Crate dendron

Expand description

Tree data structure.

Concepts

Tree

Tree consists of one or more nodes. Empty tree cannot exist since a tree must have just one root node.

Lifetime of a tree is managed by references to a tree and its nodes. An object of Tree type refers to a tree and makes the tree live longer than the reference. Objects of node types (Node, FrozenNode, and HotNode types) refer to nodes. They make the node (and the tree it belongs to) live longer than the references.

These four types are similar to trees created by node types and Rc, but this crate neither leaks the nodes nor releases the nodes if they are connected to other nodes referred by the user. See examples below for detail.

use dendron::Node;

let root = Node::new_tree("root");
let grant = root.tree().grant_hierarchy_edit().unwrap();
let child0 = root.create_as_last_child(&grant, "child0");
let child1 = root.create_as_last_child(&grant, "child1");
let child1_0 = child1.create_as_last_child(&grant, "child1_0");
//  root
//  |-- child0
//  `-- child1
//      `-- child1_0

drop(grant);
drop(root);
drop(child0);
drop(child1);
// Now only `child1_0` is alive.

// `child1_0` makes the entire tree and belonging nodes alive,
// so `root` is still alive.
assert_eq!(*child1_0.root().borrow_data(), "root");

let tree = child1_0.tree();
drop(child1_0);
// Now all node references are dropped and only `tree` is alive.

// `tree` makes the entire tree and belonging nodes alive,
// so `child0` is still alive.
assert_eq!(
    *tree.root().first_child().unwrap().borrow_data(),
    "child0"
);

drop(tree);
// Now all tree references and node references are dropped, so all objects
// in the tree is released. No memory leaks!

Hierarchy edit prohibitions and grants

Sometimes users may wish tree hierarchy to be preserved, especially when they are iterating nodes.

use dendron::tree_node;

let root = tree_node! {
    "root", [
        "0",
        "1",
        "2",
    ]
};
let grant = root.tree().grant_hierarchy_edit().unwrap();
let mut children = root.children();
// Detach the first child from `root`.
root.first_child().unwrap().detach_subtree(&grant);

// Next node is... not "1"! This is because `children` have a cache of
// "next node" at the creation of the iterator.
assert_eq!(children.next().map(|node| *node.borrow_data()), Some("0"));
assert_eq!(children.next(), None);

To enable “freezing” hierarchy, this crate requires users to have a “grant” to edit hierarchy, and also provides capability to “prohibit” hierarchy editing (for arbitrary duration).

use dendron::tree;

let tree = tree! { "root" };

// This "prohibition" prohibits tree hierarchy editing
// (and grant acquisition).
let freezer = tree.prohibit_hierarchy_edit().unwrap();

// This `grant_hierarchy_edit()` fails, so you can ensure
// the hierarchy is stable until `freezer` is dropped!
let grant = tree.grant_hierarchy_edit().unwrap();

“Grant” (HierarchyEditGrant) and “prohibition” (HierarchyEditProhibition) works in ways similar to MutexGuard. The grants and prohibitions work like scoped locks; trees are editable or non-editable during grants or prohibitions exist, and once they are dropped, trees become neutral.

Note that grants and prohibitions are mutually exclusive, but only multiple grants or only multiple prohibitions can coexist for a tree, like reader locks. In other words, you can have multiple grants for a tree, and also can have multiple prohibitions for a tree, but not both at the same time.

use dendron::tree;

let tree = tree! { "root" };

let freezer1 = tree.prohibit_hierarchy_edit().unwrap();
// You can have multiple prohibitions at the same time.
let freezer2 = tree.prohibit_hierarchy_edit().unwrap();
let freezer3 = tree.prohibit_hierarchy_edit().unwrap();

// But not both prohibitions and grants.
assert!(tree.grant_hierarchy_edit().is_err());

// Release the locks.
drop(freezer1);
drop(freezer2);
drop(freezer3);

// Now the tree is neutral. You can re-prohibit or grant hierarchy edit.
let grant = tree.grant_hierarchy_edit().unwrap();

// Again, prohibitions and grants cannot coexist for the same tree.
assert!(tree.prohibit_hierarchy_edit().is_err());

Nodes

The crate provides three node types: Node, FrozenNode, and HotNode. They all have shared ownerships of the tree they belong to.

Node is the basic neutral plain node. It can be turned into FrozenNode or HotNode by generating or bundling a prohibition or a grant to edit hierarchy.

FrozenNode is a Node with a hierarchy edit prohibition bundled. This exposes some extra methods to create tree traversal iterators, for example FrozenNode::children_stable and FrozenNode::allocating_breadth_first_traverse_stable.

HotNode is a Node with a hierarchy edit grant bundled. This exposes some extra methods to edit tree hierarchy without passing grants as arguments, for example HotNode::detach_subtree and HotNode::try_create_node_as.

Usage

To create tree directly, use tree_node! or tree!.

use dendron::{tree, tree_node};

let root1 = tree_node! {
    "root", [
        "0",
        /("1", [
            /("1-0", [
                "1-0-0"
            ]),
            "1-1",
        ]),
    ]
};
let tree1 = root1.tree();

let tree2 = tree! {
    "root", [
        "0",
        /("1", [
            /("1-0", [
                "1-0-0"
            ]),
            "1-1",
        ]),
    ]
};
let root2 = tree2.root();

assert_eq!(root1, root2);
assert_eq!(tree1, tree2);

To create a new orphan (root) node, use Node::new_tree or HotNode::new_tree.

use dendron::Node;

let root = Node::new_tree("root");

For other operations (creating a node that is connected to other nodes, moving nodes to other places, iterating nodes, getting and setting data, etc.), see methods of node types (Node, FrozenNode, and HotNode).