update stack (again)

util.d

@@ -199,15 +199,30 @@ DLLPush(T, U)(T* list, U* node, U* nil)
 struct Stack(T)
 {
 	Node!(T)* top;
+	Node!(T)* free;
+	Node!(T)* nil;
 }
 
 void
-SPush(T)(Stack!(T)* stack, Node!(T)* node, Node!(T)* nil)
+SPush(T)(Arena* arena, Stack!(T)* stack, T value)
 {
-	if (CheckNil(nil, stack.top))
+	Node!(T)* node;
+	if (!CheckNil(stack.nil, stack.free))
+	{
+		node = stack.free;
+		stack.free = node.next;
+	}
+	else
+	{
+		node = Alloc!(Node!(T))(arena);
+	}
+
+	node.value = value;
+
+	if (CheckNil(stack.nil, stack.top))
 	{
 		stack.top = node;
-		node.next = nil;
+		node.next = stack.nil;
 	}
 	else
 	{
@@ -216,15 +231,22 @@ SPush(T)(Stack!(T)* stack, Node!(T)* node, Node!(T)* nil)
 	}
 }
 
-Node!(T)*
-SPop(T)(Stack!(T)* stack, Node!(T)* nil)
+T
+SPop(T)(Stack!(T)* stack)
 {
-	Node!(T)* node = stack.top;
-	if (!CheckNil(nil, stack.top))
+	T result;
+
+	if (!CheckNil(stack.nil, stack.top))
 	{
-		stack.top = stack.top.next;
+		result = stack.top.value;
+		
+		Node!(T)* free_node = stack.top;
+		stack.top = free_node.next;
+		free_node.next = stack.free;
+		stack.free = free_node;
 	}
-	return node;
+
+	return result;
 }
 
 struct Node(T)
@@ -967,13 +989,19 @@ unittest
 
 	{ // Stack
 		Stack!(u32) stack;
-		Node!(u32) n1 = { value: 1 };
-		Node!(u32) n2 = { value: 2 };
-		Node!(u32) n3 = { value: 3 };
+		Arena arena = CreateArena(MB(1));
 
-		SPush(&stack, &n1, null);
-		SPush(&stack, &n2, null);
-		SPush(&stack, &n3, null);
+		u32 v1 = 1;
+		u32 v2 = 2;
+		u32 v3 = 3;
+
+		Node!(u32) nil;
+
+		stack.nil = &nil;
+
+		SPush(&arena, &stack, v1);
+		SPush(&arena, &stack, v2);
+		SPush(&arena, &stack, v3);
 
 		u32 count = 3;
 		for (auto n = stack.top; !CheckNil(null, n); n = n.next, count -= 1)
@@ -982,11 +1010,11 @@ unittest
 		}
 
 		count = 3;
-		for (auto n = SPop(&stack, cast(Node!(u32)*)null); !CheckNil(null, n); n = SPop(&stack, cast(Node!(u32)*)null), count -= 1)
+		for (u32 n = SPop(&stack); n != 0; n = SPop(&stack), count -= 1)
 		{
-			assert(n.value == count);
+			assert(n == count);
 		}
 
-		assert(stack.top == null);
+		assert(stack.top == &nil);
 	}
 }