mirror of https://github.com/smol-rs/vec-arena
188 lines
4.9 KiB
Rust
188 lines
4.9 KiB
Rust
use vec_arena::Arena;
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/// The null index, akin to null pointers.
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///
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/// Just like a null pointer indicates an address no object is ever stored at,
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/// the null index indicates an index no object is ever stored at.
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///
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/// Number `!0` is the largest possible value representable by `usize`.
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const NULL: usize = !0;
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struct Node<T> {
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/// Parent node.
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parent: usize,
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/// Left and right child.
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children: [usize; 2],
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/// Actual value stored in node.
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value: T,
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}
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impl<T> Node<T> {
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fn new(value: T) -> Node<T> {
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Node {
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parent: NULL,
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children: [NULL, NULL],
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value: value,
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}
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}
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}
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struct Splay<T> {
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/// This is where nodes are stored.
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arena: Arena<Node<T>>,
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/// The root node.
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root: usize,
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}
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impl<T: Ord> Splay<T> {
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/// Constructs a new, empty splay tree.
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fn new() -> Splay<T> {
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Splay {
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arena: Arena::new(),
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root: NULL,
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}
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}
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/// Links nodes `p` and `c` as parent and child with the specified direction.
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#[inline(always)]
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fn link(&mut self, p: usize, c: usize, dir: usize) {
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self.arena[p].children[dir] = c;
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if c != NULL {
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self.arena[c].parent = p;
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}
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}
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/// Performs a rotation on node `c`, whose parent is node `p`.
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#[inline(always)]
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fn rotate(&mut self, p: usize, c: usize) {
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// Variables:
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// - `c` is the child node
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// - `p` is it's parent
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// - `g` is it's grandparent
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// Find the grandparent.
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let g = self.arena[p].parent;
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// The direction of p-c relationship.
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let dir = if self.arena[p].children[0] == c { 0 } else { 1 };
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// This is the child of `c` that needs to be reassigned to `p`.
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let t = self.arena[c].children[dir ^ 1];
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self.link(p, t, dir);
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self.link(c, p, dir ^ 1);
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if g == NULL {
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// There is no grandparent, so `c` becomes the root.
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self.root = c;
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self.arena[c].parent = NULL;
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} else {
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// Link `g` and `c` together.
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let dir = if self.arena[g].children[0] == p { 0 } else { 1 };
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self.link(g, c, dir);
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}
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}
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/// Splays a node, rebalancing the tree in process.
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fn splay(&mut self, c: usize) {
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loop {
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// Variables:
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// - `c` is the current node
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// - `p` is it's parent
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// - `g` is it's grandparent
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// Find the parent.
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let p = self.arena[c].parent;
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if p == NULL {
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// There is no parent. That means `c` is the root.
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break;
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}
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// Find the grandparent.
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let g = self.arena[p].parent;
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if g == NULL {
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// There is no grandparent. Just one more rotation is left.
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// Zig step.
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self.rotate(p, c);
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break;
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}
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if (self.arena[g].children[0] == p) == (self.arena[p].children[0] == c) {
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// Zig-zig step.
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self.rotate(g, p);
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self.rotate(p, c);
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} else {
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// Zig-zag step.
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self.rotate(p, c);
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self.rotate(g, c);
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}
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}
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}
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/// Inserts a new node with specified `value`.
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fn insert(&mut self, value: T) {
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// Variables:
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// - `n` is the new node
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// - `p` will be it's parent
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// - `c` is the present child of `p`
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let n = self.arena.insert(Node::new(value));
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if self.root == NULL {
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self.root = n;
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} else {
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let mut p = self.root;
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loop {
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// Decide whether to go left or right.
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let dir = if self.arena[n].value < self.arena[p].value {
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0
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} else {
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1
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};
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let c = self.arena[p].children[dir];
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if c == NULL {
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self.link(p, n, dir);
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self.splay(n);
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break;
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}
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p = c;
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}
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}
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}
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/// Pretty-prints the subtree rooted at `node`, indented by `indent` spaces.
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fn print(&self, node: usize, indent: usize)
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where
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T: std::fmt::Display,
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{
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if node != NULL {
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// Print the left subtree.
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self.print(self.arena[node].children[0], indent + 3);
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// Print the current node.
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println!("{:width$}{}", "", self.arena[node].value, width = indent);
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// Print the right subtree.
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self.print(self.arena[node].children[1], indent + 3);
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}
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}
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}
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fn main() {
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let mut splay = Splay::new();
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// Insert a bunch of pseudorandom numbers.
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let mut num = 1u32;
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for _ in 0..30 {
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num = num.wrapping_mul(17).wrapping_add(255);
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splay.insert(num);
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}
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// Display the whole splay tree.
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splay.print(splay.root, 0);
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}
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