A very basic text editor in ruby, copied from Kilo and adapted in Ruby

kilo.rb

require 'termios'

s = [0, 0, 0, 0].pack("S_S_S_S_")
STDOUT.ioctl(Termios::TIOCGWINSZ, s)

HEIGHT, WIDTH, _, _ = s.unpack("S_S_S_S_")

# Raw mode
current = Termios.tcgetattr(STDIN)
t = current.dup
t.c_iflag &= ~(Termios::BRKINT | Termios::ICRNL | Termios::INPCK | Termios::ISTRIP | Termios::IXON)
t.c_oflag &= ~(Termios::OPOST)
t.c_cflag |= (Termios::CS8)
t.c_lflag &= ~(Termios::ECHO | Termios::ICANON | Termios::IEXTEN | Termios::ISIG)
t.c_cc[Termios::VMIN] = 1 # Setting 0 as in Kilo raises EOF errors
Termios.tcsetattr(STDIN, Termios::TCSANOW, t)

trap("INT") {
  Termios.tcsetattr(STDIN, Termios::TCSANOW, current)
  exit(0)
}

TEXT_CONTENT = [""]
CURSOR_POSITION = [1, 1]

def refresh
  append_buffer = ""
  append_buffer << "\x1b[?25l" # Hide cursor
  append_buffer << "\x1b[H"
  # stderr_log TEXT_CONTENT
  HEIGHT.times do |row_index|

    if row_index >= TEXT_CONTENT.count
      append_buffer << "~\x1b[0K\r\n"
      # stderr_log("Row index: #{row_index}")
      # stderr_log("Line count: #{TEXT_CONTENT.count}")
      next
    end

    row = TEXT_CONTENT[row_index] || ""
    # stderr_log "'#{row}'"
    append_buffer << row
    append_buffer << "\x1b[39m"
    append_buffer << "\x1b[0K"
    append_buffer << "\r\n"
  end
  append_buffer.strip!
  append_buffer << "\x1b[H"
  x, y = CURSOR_POSITION
  # x += 1 if x > 0
  append_buffer << "\x1b[#{y};#{x}H"
  # append_buffer << "\x1b[;1H"
  append_buffer << "\x1b[?25h" # Show cursor
  stderr_log("'#{append_buffer}'".inspect)
  stderr_log("Cursor postition: x: #{CURSOR_POSITION[0]}, y: #{CURSOR_POSITION[1]}: #{y};#{x}H")
  STDOUT.write(append_buffer)
end

def current_row
  TEXT_CONTENT[CURSOR_POSITION[1] - 1]
end

def stderr_log(message)
  unless STDERR.tty? # true when not redirecting to a file, a little janky but works for what I want
    STDERR.puts(message)
  end
end

loop do
  refresh
  c = STDIN.readpartial(1)
  if c == "q"
    Process.kill("INT", Process.pid)
  end
  # stderr_log("ord: #{c.ord}")
  if c.ord == 13 # enter
    if current_row.length > (CURSOR_POSITION[0] - 1)
      carry = current_row[(CURSOR_POSITION[0] - 1)..-1]
      current_row.slice!((CURSOR_POSITION[0] - 1)..-1)
    else
      carry = ""
    end
    TEXT_CONTENT.insert(CURSOR_POSITION[1], carry)
    CURSOR_POSITION[0] = 1
    CURSOR_POSITION[1] += 1
  elsif c.ord == 127 # backspace
    next if CURSOR_POSITION[0] == 1 && CURSOR_POSITION[1] == 1

