Add support for Python-like negative indexing

Negative indexing is a useful feature in Python, especially when combined
with array slicing. Array slicing will hopefully be implemented later, but
negative indexing is useful in its own right.

A negative index is indexing from the end of an array,
"array[-1] == array[array.size()-1]", using a negative index
larger/smaller than the length of the array is still an error.

While primarily useful for arrays and strings, support is also added to
"array like" structures like Vector3 and Color. This is done just
to be consistent; vector3[2] is much clearer than vector3[-1], but disallowing
it while allowing it for an array with 3 elements seems confusing.

core/variant_op.cpp

@@ -861,7 +861,6 @@ void Variant::evaluate(const Operator& p_op, const Variant& p_a, const Variant&
 		} break;
 		//logic
 		case OP_AND: {
-
 			bool l = p_a.booleanize(r_valid);
 			if (!r_valid)
 				return;
@@ -978,6 +977,8 @@ Variant Variant::get_named(const StringName& p_index, bool *r_valid) const {
 			int index = p_index;\
 			m_type *arr=reinterpret_cast<m_type* >(_data._mem);\
 \
+			if (index<0)\
+				index += arr->size();\
 			if (index>=0 && index<arr->size()) {\
 				valid=true;\
 				cmd;\
@@ -1011,7 +1012,10 @@ void Variant::set(const Variant& p_index, const Variant& p_value, bool *r_valid)
 
 			int idx=p_index;
 			String *str=reinterpret_cast<String*>(_data._mem);
-			if (idx <0 || idx>=str->length())
+			int len = str->length();
+			if (idx<0)
+				idx += len;
+			if (idx<0 || idx>=len)
 				return;
 
 			String chr;
@@ -1025,7 +1029,7 @@ void Variant::set(const Variant& p_index, const Variant& p_value, bool *r_valid)
 				return;
 			}
 
-			*str = str->substr(0,idx)+chr+str->substr(idx+1,str->length());
+			*str = str->substr(0,idx)+chr+str->substr(idx+1, len);
 			valid=true;
 			return;
 
@@ -1040,6 +1044,8 @@ void Variant::set(const Variant& p_index, const Variant& p_value, bool *r_valid)
 				// scalar index
 				int idx=p_index;
 
+				if (idx<0)
+					idx += 2;
 				if (idx>=0 && idx<2) {
 
 					Vector2 *v=reinterpret_cast<Vector2*>(_data._mem);
@@ -1098,6 +1104,8 @@ void Variant::set(const Variant& p_index, const Variant& p_value, bool *r_valid)
 
 				int index = p_index;
 
+				if (index<0)
+					index += 3;
 				if (index>=0 && index<3) {
 					Matrix32 *v=_data._matrix32;
 
@@ -1134,6 +1142,8 @@ void Variant::set(const Variant& p_index, const Variant& p_value, bool *r_valid)
 			if (p_index.get_type()==Variant::INT || p_index.get_type()==Variant::REAL) {
 				//scalar index
 				int idx=p_index;
+				if (idx<0)
+					idx += 3;
 				if (idx>=0 && idx<3) {
 
 					Vector3 *v=reinterpret_cast<Vector3*>(_data._mem);
@@ -1268,6 +1278,8 @@ void Variant::set(const Variant& p_index, const Variant& p_value, bool *r_valid)
 
 				int index = p_index;
 
+				if (index<0)
+					index += 3;
 				if (index>=0 && index<3) {
 					Matrix3 *v=_data._matrix3;
 
@@ -1306,6 +1318,8 @@ void Variant::set(const Variant& p_index, const Variant& p_value, bool *r_valid)
 
 				int index = p_index;
 
+				if (index<0)
+					index += 4;
 				if (index>=0 && index<4) {
 					Transform *v=_data._transform;
 					valid=true;
@@ -1394,6 +1408,8 @@ void Variant::set(const Variant& p_index, const Variant& p_value, bool *r_valid)
 			} else if (p_index.get_type()==Variant::INT) {
 
 				int idx = p_index;
+				if (idx<0)
+					idx += 4;
 				if (idx>=0 || idx<4) {
 					Color *v=reinterpret_cast<Color*>(_data._mem);
 					(*v)[idx]=p_value;
@@ -1841,6 +1857,8 @@ Variant Variant::get(const Variant& p_index, bool *r_valid) const {
 
 				int idx=p_index;
 				const String *str=reinterpret_cast<const String*>(_data._mem);
+				if (idx<0)
+					idx += str->length();
 				if (idx >=0 && idx<str->length()) {
 
 					valid=true;
@@ -1854,6 +1872,8 @@ Variant Variant::get(const Variant& p_index, bool *r_valid) const {
 			if (p_index.get_type()==Variant::INT || p_index.get_type()==Variant::REAL) {
 				// scalar index
 				int idx=p_index;
+				if (idx<0)
+					idx += 2;
 				if (idx>=0 && idx<2) {
 
 					const Vector2 *v=reinterpret_cast<const Vector2*>(_data._mem);
@@ -1899,6 +1919,8 @@ Variant Variant::get(const Variant& p_index, bool *r_valid) const {
 			if (p_index.get_type()==Variant::INT || p_index.get_type()==Variant::REAL) {
 				//scalar index
 				int idx=p_index;
+				if (idx<0)
+					idx += 3;
 				if (idx>=0 && idx<3) {
 
 					const Vector3 *v=reinterpret_cast<const Vector3*>(_data._mem);
@@ -1929,6 +1951,8 @@ Variant Variant::get(const Variant& p_index, bool *r_valid) const {
 
 				int index = p_index;
 
+				if (index<0)
+					index += 3;
 				if (index>=0 && index<3) {
 					const Matrix32 *v=_data._matrix32;
 
@@ -2024,7 +2048,8 @@ Variant Variant::get(const Variant& p_index, bool *r_valid) const {
 			if (p_index.get_type()==Variant::INT || p_index.get_type()==Variant::REAL) {
 
 				int index = p_index;
-
+				if (index<0)
+					index += 3;
 				if (index>=0 && index<3) {
 					const Matrix3 *v=_data._matrix3;
 
@@ -2054,7 +2079,8 @@ Variant Variant::get(const Variant& p_index, bool *r_valid) const {
 			if (p_index.get_type()==Variant::INT || p_index.get_type()==Variant::REAL) {
 
 				int index = p_index;
-
+				if (index<0)
+					index += 4;
 				if (index>=0 && index<4) {
 					const Transform *v=_data._transform;
 					valid=true;
@@ -2118,6 +2144,8 @@ Variant Variant::get(const Variant& p_index, bool *r_valid) const {
 			}  else if (p_index.get_type()==Variant::INT) {
 
 				int idx = p_index;
+				if (idx<0)
+					idx += 4;
 				if (idx>=0 || idx<4) {
 					const Color *v=reinterpret_cast<const Color*>(_data._mem);
 					valid=true;

doc/base/classes.xml

@@ -4325,7 +4325,7 @@
 	Generic array datatype.
 	</brief_description>
 	<description>
-	Generic array, contains several elements of any type, accessible by numerical index starting at 0. Arrays are always passed by reference.
+	Generic array, contains several elements of any type, accessible by numerical index starting at 0. Negative indices can be used to count from the right, like in Python. Arrays are always passed by reference.
 	</description>
 	<methods>
 		<method name="append">