13.1 BinaryTrie: A digital search tree

A BinaryTrie encodes a set of $\mathtt{w}$ bit integers in a binary tree. All leaves in the tree have depth $\mathtt{w}$ and each integer is encoded as a root-to-leaf path. The path for the integer $\mathtt{x}$ turns left at level $\mathtt{i}$ if the $\mathtt{i}$th most significant bit of $\mathtt{x}$ is a 0 and turns right if it is a 1. Figure 13.1 shows an example for the case $\ensuremath{\mathtt{w}}=4$, in which the trie stores the integers 3(0011), 9(1001), 12(1100), and 13(1101).

Figure 13.1: The integers stored in a binary trie are encoded as root-to-leaf paths.

Because the search path for a value $\mathtt{x}$ depends on the bits of $\mathtt{x}$, it will be helpful to name the children of a node, $\mathtt{u}$, $\mathtt{u.child[0]}$ ( $\mathtt{left}$) and $\mathtt{u.child[1]}$ ( $\mathtt{right}$). These child pointers will actually serve double-duty. Since the leaves in a binary trie have no children, the pointers are used to string the leaves together into a doubly-linked list. For a leaf in the binary trie $\mathtt{u.child[0]}$ ( $\mathtt{prev}$) is the node that comes before $\mathtt{u}$ in the list and $\mathtt{u.child[1]}$ ( $\mathtt{next}$) is the node that follows $\mathtt{u}$ in the list. A special node, $\mathtt{dummy}$, is used both before the first node and after the last node in the list (see Section 3.2).

Each node, $\mathtt{u}$, also contains an additional pointer $\mathtt{u.jump}$. If $\mathtt{u}$'s left child is missing, then $\mathtt{u.jump}$ points to the smallest leaf in $\mathtt{u}$'s subtree. If $\mathtt{u}$'s right child is missing, then $\mathtt{u.jump}$ points to the largest leaf in $\mathtt{u}$'s subtree. An example of a BinaryTrie, showing $\mathtt{jump}$ pointers and the doubly-linked list at the leaves, is shown in Figure 13.2.

Figure 13.2: A BinaryTrie with $\mathtt{jump}$ pointers shown as curved dashed edges.

The $\mathtt{find(x)}$ operation in a BinaryTrie is fairly straightforward. We try to follow the search path for $\mathtt{x}$ in the trie. If we reach a leaf, then we have found $\mathtt{x}$. If we reach a node $\mathtt{u}$ where we cannot proceed (because $\mathtt{u}$ is missing a child), then we follow $\mathtt{u.jump}$, which takes us either to the smallest leaf larger than $\mathtt{x}$ or the largest leaf smaller than $\mathtt{x}$. Which of these two cases occurs depends on whether $\mathtt{u}$ is missing its left or right child, respectively. In the former case ( $\mathtt{u}$ is missing its left child), we have found the node we want. In the latter case ( $\mathtt{u}$ is missing its right child), we can use the linked list to reach the node we want. Each of these cases is illustrated in Figure 13.3.

    T find(T x) {
        int i, c = 0, ix = it.intValue(x);
        Node u = r;
        for (i = 0; i < w; i++) {
            c = (ix >>> w-i-1) & 1;
            if (u.child[c] == null) break;
            u = u.child[c];
        }
        if (i == w) return u.x;  // found it
        u = (c == 0) ? u.jump : u.jump.child[next]; 
        return u == dummy ? null : u.x;
    }

Figure 13.3: The paths followed by $\mathtt{find(5)}$ and $\mathtt{find(8)}$.

The running-time of the $\mathtt{find(x)}$ method is dominated by the time it takes to follow a root-to-leaf path, so it runs in $O(\ensuremath{\mathtt{w}})$ time.

The $\mathtt{add(x)}$ operation in a BinaryTrie is also fairly straightforward, but has a lot of work to do:

1. It follows the search path for $\mathtt{x}$ until reaching a node $\mathtt{u}$ where it can no longer proceed.
2. It creates the remainder of the search path from $\mathtt{u}$ to a leaf that contains $\mathtt{x}$.
3. It adds the node, $\mathtt{u'}$, containing $\mathtt{x}$ to the linked list of leaves (it has access to the predecessor, $\mathtt{pred}$, of $\mathtt{u'}$ in the linked list from the $\mathtt{jump}$ pointer of the last node, $\mathtt{u}$, encountered during step 1.)
4. It walks back up the search path for $\mathtt{x}$ adjusting $\mathtt{jump}$ pointers at the nodes whose $\mathtt{jump}$ pointer should now point to $\mathtt{x}$.

