Parse binary expressions

Author: mudge

lib/expression_parser.rb

@@ -13,6 +13,14 @@ def emit(vm_writer, symbol_table)
     end
   end
 
+  BinaryOperation = Struct.new(:operation, :left_node, :right_node) do
+    def emit(vm_writer, symbol_table)
+      left_node.emit(vm_writer, symbol_table)
+      right_node.emit(vm_writer, symbol_table)
+      vm_writer.write_operation(operation)
+    end
+  end
+
   attr_reader :tokenizer
   private :tokenizer
 
@@ -21,15 +29,30 @@ def initialize(tokenizer)
   end
 
   def parse
-    case tokenizer.token_type
+    left_node = case tokenizer.token_type
     when Tokenizer::INT_CONST
       number = tokenizer.int_val
 
       Number.new(number)
-    else
+    when Tokenizer::IDENTIFIER
       identifier = tokenizer.identifier
 
       Variable.new(identifier)
     end
+
+    return left_node unless tokenizer.has_more_tokens?
+
+    tokenizer.advance
+
+    if %w[+ - * / & | < > =].include?(tokenizer.symbol)
+      operation = tokenizer.symbol
+      tokenizer.advance
+
+      right_node = parse
+
+      BinaryOperation.new(operation, left_node, right_node)
+    else
+      left_node
+    end
   end
 end

lib/vm_writer.rb

@@ -14,6 +14,10 @@ def write_pop(segment, index)
     io.puts("pop #{segment} #{index}")
   end
 
+  def write_operation(operation)
+    write_arithmetic({ '+' => 'add' }.fetch(operation))
+  end
+
   def write_arithmetic(command)
     io.puts(command)
   end

spec/unit/expression_parser_spec.rb

@@ -41,4 +41,42 @@
 
     expect(output.string).to eq("push static 0\n")
   end
+
+  it 'emits VM code for binary expressions' do
+    tokenizer = Tokenizer.new('1 + 1')
+    tokenizer.advance
+
+    symbol_table = SymbolTable.new
+
+    result = ExpressionParser.new(tokenizer).parse
+    output = StringIO.new
+    vm_writer = VMWriter.new(output)
+    result.emit(vm_writer, symbol_table)
+
+    expect(output.string).to eq(<<-VM)
+push constant 1
+push constant 1
+add
+    VM
+  end
+
+  it 'emits VM code for compound binary expressions' do
+    tokenizer = Tokenizer.new('1 + 2 + 3')
+    tokenizer.advance
+
+    symbol_table = SymbolTable.new
+
+    result = ExpressionParser.new(tokenizer).parse
+    output = StringIO.new
+    vm_writer = VMWriter.new(output)
+    result.emit(vm_writer, symbol_table)
+
+    expect(output.string).to eq(<<-VM)
+push constant 1
+push constant 2
+push constant 3
+add
+add
+    VM
+  end
 end