Cubically Interpreter

-- Cube
local Cube = {}
function Cube.new()
  local faces = {}
  for i = 0, 5 do
    faces[i] = {}
    for n = 0, 8 do
      faces[i][n] = i
    end
  end

  return setmetatable({
    faces = faces
  }, {
    __index = Cube,
    __tostring = Cube.tostring
  })
end

function Cube:R(n)
  n = n % 3

  local t = 0
  local f = self.faces
  for _ = 1, n do
    for i = 2, 8, 3 do
      t = f[0][i]
      f[0][i] = f[2][i]
      f[2][i] = f[5][10 - i]
      f[5][10 - i] = f[4][10 - i - 2]
      f[4][10 - i - 2] = t
    end
  end

  -- Rotate face 3 CW
  f = f[3]
  t = f[0]
  f[0] = f[6]
  f[6] = f[8]
  f[8] = f[2]
  f[2] = t
  t = f[1]
  f[1] = f[3]
  f[3] = f[7]
  f[7] = f[5]
  f[5] = t
end

function Cube:L(n)
  n = n % 3

  local t = 0
  local f = self.faces
  for _ = 1, n do
    for i = 0, 6, 3 do
      t = f[0][i]
      f[0][i] = f[4][6 - i + 2]
      f[4][6 - i + 2] = f[5][6 - i]
      f[5][6 - i] = f[2][i]
      f[2][i] = t
    end
  end

  -- Rotate face 1 CW
  f = f[1]
  t = f[0]
  f[0] = f[6]
  f[6] = f[8]
  f[8] = f[2]
  f[2] = t
  t = f[1]
  f[1] = f[3]
  f[3] = f[7]
  f[7] = f[5]
  f[5] = t
end

function Cube:U(n)
  n = n % 3

  local t = 0
  local f = self.faces
  for _ = 1, n do
    for i = 0, 2 do
      t = f[1][i]
      f[1][i] = f[2][i]
      f[2][i] = f[3][i]
      f[3][i] = f[4][i]
      f[4][i] = t
    end
  end

  -- Rotate face 1 CW
  f = f[0]
  t = f[0]
  f[0] = f[6]
  f[6] = f[8]
  f[8] = f[2]
  f[2] = t
  t = f[1]
  f[1] = f[3]
  f[3] = f[7]
  f[7] = f[5]
  f[5] = t
end

function Cube:D(n)
  n = n % 3

  local t = 0
  local f = self.faces
  for _ = 1, n do
    for i = 6, 8 do
      t = f[4][i]
      f[4][i] = f[3][i]
      f[3][i] = f[2][i]
      f[2][i] = f[1][i]
      f[1][i] = t
    end
  end

  -- Rotate face 1 CW
  f = f[5]
  t = f[0]
  f[0] = f[6]
  f[6] = f[8]
  f[8] = f[2]
  f[2] = t
  t = f[1]
  f[1] = f[3]
  f[3] = f[7]
  f[7] = f[5]
  f[5] = t
end

function Cube:F(n)
  n = n % 3

  local t = 0
  local f = self.faces
  for _ = 1, n do
    for i = 0, 2 do
      t = f[0][6 + i]
      f[0][6 + i] = f[1][6 - 3 * i + 2]
      f[1][6 - 3 * i + 2] = f[5][8 - i]
      f[5][8 - i] = f[3][3 * i]
      f[3][3 * i] = t
    end
  end

  -- Rotate face 2 CW
  f = f[2]
  t = f[0]
  f[0] = f[6]
  f[6] = f[8]
  f[8] = f[2]
  f[2] = t
  t = f[1]
  f[1] = f[3]
  f[3] = f[7]
  f[7] = f[5]
  f[5] = t
end

function Cube:B(n)
  n = n % 3

  local t = 0
  local f = self.faces
  for _ = 1, n do
    for i = 0, 2 do
      t = f[0][i]
      f[0][i] = f[3][3 * i + 2]
      f[3][3 * i + 2] = f[5][2 - i]
      f[5][2 - i] = f[1][6 - 3 * i]
      f[1][6 - 3 * i] = t
    end
  end

