Transform `-x + y` to `y - x` unless both have side effects

deob-ast/src/main/java/dev/openrs2/deob/ast/transform/AddSubTransformer.java

@@ -38,9 +38,11 @@ public final class AddSubTransformer extends Transformer {
 				/* x + -y => x - y */
 				expr.setOperator(BinaryExpr.Operator.MINUS);
 				expr.setRight(ExprUtils.negate(right));
-			} else if (op == BinaryExpr.Operator.PLUS && isNegative(left)) {
+			} else if (op == BinaryExpr.Operator.PLUS && isNegative(left) && !(ExprUtils.hasSideEffects(left) && ExprUtils.hasSideEffects(right))) {
 				/* -x + y => y - x */
-				// TODO(gpe): check for side effects before applying this transform
+				expr.setOperator(BinaryExpr.Operator.MINUS);
+				expr.setLeft(right.clone());
+				expr.setRight(ExprUtils.negate(left));
 			} else if (op == BinaryExpr.Operator.MINUS && isNegative(right)) {
 				/* x - -y => x + y */
 				expr.setOperator(BinaryExpr.Operator.PLUS);

deob-ast/src/main/java/dev/openrs2/deob/ast/util/ExprUtils.java

@@ -71,6 +71,24 @@ public final class ExprUtils {
 		return new UnaryExpr(expr.clone(), UnaryExpr.Operator.LOGICAL_COMPLEMENT);
 	}
 
+	public static boolean hasSideEffects(Expression expr) {
+		if (expr.isLiteralExpr() || expr.isNameExpr() | expr.isFieldAccessExpr()) {
+			return false;
+		} else if (expr.isEnclosedExpr()) {
+			return hasSideEffects(expr.asEnclosedExpr().getInner());
+		} else if (expr.isUnaryExpr()) {
+			return hasSideEffects(expr.asUnaryExpr().getExpression());
+		} else if (expr.isBinaryExpr()) {
+			var binary = expr.asBinaryExpr();
+			return hasSideEffects(binary.getLeft()) || hasSideEffects(binary.getRight());
+		} else if (expr.isArrayAccessExpr()) {
+			var access = expr.asArrayAccessExpr();
+			return hasSideEffects(access.getName()) || hasSideEffects(access.getIndex());
+		}
+		// TODO(gpe): more cases
+		return true;
+	}
+
 	private ExprUtils() {
 		/* empty */
 	}