Adding paths

The algorithm for inserting a string into a trie can be described in only a few sentences: go through each character of the word and trace it down the trie. If a node does not yet exist, create it, and add the object with the last entry.

Of course, there are special cases that need to be decided as well: what happens when a string already exists? Overwrite or ignore? In the case of this implementation, the last write will win—that is, it's overwriting whatever existed previously:

pub fn add(&mut self, device: IoTDevice) {
    let p = device.path.clone();
    let mut path = p.chars();
    if let Some(start) = path.next() {
        self.length += 1;
        let mut n = self.root
                .entry(start)
                .or_insert(Node::new(start, None));
        for c in path {
            let tmp = n.next
                    .entry(c)
                    .or_insert(Node::new(c, None));
            n = tmp;
        }
        n.value = Some(device);
    }
}

Another special...