v/examples/binary_search_tree.v

// Binary Search Tree example by @SleepyRoy

// TODO: make Node.value generic once it's robust enough
// TODO: `return match` instead of returns everywhere inside match 

struct Empty {}

struct Node {
	value f64
	left  Tree
	right Tree
}

type Tree = Empty | Node

// return size(number of nodes) of BST
fn size(tree Tree) int {
	return match tree {
		// TODO: remove int() once match gets smarter
		Empty { int(0) }
		Node { 1 + size(tree.left) + size(tree.right) }
	}
}

// insert a value to BST
fn insert(tree Tree, x f64) Tree {
	match tree {
		Empty { return Node{x, tree, tree} }
		Node { 
			return if x == tree.value {
				tree
			} else if x < tree.value {
				Node{...tree, left: insert(tree.left, x)}
			} else {
				Node{...tree, right: insert(tree.right, x)}
			} 
		}
	}
}

// whether able to find a value in BST
fn search(tree Tree, x f64) bool {
	match tree {
		Empty { return false }
		Node { 
			return if x == tree.value {
				true
			} else if x < tree.value {
				search(tree.left, x)
			} else {
				search(tree.right, x)
			} 
		}
	}
}

// find the minimal value of a BST
fn min(tree Tree) f64 {
	match tree {
		Empty { return 1e100 }
		Node { return if tree.value < min(tree.left) { tree.value } else { min(tree.left) } }
	}
}

// delete a value in BST (if nonexistant do nothing)
fn delete(tree Tree, x f64) Tree {
	match tree {
		Empty { return tree }
		Node {
			if tree.left is Node && tree.right is Node {
				return if x < tree.value { 
					Node{...tree, left: delete(tree.left, x)}
				} else if x > tree.value {
					Node{...tree, right: delete(tree.right, x)}
				} else {
					Node{...tree, value: min(tree.right), right: delete(tree.right, min(tree.right))}
				}	
			} else if tree.left is Node {
				return if x == tree.value { tree.left } else { Node{...tree, left: delete(tree.left, x)} } 
			} else {
				if x == tree.value { return tree.right } else { return Node{...tree, right: delete(tree.right, x)} }  
			}
		}
	}
}

fn main() {
	mut tree := Tree(Empty{})
	input := [0.3, 0.2, 0.5, 0.0, 0.6, 0.8, 0.9, 1.0, 0.1, 0.4, 0.7]
	for i in input {
		tree = insert(tree, i)
	}
	println('[1] after insertion tree size is ${size(tree)}')  // 11
	del := [-0.3, 0.0, 0.3, 0.6, 1.0, 1.5]
	for i in del {
		tree = delete(tree, i)
	}
	print('[2] after deletion tree size is ${size(tree)}, ')  // 7
	print('and these elements were deleted: ')  // 0.0 0.3 0.6 1.0
	for i in input {
		if !search(tree, i) {
			print('$i ')
		}
	}
	println('')
}