line_input.cr

def key
  STDIN.raw do |io|
    loop do
      buffer = Bytes.new(3)
      bytes_read = io.read(buffer)
      if bytes_read > 0
        return String.new(buffer[0, bytes_read])
      end
    end
  end
end

string = ""
cursor_pos = 0
loop do
  input = key()
  case input
  when "\x7f" # backspace
    if cursor_pos > 0
      cursor_pos -= 1
      string = string.sub(cursor_pos, "")
    end
  when "\e[D" # left
    cursor_pos = {cursor_pos - 1, 0}.max
  when "\e[C" # right
    cursor_pos = {cursor_pos + 1, string.size}.min
  when "\t"
    exit
  else
    if input[0] >= ' ' # non-control characters
      string = string.sub(cursor_pos...cursor_pos, input)
      cursor_pos += input.size
    end
  end
  print "\r" + string + " \r" + string[0, cursor_pos]
end