Add Epson TM-T20II receipt printing (title/body/QR, no ESC/POS knowledge needed)

Custom ReceiptBuilder instead of the escpos gem -- its diacritics handling
needed the same manual UTF-8->CP852 encode call anyway, based on earlier
experiments in ~/test/epson/.

lib/receipt_builder.rb

@@ -0,0 +1,106 @@
+# Minimal chainable ESC/POS builder for the Epson TM-T20II.
+#
+# Written instead of pulling in the `escpos` gem: the gem's diacritics
+# handling needed the same manual UTF-8->CP852 `.encode` call anyway, and a
+# couple of its helpers didn't behave as documented against this printer --
+# fewer moving parts doing it directly.
+class ReceiptBuilder
+  INIT       = "\x1B\x40".freeze
+  CP852      = "\x1B\x74\x12".freeze # code page for Slovak diacritics
+
+  ALIGN_LEFT = "\x1B\x61\x00".freeze
+  ALIGN_CENT = "\x1B\x61\x01".freeze
+  ALIGN_RIGH = "\x1B\x61\x02".freeze
+
+  BOLD_ON    = "\x1B\x45\x01".freeze
+  BOLD_OFF   = "\x1B\x45\x00".freeze
+  UNDER_ON   = "\x1B\x2D\x01".freeze
+  UNDER_OFF  = "\x1B\x2D\x00".freeze
+
+  TXT_NORM   = "\x1D\x21\x00".freeze
+  TXT_BIG    = "\x1D\x21\x11".freeze # double height and width
+  TXT_HIGH   = "\x1D\x21\x01".freeze # double height
+  TXT_WIDE   = "\x1D\x21\x10".freeze # double width
+
+  CUT        = "\x1D\x56\x00".freeze
+
+  def initialize
+    @buffer = "".b
+    init
+  end
+
+  def init
+    @buffer << INIT << CP852
+    self
+  end
+
+  def text(str)
+    @buffer << str.encode('CP852', invalid: :replace, undef: :replace, replace: '?')
+    self
+  end
+
+  def align(mode)
+    @buffer << case mode
+               when :center then ALIGN_CENT
+               when :right  then ALIGN_RIGH
+               else ALIGN_LEFT
+               end
+    self
+  end
+
+  def bold(enable = true)
+    @buffer << (enable ? BOLD_ON : BOLD_OFF)
+    self
+  end
+
+  def underline(enable = true)
+    @buffer << (enable ? UNDER_ON : UNDER_OFF)
+    self
+  end
+
+  def size(mode)
+    @buffer << case mode
+               when :big  then TXT_BIG
+               when :high then TXT_HIGH
+               when :wide then TXT_WIDE
+               else TXT_NORM
+               end
+    self
+  end
+
+  # Native ESC/POS QR code
+  def qr(data, size: 6)
+    qr_data = data.b
+    store_len = qr_data.bytesize + 3
+
+    @buffer << "\x1D\x28\x6B\x03\x00\x31\x43" << size.chr
+    @buffer << "\x1D\x28\x6B\x03\x00\x31\x45\x30" # error correction L
+    @buffer << "\x1D\x28\x6B" << [store_len % 256, store_len / 256].pack('CC') << "\x31\x50\x30" << qr_data
+    @buffer << "\x1D\x28\x6B\x03\x00\x31\x51\x30" # print
+    self
+  end
+
+  # Native ESC/POS EAN-13
+  def ean13(code)
+    @buffer << "\x1D\x6B\x02" << code.b << "\x00"
+    self
+  end
+
+  def feed(lines = 3)
+    @buffer << ("\n" * lines)
+    self
+  end
+
+  def cut
+    @buffer << CUT
+    self
+  end
+
+  def render
+    @buffer
+  end
+
+  def print!(device)
+    File.open(device, 'wb') { |f| f.write(@buffer) }
+  end
+end