borrow link def url if on single line

src/lib.rs

@@ -268,7 +268,7 @@ pub struct Parser<'s> {
     tree: tree::Branch<block::Node, block::Atom>,
     inlines: span::InlineSpans<'s>,
     inline_parser: Option<inline::Parser<span::InlineCharsIter<'s>>>,
-    link_definitions: std::collections::HashMap<&'s str, String>,
+    link_definitions: std::collections::HashMap<&'s str, CowStr<'s>>,
 
     _tree_data: block::Tree,
 }
@@ -286,8 +286,11 @@ impl<'s> Parser<'s> {
                     e.kind
                 {
                     let tag = e.span.of(src);
-                    // TODO borrow url string if single inline
-                    let url = branch.take_inlines().map(|sp| sp.of(src).trim()).collect();
+                    let url = match branch.count_children() {
+                        0 => "".into(),
+                        1 => branch.take_inlines().next().unwrap().of(src).trim().into(),
+                        _ => branch.take_inlines().map(|sp| sp.of(src).trim()).collect(),
+                    };
                     defs.insert(tag, url);
                 }
             }

src/tree.rs

@@ -50,6 +50,18 @@ pub struct Branch<C: 'static, A: 'static> {
 }
 
 impl<C, A> Branch<C, A> {
+    /// Count number of direct children nodes.
+    pub fn count_children(&self) -> usize {
+        let mut head = self.head;
+        let mut count = 0;
+        while let Some(h) = head {
+            let n = &self.nodes[h.index()];
+            head = n.next;
+            count += 1;
+        }
+        count
+    }
+
     /// Retrieve all inlines until the end of the current container. Panics if any upcoming node is
     /// not an inline node.
     pub fn take_inlines(&mut self) -> impl Iterator<Item = Span> + '_ {