  -- Rotate face 4 CW
  f = f[4]
  t = f[0]
  f[0] = f[6]
  f[6] = f[8]
  f[8] = f[2]
  f[2] = t
  t = f[1]
  f[1] = f[3]
  f[3] = f[7]
  f[7] = f[5]
  f[5] = t
end

function Cube:value(n)
  if n < 0 or n > 5 or n % 1 ~= 0 then
    return 0
  end

  local sum = 0
  for i = 0, 8 do
    sum = sum + self.faces[n][i]
  end
  return sum
end

function Cube:tostring()
  local s = ""
  for y = 0, 2 do
    s = s .. "   "
    for x = 0, 2 do
      s = s .. self.faces[0][y * 3 + x]
    end
    s = s .. "\n"
  end

  for y = 0, 2 do
    for i = 1, 4 do
      for x = 0, 2 do
        s = s .. self.faces[i][y * 3 + x]
      end
    end
    s = s .. "\n"
  end

  for y = 2, 0, -1 do
    s = s .. "   "
    for x = 0, 2 do
      s = s .. self.faces[5][y * 3 + x]
    end
    s = s .. "\n"
  end

  return s:sub(1, #s - 1)
end

-- Interpreter

local C = {}
setmetatable(C, {__call = C.new})

function C.new()
  return setmetatable({
    instance = true,
    cube = Cube.new(),
    notepad = 0,
    input = 0,
    command = nil
  }, {__index = C})
end

function C:exec(program)
  assert(self.instance, "Must be an instance, call new() first")
  assert(type(program) == "string", "program must be a string")

  self.program = program
  for i = 1, #self.program do
    local c = self.program:sub(i, i)
    local n = tonumber(c)

    if n then
      if not self.command then
        print(self.cube:tostring())
        return
      end

      if self.command then
        self:command(n)
      end
    else
      self.command = self.commands[c]
    end
  end
end

function C:value(n)
  if n % 1 ~= 0 then
    return 0
  end

  if n >= 0 and n <= 5 then
    return self.cube:value(n)
  elseif n == 6 then
    return self.notepad
  elseif n == 7 then
    return self.input
  else
    return 0
  end
end

C.commands = {
  ['R'] = function(self, n)
    self.cube.R(n)
  end,
  ['L'] = function(self, n)
    self.cube.L(n)
  end,
  ['U'] = function(self, n)
    self.cube.U(n)
  end,
  ['D'] = function(self, n)
    self.cube.D(n)
  end,
  ['F'] = function(self, n)
    self.cube.F(n)
  end,
  ['B'] = function(self, n)
    self.cube.B(n)
  end,

  ['&'] = function(self, n)
    self.program = ""
  end,
  ['+'] = function(self, n)
    self.notepad = self.notepad + self:value(n)
  end,
  ['-'] = function(self, n)
    self.notepad = self.notepad - self:value(n)
  end,
  ['*'] = function(self, n)
    self.notepad = self.notepad * self:value(n)
  end,
  ['/'] = function(self, n)
    self.notepad = self.notepad / self:value(n)
  end,
  ['^'] = function(self, n)
    self.notepad = self.notepad ^ self:value(n)
  end,
  ['='] = function(self, n)
    self.notepad = (self.notepad == self:value(n)) and 1 or 0
  end,
  [':'] = function(self, n)
    self.notepad = self:value(n)
  end,

  ['@'] = function(self, n)
    io.write(string.char(self:value(n) % 256))
  end,
  ['%'] = function(self, n)
    io.write(self:value(n))
  end,
  ['$'] = function(self, n)
    local inp = io.read("*n")
    self.input = inp or self.input
  end,
  ['~'] = function(self, n)
    local inp = io.read(1)
    self.input = inp and string.byte(inp) or -1
  end
}

local cubically = C.new()
cubically:exec(select(1, ...))

print()
print("============")
print("Final state:")
print(cubically.cube:tostring())
print("Notepad: " .. cubically.notepad)
print("Input: " .. cubically.input)