impl Display for Tree

.gitignore

@@ -1,2 +1,3 @@
 target
 Cargo.lock
+.vscode

src/display.rs

@@ -0,0 +1,89 @@
+use std::fmt::Display;
+
+/// Indentation token
+#[derive(Debug)]
+struct Token {
+    /// Is followed by a brother
+    siblings: bool,
+    /// Is intermediate while printing children
+    children: bool,
+}
+
+impl Display for Token {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        let Token { siblings, children } = self;
+
+        write!(
+            f,
+            "{}",
+            match (siblings, children) {
+                (true, true) => "│   ",
+                (true, false) => "├── ",
+                (false, true) => "    ",
+                (false, false) => "└── ",
+            }
+        )
+    }
+}
+
+impl Token {
+    /// Create a new indentation token
+    fn new(siblings: bool) -> Self {
+        Token {
+            siblings,
+            children: false,
+        }
+    }
+
+    /// Set children flag before starting displaying children
+    fn set_children(&mut self) {
+        self.children = true;
+    }
+}
+
+/// Manages the state during the display operation
+#[derive(Debug)]
+pub struct Indentation {
+    tokens: Vec<Token>,
+    ignore_root: bool,
+}
+
+impl Display for Indentation {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        let first: usize = if self.ignore_root { 1 } else { 0 };
+
+        for token in &self.tokens[first..] {
+            write!(f, "{token}")?;
+        }
+
+        Ok(())
+    }
+}
+
+impl Indentation {
+    /// Creates a new indentation handler
+    pub fn new(ignore_root: bool) -> Self {
+        Indentation {
+            tokens: Vec::new(),
+            ignore_root,
+        }
+    }
+
+    /// Adds a new layer of indentation
+    pub fn indent(&mut self, siblings: bool) -> &mut Self {
+        // Setup children mode for previous tokens
+        let len = self.tokens.len();
+        if len > 0 {
+            self.tokens[len - 1].set_children();
+        }
+
+        self.tokens.push(Token::new(siblings));
+        self
+    }
+
+    /// Removes the last layer of indentation
+    pub fn deindent(&mut self) -> &mut Self {
+        self.tokens.pop();
+        self
+    }
+}

src/lib.rs

@@ -35,7 +35,7 @@
     missing_copy_implementations
 )]
 
-use std::fmt::{self, Debug, Formatter};
+use std::fmt::{self, Debug, Display, Formatter};
 use std::num::NonZeroUsize;
 
 /// Vec-backed ID-tree.
@@ -718,3 +718,34 @@ impl<T: Debug> Debug for Tree<T> {
         }
     }
 }
+
+// Handles display
+mod display;
+
+impl<T: Display> Display for Tree<T> {
+    fn fmt(&self, f: &mut Formatter) -> Result<(), fmt::Error> {
+        use display::Indentation;
+        use iter::Edge;
+
+        let mut indent: Indentation = Indentation::new(true);
+
+        for edge in self.root().traverse() {
+            match edge {
+                Edge::Open(node) if node.has_children() => {
+                    indent.indent(node.next_sibling().is_some());
+                    writeln!(f, "{indent}{}", node.value())?;
+                }
+                Edge::Open(node) => {
+                    indent.indent(node.next_sibling().is_some());
+                    writeln!(f, "{indent}{}", node.value())?;
+                    indent.deindent();
+                }
+                Edge::Close(node) if node.has_children() => {
+                    indent.deindent();
+                }
+                _ => {}
+            }
+        }
+        Ok(())
+    }
+}

tests/tree.rs

@@ -156,3 +156,35 @@ fn insert_id_before() {
     );
     assert_eq!(3, tree.root().children().count());
 }
+
+#[test]
+fn test_display() {
+    let tree = tree! {
+        "root" => {
+            "a" => {
+                "child 1",
+            },
+            "b" => {
+                "child 2",
+            },
+        }
+    };
+
+    let repr = format!("{tree}");
+    let expected = "root\n├── a\n│   └── child 1\n└── b\n    └── child 2\n";
+
+    assert_eq!(repr, expected);
+
+    let tree = tree! {
+        "root" => {
+            "a", "b" => {
+                "x", "y"
+            }, "c"
+        }
+    };
+
+    let repr = format!("{tree}");
+    let expected = "root\n├── a\n├── b\n│   ├── x\n│   └── y\n└── c\n";
+
+    assert_eq!(repr, expected);
+}