better cache management for dir size computation

Makes it faster in 2 common cases:
- compute the root size when children already computed
- going up one level in the tree

1 files changed, 2 insertions, 0 deletions

diff --git a/src/tree/tree.rs b/src/tree/tree.rs
index c7670e1..977d6ad 100644
--- a/src/tree/tree.rs
+++ b/src/tree/tree.rs
@@ -366,6 +366,8 @@ impl Tree {
     /// To compute the size of all of them, this should be called until
     ///  has_dir_missing_sum returns false
     pub fn fetch_some_missing_dir_sum(&mut self, dam: &Dam) {
+        // we prefer to compute the root directory last: its computation
+        // is faster when its first level children are already computed
         for i in (0..self.lines.len()).rev() {
             if self.lines[i].sum.is_none() && self.lines[i].line_type == TreeLineType::Dir {
                 self.lines[i].sum = FileSum::from_dir(&self.lines[i].path, dam);