    if CURSOR_POSITION[0] == 1
      if current_row.nil?
        TEXT_CONTENT.delete_at(CURSOR_POSITION[1] - 1)
        CURSOR_POSITION[1] -= 1
        CURSOR_POSITION[0] = current_row.length + 1
      elsif current_row.empty?
        TEXT_CONTENT.delete_at(CURSOR_POSITION[1] - 1)
        CURSOR_POSITION[1] -= 1
        CURSOR_POSITION[0] = current_row.length + 1
      else
        previous_row = TEXT_CONTENT[CURSOR_POSITION[1] - 2]
        CURSOR_POSITION[0] = previous_row.length + 1
        TEXT_CONTENT[CURSOR_POSITION[1] - 2] = previous_row + current_row
        TEXT_CONTENT.delete_at(CURSOR_POSITION[1] - 1)
        CURSOR_POSITION[1] -= 1
      end
    else
      deletion_index = CURSOR_POSITION[0] - 2
      current_row.slice!(deletion_index)
      CURSOR_POSITION[0] -= 1
    end
  elsif c.ord == 27 # ESC
    second_char = STDIN.read_nonblock(1, exception: false)
    next if second_char == :wait_readable

    third_char = STDIN.read_nonblock(1, exception: false)
    next if third_char == :wait_readable

    if second_char == "["
      case third_char
      when "A" # Up
        CURSOR_POSITION[1] -= 1 unless CURSOR_POSITION[1] == 1
        if current_row && CURSOR_POSITION[0] > current_row.length + 1
          CURSOR_POSITION[0] = current_row.length + 1
        end
      when "B" # Down
        if CURSOR_POSITION[1] == TEXT_CONTENT.length
          CURSOR_POSITION[0] = 1
        end
        CURSOR_POSITION[1] += 1 unless CURSOR_POSITION[1] == TEXT_CONTENT.length + 1
        if current_row && CURSOR_POSITION[0] > current_row.length + 1
          CURSOR_POSITION[0] = current_row.length + 1
        end
      when "C" # Right
        # stderr_log("Current row: #{current_row}\n")
        if current_row && CURSOR_POSITION[0] > current_row.length
          if CURSOR_POSITION[1] <= TEXT_CONTENT.length + 1
            CURSOR_POSITION[0] = 1
            CURSOR_POSITION[1] += 1
          end
        elsif current_row
          CURSOR_POSITION[0] += 1
        end
      when "D" # Left
        if CURSOR_POSITION[0] == 1
          if CURSOR_POSITION[1] > 1
            CURSOR_POSITION[1] -= 1
            CURSOR_POSITION[0] = current_row.length + 1
          end
        else
          CURSOR_POSITION[0] -= 1
        end
      when "H" then "H" # Home
      when "F" then "F" # End
      end
    end
    # TEXT_CONTENT.last << third_char
  elsif c.ord >= 32 && c.ord <= 126
    if current_row.nil?
      TEXT_CONTENT << ""
    end
    current_row.insert(CURSOR_POSITION[0] - 1, c)
    CURSOR_POSITION[0] += 1
  end
end

minimal.rb

require 'termios'

s = [0, 0, 0, 0].pack("S_S_S_S_")
STDOUT.ioctl(Termios::TIOCGWINSZ, s)

HEIGHT, WIDTH, _, _ = s.unpack("S_S_S_S_")

TEXT_CONTENT = [""]

current = Termios.tcgetattr(STDIN)
t = current.dup
# https://man7.org/linux/man-pages/man3/tcflush.3.html
t.c_lflag &= ~(Termios::ICANON);
Termios.tcsetattr(STDIN, Termios::TCSANOW, t)

def refresh
  append_buffer = ""
  append_buffer << "\x1b[?25l" # Hide cursor
  append_buffer << "\x1b[H"
  HEIGHT.times do |row_index|    
    if row_index >= TEXT_CONTENT.count
      append_buffer << "~\x1b[0K\r\n"
      next
    end
  end
  append_buffer << "\x1b[H"
  append_buffer << TEXT_CONTENT.join("")
  STDOUT.write(append_buffer)
end

loop do
  refresh
  c = STDIN.readpartial(1)
  exit(0) if c == "q"

  if c.ord >= 32 && c.ord <= 126
    TEXT_CONTENT.last << c
  end
end