Refactor MFM

Co-authored-by: syuilo syuilotan@yahoo.co.jp

src/prelude/tree.ts

@@ -0,0 +1,36 @@
+import { concat, sum } from './array';
+
+export type Tree<T> = {
+	node: T,
+	children: Forest<T>;
+};
+
+export type Forest<T> = Tree<T>[];
+
+export function createLeaf<T>(node: T): Tree<T> {
+	return { node, children: [] };
+}
+
+export function createTree<T>(node: T, children: Forest<T>): Tree<T> {
+	return { node, children };
+}
+
+export function hasChildren<T>(t: Tree<T>): boolean {
+	return t.children.length !== 0;
+}
+
+export function preorder<T>(t: Tree<T>): T[] {
+	return [t.node, ...preorderF(t.children)];
+}
+
+export function preorderF<T>(ts: Forest<T>): T[] {
+	return concat(ts.map(preorder));
+}
+
+export function countNodes<T>(t: Tree<T>): number {
+	return preorder(t).length;
+}
+
+export function countNodesF<T>(ts: Forest<T>): number {
+	return sum(ts.map(countNodes));
+}