else statements

Lang/Block.cs

@@ -32,14 +32,32 @@ public Block(List<Instruction> buffer) {
 	public class IfBlock : Block {
 		public BranchInstruction branch;
 		public int branchIndex;
+		public JumpInstruction elseJump = null;
 
 		public IfBlock(Expression condition, List<Instruction> buffer) : base(buffer) {
 			branchIndex = buffer.Count;
 			branch = new BranchInstruction(condition, branchIndex+1, -1);
+			buffer.Add(branch);
 		}
 
-		override public void End() {
+		public bool ElseIf(Expression condition) {
+			return false;
+		}
+
+		public bool Else() {
+			if (elseJump != null) { return false; }
+			elseJump = new JumpInstruction(-1);
+			buffer.Add(elseJump);
 			branch.indexFalse = buffer.Count;
+			return true;
+		}
+
+		override public void End() {
+			if (elseJump != null) {
+				elseJump.index = buffer.Count;
+			} else {
+				branch.indexFalse = buffer.Count;
+			}
 		}
 
 		// todo add elseif, else

Lang/InstructionParser.cs

@@ -97,6 +97,12 @@ State Idle(Token t) {
 						if (mBlock.Count == 0) { return null; }
 						mBlock.Pop().End();
 						return Idle;
+					case "else":
+						if (mBlock.Count == 0) { return null; }
+						var block = mBlock.Peek() as IfBlock;
+						if (block == null) { return null; }
+						if (!block.Else()) { return null; }
+						return Idle;
 					default: break;
 				}
 			}

Main.cs

@@ -41,6 +41,11 @@ turn left
 					move backward 5
 					stop stroke
 				end
+				if 0
+					move forward 100
+				else
+					move backward 100
+				end
 			");
 			var interpreter = new Interpreter(program);
 			while(interpreter.ExecuteNextInstruction()) {}