example/test: add a binary search tree example and test (#8145)

examples/binary_search_tree.v

@@ -0,0 +1,106 @@
+// 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 nonexist 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('')
+}