refactor(components, hooks): rewrite ToC and useHeadingsTree

* replace TableOfContents component with TocWidget to keep the name of
widget components coherent
* replace `wrapper` prop with `tree` prop (the component no longer uses
the hook, it is up to the consumer to provide the headings tree)
* let consumer handle the widget title
* add options to useHeadingsTree hook to retrieve only the wanted
headings (and do not assume that h1 is unwanted)
* expect an ref object instead of an element in useHeadingsTree hook
* rename most of the types involved

4 files changed, 228 insertions, 162 deletions

diff --git a/src/utils/hooks/use-headings-tree.tsx b/src/utils/hooks/use-headings-tree.tsx
deleted file mode 100644
index 049ab29..0000000
--- a/src/utils/hooks/use-headings-tree.tsx
+++ /dev/null
@@ -1,162 +0,0 @@
-import { useCallback, useEffect, useMemo, useState } from 'react';
-import { slugify } from '../helpers';
-import { useMutationObserver } from './use-mutation-observer';
-
-export type Heading = {
-  /**
-   * The heading depth.
-   */
-  depth: number;
-  /**
-   * The heading id.
-   */
-  id: string;
-  /**
-   * The heading children.
-   */
-  children: Heading[];
-  /**
-   * The heading title.
-   */
-  title: string;
-};
-
-/**
- * Get the headings tree of the given HTML element.
- *
- * @param {HTMLElement} wrapper - An HTML element that contains the headings.
- * @returns {Heading[]} The headings tree.
- */
-export const useHeadingsTree = (wrapper: HTMLElement): Heading[] => {
-  const depths = useMemo(() => ['h2', 'h3', 'h4', 'h5', 'h6'], []);
-  const [allHeadings, setAllHeadings] =
-    useState<NodeListOf<HTMLHeadingElement>>();
-  const [headingsTree, setHeadingsTree] = useState<Heading[]>([]);
-
-  const getHeadingsInWrapper = useCallback(() => {
-    const query = depths.join(', ');
-    const result: NodeListOf<HTMLHeadingElement> =
-      wrapper.querySelectorAll(query);
-    setAllHeadings(result);
-  }, [depths, wrapper]);
-
-  useEffect(() => {
-    getHeadingsInWrapper();
-  }, [getHeadingsInWrapper]);
-
-  useMutationObserver({
-    callback: getHeadingsInWrapper,
-    options: { childList: true },
-    nodeOrSelector: wrapper,
-  });
-
-  const getDepth = useCallback(
-    /**
-     * Retrieve the heading element depth.
-     *
-     * @param {HTMLHeadingElement} el - An heading element.
-     * @returns {number} The heading depth.
-     */
-    (el: HTMLHeadingElement): number => {
-      return depths.findIndex((depth) => depth === el.localName);
-    },
-    [depths]
-  );
-
-  const formatHeadings = useCallback(
-    /**
-     * Convert a list of headings into an array of Heading objects.
-     *
-     * @param {NodeListOf<HTMLHeadingElement>} headings - A list of headings.
-     * @returns {Heading[]} An array of Heading objects.
-     */
-    (headings: NodeListOf<HTMLHeadingElement>): Heading[] => {
-      const formattedHeadings: Heading[] = [];
-
-      Array.from(headings).forEach((heading) => {
-        const title: string = heading.textContent!;
-        const id = slugify(title);
-        const depth = getDepth(heading);
-        const children: Heading[] = [];
-
-        heading.id = id;
-
-        formattedHeadings.push({
-          depth,
-          id,
-          children,
-          title,
-        });
-      });
-
-      return formattedHeadings;
-    },
-    [getDepth]
-  );
-
-  const buildSubTree = useCallback(
-    /**
-     * Build the heading subtree.
-     *
-     * @param {Heading} parent - The heading parent.
-     * @param {Heading} currentHeading - The current heading element.
-     */
-    (parent: Heading, currentHeading: Heading): void => {
-      if (parent.depth === currentHeading.depth - 1) {
-        parent.children.push(currentHeading);
-      } else {
-        const lastItem = parent.children[parent.children.length - 1];
-        buildSubTree(lastItem, currentHeading);
-      }
-    },
-    []
-  );
-
-  const buildTree = useCallback(
-    /**
-     * Build a heading tree.
-     *
-     * @param {Heading[]} headings - An array of Heading objects.
-     * @returns {Heading[]} The headings tree.
-     */
-    (headings: Heading[]): Heading[] => {
-      const tree: Heading[] = [];
-
-      headings.forEach((heading) => {
-        if (heading.depth === 0) {
-          tree.push(heading);
-        } else {
-          const lastItem = tree[tree.length - 1];
-          buildSubTree(lastItem, heading);
-        }
-      });
-