An addition is illustrated in Figure 13.4.

Figure 13.4: Adding the values 2 and 15 to the BinaryTrie in Figure 13.2.

    boolean add(T x) {
        int i, c = 0, ix = it.intValue(x);
        Node u = r;
        // 1 - search for ix until falling out of the trie
        for (i = 0; i < w; i++) {
            c = (ix >>> w-i-1) & 1;
            if (u.child[c] == null) break;
            u = u.child[c];
        }        
        if (i == w) return false; // already contains x - abort
        Node pred = (c == right) ? u.jump : u.jump.child[0];
        u.jump = null;  // u will have two children shortly
        // 2 - add path to ix
        for (; i < w; i++) {
            c = (ix >>> w-i-1) & 1;
            u.child[c] = newNode();
            u.child[c].parent = u;
            u = u.child[c];
        }
        u.x = x;
        // 3 - add u to linked list
        u.child[prev] = pred;
        u.child[next] = pred.child[next];
        u.child[prev].child[next] = u;
        u.child[next].child[prev] = u;
        // 4 - walk back up, updating jump pointers
        Node v = u.parent;
        while (v != null) {
            if ((v.child[left] == null 
                    && (v.jump == null || it.intValue(v.jump.x) > ix))
            || (v.child[right] == null 
                    && (v.jump == null || it.intValue(v.jump.x) < ix)))
                v.jump = u;
            v = v.parent;
        }
        n++;
        return true;
    }

This method performs one walk down the search path for $\mathtt{x}$ and one walk back up. Each step of these walks takes constant time, so the $\mathtt{add(x)}$ method runs in $O(\ensuremath{\mathtt{w}})$ time.

The $\mathtt{remove(x)}$ operation undoes the work of $\mathtt{add(x)}$. Like $\mathtt{add(x)}$, it has a lot of work to do:

1. It follows the search path for $\mathtt{x}$ until reaching the leaf, $\mathtt{u}$, containing $\mathtt{x}$.
2. It removes $\mathtt{u}$ from the doubly-linked list.
3. It deletes $\mathtt{u}$ and then walks back up the search path for $\mathtt{x}$ deleting nodes until reaching a node $\mathtt{v}$ that has a child that is not on the search path for $\mathtt{x}$.
4. It walks upwards from $\mathtt{v}$ to the root updating any $\mathtt{jump}$ pointers that point to $\mathtt{u}$.

A removal is illustrated in Figure 13.5.

Figure 13.5: Removing the value 9 from the BinaryTrie in Figure 13.2.

    boolean remove(T x) {
        // 1 - find leaf, u, containing x
        int i, c, ix = it.intValue(x);
        Node u = r;
        for (i = 0; i < w; i++) {
            c = (ix >>> w-i-1) & 1;
            if (u.child[c] == null) return false;
            u = u.child[c];
        }
        // 2 - remove u from linked list
        u.child[prev].child[next] = u.child[next];
        u.child[next].child[prev] = u.child[prev];
        Node v = u;
        // 3 - delete nodes on path to u
        for (i = w-1; i >= 0; i--) {
            c = (ix >>> w-i-1) & 1;
            v = v.parent;
            v.child[c] = null;
            if (v.child[1-c] != null) break;
        }
        // 4 - update jump pointers
        c = (ix >>> w-i-1) & 1;
        v.jump = u.child[1-c];
        v = v.parent;
        i--;
        for (; i >= 0; i--) {
            c = (ix >>> w-i-1) & 1;
            if (v.jump == u) 
                v.jump = u.child[1-c];
            v = v.parent;
        }
        n--;
        return true;
    }

Theorem 13..1   A BinaryTrie implements the SSet interface for $\mathtt{w}$-bit integers. A BinaryTrie supports the operations $\mathtt{add(x)}$, $\mathtt{remove(x)}$, and $\mathtt{find(x)}$ in $O(\ensuremath{\mathtt{w}})$ time per operation. The space used by a BinaryTrie that stores $\mathtt{n}$ values is $O(\ensuremath{\mathtt{n}}\cdot\ensuremath{\mathtt{w}})$.