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Brandon Rozek

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PhD Student @ RPI studying Automated Reasoning in AI and Linux Enthusiast.

Immutable Traversals with Unfold

Published on

2 minute reading time

Let’s consider the following binary tree:

b a c d

We can encode this with the following Scala code:

final case class BinNode(
    val label: String,
    val left: Option[BinNode],
    val right: Option[BinNode]
)

// Leaf Nodes
val c_node = BinNode("c", None, None)
val d_node = BinNode("d", None, None)
// Rest of nodes
val b_node = BinNode("b", None, Some(c_node))
val a_node = BinNode("a", Some(b_node), Some(d_node))

For depth first search, an intuitive immutable implementation would be a recursive function.

// Using Preorder traversal
def DFS(node: BinNode): Iterator[BinNode] = 
    lazy val left_side = node.left.fold(Iterator.empty[BinNode])(DFS)
    lazy val right_side = node.right.fold(Iterator.empty[BinNode])(DFS)
    Iterator(node) ++ left_side ++ right_side

Let’s evaluate this using our example above:

DFS(a_node).toList.map(_.label)
// List(a, b, c, d)

The recursive implementation inherently uses the system stack to keep track of the nodes. This means that the last element gets evaluated in each step. Otherwise called last-in-first-out (LIFO). Breadth first search, however, uses a queue based approach where the first one added to the data structure is the first one considered (FIFO).

To preserve immutability in our code, we can use unfold. Here our state is the queue of nodes.

def BFS(node: BinNode): Iterator[BinNode] =
    Iterator.unfold(List(node))(q =>
        if q.isEmpty then
            None
        else
            val crnt_node = q.head
            val next_q = q.tail ++ crnt_node.left ++ crnt_node.right
            Some(crnt_node, next_q)
    )

Evaluating on our example:

BFS(a_node).toList.map(_.label)
// List(a, b, d, c)

We can also use unfold for the depth first search approach as well. We can replace the list used with a stack.

import scala.collection.mutable.Stack
def DFS2(node: BinNode): Iterator[BinNode] = 
    Iterator.unfold(Stack(node))(s =>
        if s.isEmpty then
            None
        else
            val crnt_node = s.pop()
            s.pushAll(crnt_node.right)
            s.pushAll(crnt_node.left)
            Some(crnt_node, s)
    )

Using a stack introduces some mutability. We can use the immutable list data structure instead, as long as we satisfy the LIFO ordering.

def DFS3(node: BinNode): Iterator[BinNode] = 
    Iterator.unfold(List(node))(s =>
        if s.isEmpty then
            None
        else
            val crnt_node = s.last
            val next_s = s.init ++ crnt_node.right ++ crnt_node.left
            Some(crnt_node, next_s)
    )
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