Simple text editor in Ruby

#! /usr/bin/env ruby

require 'curses'
include Curses
include Curses::Key

class Fixnum
  def clamp(low, high)
    v = self
    v = low if self < low
    v = high if self > high
    v
  end
end

class Cursor
  attr_reader :row, :col

  def initialize(row=0, col=0)
    @row = row
    @col = col
  end

  def right; Cursor.new @row, @col + 1; end
  def left;  Cursor.new @row, @col - 1; end
  def down;  Cursor.new @row + 1, @col; end
  def up;    Cursor.new @row - 1, @col; end
end

class Buffer
  attr_reader :lines, :cursor

  def initialize(lines=[], cursor=Cursor.new)
    @lines = lines
    @cursor = cursor
  end

  def cur_row; @cursor.row; end
  def cur_col; @cursor.col; end
  def current_line; @lines[cur_row]; end

  def add_char(char)
    lines = @lines.dup
    line = lines[cur_row] || []
    line.insert cur_col, char
    lines[cur_row] = line

    Buffer.new lines, @cursor.right
  end

  def remove_char_before_cursor
    return Buffer.new @lines, @cursor if cur_col == 0

    lines = @lines.dup
    line = lines[cur_row] || []
    line.delete_at(cur_col - 1)
    lines[@cursor.row] = line

    Buffer.new lines, @cursor.left
  end

  def new_line
    lines = @lines.dup
    line, next_line = current_line.partition.with_index { |_, i| i < cur_col }
    lines[cur_row] = line
    lines = lines.insert cur_row + 1, next_line
    Buffer.new lines, Cursor.new(cur_row + 1, 0)
  end

  def beginning_of_line
    Buffer.new lines, Cursor.new(cur_row, 0)
  end

  def end_of_line
    Buffer.new lines, Cursor.new(cur_row, current_line.length)
  end

  def cursor_left
    Buffer.new lines, clamp(@cursor.left)
  end

  def cursor_right
    Buffer.new lines, clamp(@cursor.right)
  end

  def cursor_up
    Buffer.new lines, clamp(@cursor.up)
  end

  def cursor_down
    Buffer.new lines, clamp(@cursor.down)
  end

  def clamp(cursor)
    r = cursor.row.clamp(0, @lines.count - 1)
    c = cursor.col.clamp(0, @lines[r].length)

    Cursor.new r, c
  end
end


class Editor
  BACKSPACE = 127
  CTRLA = 1
  CTRLC = 3
  CTRLE = 5
  ENTER = 13

  def main
    raw
    noecho
    nonl
    stdscr.keypad = true
    init_screen

    buffer = Buffer.new

    begin
      loop do
        char = getch
        buffer = case char
        when CTRLC then break
        when CTRLA then buffer.beginning_of_line
        when CTRLE then buffer.end_of_line
        when KEY_UP then buffer.cursor_up
        when KEY_DOWN then buffer.cursor_down
        when KEY_LEFT then buffer.cursor_left
        when KEY_RIGHT then buffer.cursor_right
        when BACKSPACE then buffer.remove_char_before_cursor
        when ENTER then buffer.new_line
        when /[[:print:]]/ then buffer.add_char(char)
        else buffer
        end

        render buffer
      end
    ensure
      close_screen
    end
  end

  def render(buffer)
    clear
    buffer.lines.each_with_index do |line, index|
      setpos index, 0
      addstr(line.join)
    end
    setpos buffer.cursor.row, buffer.cursor.col
    refresh
  end
end

Editor.new.main

I modified it a little to make it edit a file. https://gist.github.com/nroose/32997db80a45b570bc53174dbe7a5e66