servo: Extend the traversal interface to support passing a value from parents to children.

Source-Repo: https://github.com/servo/servo
Source-Revision: 0b7d020174197e40a27107fd45e1c8657c618874

servo/src/servo/layout/traverse.rs

@@ -6,6 +6,8 @@ import intrinsic::tydesc;
 export full_traversal;
 export top_down_traversal;
 export bottom_up_traversal;
+export extended_full_traversal;
+export extended_top_down_traversal;
 
 // The underlying representation of an @T.  We don't actually care
 // what it is, just that we can transform to and from this
@@ -35,18 +37,25 @@ fn rewrap_box(-b : *shared_box<Box>) -> @Box unsafe {
 
 Iterate down and then up a tree of layout boxes in parallel and apply
 the given functions to each box.  Each box applies the first function,
-spawns a task to complete all of its children in parallel, waits for
-them to finish, and then applies the second function.
+spawns a task to complete all of its children in parallel, passing
+each child the result of the ifrst funciton. It waits for them to
+finish, and then applies the second function to the current box.
 
 # Arguments 
 
-* `root` - The current top of the tree, the functions will be applied to it and its children.
-* `top-down` - A function that is applied to each node after it is applied to that node's parent.
-* `bottom-up` - A function that is applied to each node after it is applied to that node's 
-                children
+* `root` - The current top of the tree, the functions will be applied
+           to it and its children.
+* `returned` - The value returned by applying top_down to the parent
+               of the current box, or a passed in default
+* `top_down` - A function that is applied to each node after it is
+              applied to that node's parent.
+* `bottom_up` - A function that is applied to each node after it is
+                applied to that node's children
+
 "]
-fn traverse_helper(-root : @Box, -top_down : fn~(@Box), -bottom_up : fn~(@Box)) {
-    top_down(root);
+fn traverse_helper<T : copy send>(-root : @Box, returned : T, -top_down : fn~(+T, @Box) -> T,
+                      -bottom_up : fn~(@Box)) {
+    let returned = top_down(returned, root);
 
     do listen |ack_chan| { 
         let mut count = 0;
@@ -74,7 +83,7 @@ fn traverse_helper(-root : @Box, -top_down : fn~(@Box), -bottom_up : fn~(@Box))
 
                 // Retrieve the original @Box and recurse
                 let new_kid = rewrap_box(option::unwrap(swapped_in));
-                traverse_helper(new_kid, copy top_down, copy bottom_up);
+                traverse_helper(new_kid, copy returned, copy top_down, copy bottom_up);
 
                 ack_chan.send(());
             }
@@ -92,13 +101,20 @@ fn nop(_box : @Box) {
     return;
 }
 
+#[doc= "
+   A wrapper to change a function that only acts on a box to one that
+   threasds a unit through to match travserse_helper
+"]
+fn unit_wrapper(-fun : fn~(@Box)) -> fn~(+(), @Box) {
+    fn~(+_u : (), box : @Box) { fun(box); }
+}
+
 #[doc="
    Iterate in parallel over the boxes in a tree, applying one function
    to a parent before recursing on its children and one after.
 "]
-
 fn full_traversal(+root : @Box, -top_down : fn~(@Box), -bottom_up : fn~(@Box)) {
-    traverse_helper(root, top_down, bottom_up);
+    traverse_helper(root, (), unit_wrapper(top_down), bottom_up);
 }
 
 #[doc="
@@ -106,7 +122,7 @@ fn full_traversal(+root : @Box, -top_down : fn~(@Box), -bottom_up : fn~(@Box)) {
    function to a parent before its children.
 "]
 fn top_down_traversal(+root : @Box, -top_down : fn~(@Box)) {
-    traverse_helper(root, top_down, nop);
+    traverse_helper(root, (), unit_wrapper(top_down), nop);
 }
 
 #[doc="
@@ -114,5 +130,28 @@ fn top_down_traversal(+root : @Box, -top_down : fn~(@Box)) {
    function to a parent after its children.
 "]
 fn bottom_up_traversal(+root : @Box, -bottom_up : fn~(@Box)) {
-    traverse_helper(root, nop, bottom_up);
+    traverse_helper(root, (), unit_wrapper(nop), bottom_up);
+}
+
+#[doc="
+   Iterate in parallel over the boxes in a tree, applying the given
+   function to a parent before its children, the value returned by the
+   function is passed to each child when they are recursed upon.  As
+   the recursion unwinds, the second function is applied to first the
+   children in parallel, and then the parent.
+"]
+fn extended_full_traversal<T : copy send>(+root : @Box, first_val : T, 
+                                          -top_down : fn~(+T, @Box) -> T,
+                                          -bottom_up : fn~(@Box)) {
+    traverse_helper(root, first_val, top_down, bottom_up);
+}
+
+#[doc="
+   Iterate in parallel over the boxes in a tree, applying the given
+   function to a parent before its children, the value returned by the
+   function is passed to each child when they are recursed upon.
+"]
+fn extended_top_down_traversal<T : copy send>(+root : @Box, first_val : T,
+                                              -top_down : fn~(+T, @Box) -> T) {
+    traverse_helper(root, first_val, top_down, nop);
 }