replace expressions with polynomials, add interpreter command class, rewrite parser

lib/ruby_algebra/command.rb

@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+module RubyAlgebra
+  # Команда интерпретатора
+  class Command
+  end
+
+  # Команда присвоения
+  class AssignmentCommand < Command
+    attr_reader :lhs, :operation, :operand1, :operand2
+
+    def initialize(lhs, operation, operand1, operand2)
+      @lhs = lhs
+      @operation = operation
+      @operand1 = operand1
+      @operand2 = operand2
+    end
+
+    def to_s
+      result = @lhs.is_a?(Array) ? @lhs.join(', ') : @lhs.to_s
+      result += ' := '
+      result += @operand1.to_s
+      case @operation
+      when :add
+        result += ' + '
+      when :sub
+        result += ' - '
+      when :mult, :scale
+        result += ' * '
+      when :div
+        result += ' / '
+      end
+      result += @operand2.to_s if @operation != :assign
+      result
+    end
+  end
+
+  # Команда вывода на экран
+  class DisplayCommand < Command
+    attr_reader :polynomial
+
+    def initialize(polynomial)
+      @polynomial = polynomial
+    end
+  end
+end