe would miss that listener.
  %w[127.0.0.1 ::1].all? do |host|
    server = TCPServer.new(host, port)
    server.close
    true
  rescue Errno::EADDRINUSE, Errno::EACCES
    false
  rescue Errno::EADDRNOTAVAIL, SocketError
    true # address family unavailable on this system
  end
end

def select_ports

Probes for free ports when either or both env vars are unset.
Respects existing ENV['PORT'] / ENV['SHAKAPACKER_DEV_SERVER_PORT'].
Returns { rails: Integer, webpack: Integer }. 
def select_ports
  rails_port   = explicit_rails_port
  webpack_port = explicit_webpack_port
  rails_auto   = rails_port.nil?
  webpack_auto = webpack_port.nil?
  rails_port   ||= find_available_port(DEFAULT_RAILS_PORT, exclude: webpack_port)
  webpack_port ||= find_available_port(DEFAULT_WEBPACK_PORT, exclude: rails_port)
  if (rails_auto && rails_port != DEFAULT_RAILS_PORT) ||
     (webpack_auto && webpack_port != DEFAULT_WEBPACK_PORT)
    puts "Default ports in use. Using Rails :#{rails_port}, webpack :#{webpack_port}"
  end
  { rails: rails_port, webpack: webpack_port }
end