-      return tree;
-    },
-    [buildSubTree]
-  );
-
-  const getHeadingsTree = useCallback(
-    /**
-     * Retrieve a headings tree from a list of headings element.
-     *
-     * @param {NodeListOf<HTMLHeadingElement>} headings - A headings list.
-     * @returns {Heading[]} The headings tree.
-     */
-    (headings: NodeListOf<HTMLHeadingElement>): Heading[] => {
-      const formattedHeadings = formatHeadings(headings);
-
-      return buildTree(formattedHeadings);
-    },
-    [formatHeadings, buildTree]
-  );
-
-  useEffect(() => {
-    if (allHeadings) {
-      const headingsList = getHeadingsTree(allHeadings);
-      setHeadingsTree(headingsList);
-    }
-  }, [allHeadings, getHeadingsTree]);
-
-  return headingsTree;
-};
diff --git a/src/utils/hooks/use-headings-tree/index.ts b/src/utils/hooks/use-headings-tree/index.ts
new file mode 100644
index 0000000..8f4c115
--- /dev/null
+++ b/src/utils/hooks/use-headings-tree/index.ts
@@ -0,0 +1 @@
+export * from './use-headings-tree';
diff --git a/src/utils/hooks/use-headings-tree/use-headings-tree.test.ts b/src/utils/hooks/use-headings-tree/use-headings-tree.test.ts
new file mode 100644
index 0000000..ad30a4f
--- /dev/null
+++ b/src/utils/hooks/use-headings-tree/use-headings-tree.test.ts
@@ -0,0 +1,79 @@
+import { describe, expect, it } from '@jest/globals';
+import { renderHook } from '@testing-library/react';
+import { useHeadingsTree } from './use-headings-tree';
+
+const labels = {
+  h1: 'Title 1',
+  firstH2: 'First subtitle',
+  secondH2: 'Second subtitle',
+};
+
+describe('useHeadingsTree', () => {
+  it('returns a ref object and the headings tree', () => {
+    const wrapper = document.createElement('div');
+
+    wrapper.innerHTML = `
+<h1>${labels.h1}</h1>
+<h2>${labels.firstH2}</h2>
+<p>Expedita et necessitatibus qui numquam sunt et ut et. Earum nostrum esse nemo nisi qui. Ab in iure qui repellat voluptatibus nostrum odit aut qui. Architecto eum fugit quod excepturi numquam qui maxime accusantium. Fugit ipsam harum tempora.</p>
+<h2>${labels.secondH2}</h2>
+<p>Totam cumque aut ipsum. Necessitatibus magnam necessitatibus. Qui illo nulla non ab. Accusamus voluptatem ab fugiat voluptas aspernatur velit dolore reprehenderit. Voluptatem quod minima asperiores voluptatum distinctio cumque quo.</p>`;
+
+    const wrapperRef = { current: wrapper };
+    const { result } = renderHook(() => useHeadingsTree(wrapperRef));
+
+    expect(result.current.length).toBe(1);
+    expect(result.current[0].label).toBe(labels.h1);
+    expect(result.current[0].children.length).toBe(2);
+  });
+
+  it('can return a headings tree starting at the specified level', () => {
+    const wrapper = document.createElement('div');
+
+    wrapper.innerHTML = `
+<h1>${labels.h1}</h1>
+<h2>${labels.firstH2}</h2>
+<p>Expedita et necessitatibus qui numquam sunt et ut et. Earum nostrum esse nemo nisi qui. Ab in iure qui repellat voluptatibus nostrum odit aut qui. Architecto eum fugit quod excepturi numquam qui maxime accusantium. Fugit ipsam harum tempora.</p>
+<h2>${labels.secondH2}</h2>
+<p>Totam cumque aut ipsum. Necessitatibus magnam necessitatibus. Qui illo nulla non ab. Accusamus voluptatem ab fugiat voluptas aspernatur velit dolore reprehenderit. Voluptatem quod minima asperiores voluptatum distinctio cumque quo.</p>`;
+
+    const wrapperRef = { current: wrapper };
+    const { result } = renderHook(() =>
+      useHeadingsTree(wrapperRef, { fromLevel: 2 })
+    );
+
+    expect(result.current.length).toBe(2);
+    expect(result.current[0].label).toBe(labels.firstH2);
+    expect(result.current[1].label).toBe(labels.secondH2);
+  });
+
+  it('can return a headings tree stopping at the specified level', () => {
+    const wrapper = document.createElement('div');
+
+    wrapper.innerHTML = `
+<h1>${labels.h1}</h1>
+<h2>${labels.firstH2}</h2>
+<p>Expedita et necessitatibus qui numquam sunt et ut et. Earum nostrum esse nemo nisi qui. Ab in iure qui repellat voluptatibus nostrum odit aut qui. Architecto eum fugit quod excepturi numquam qui maxime accusantium. Fugit ipsam harum tempora.</p>
+<h2>${labels.secondH2}</h2>
+<p>Totam cumque aut ipsum. Necessitatibus magnam necessitatibus. Qui illo nulla non ab. Accusamus voluptatem ab fugiat voluptas aspernatur velit dolore reprehenderit. Voluptatem quod minima asperiores voluptatum distinctio cumque quo.</p>`;
+
+    const wrapperRef = { current: wrapper };
+    const { result } = renderHook(() =>
+      useHeadingsTree(wrapperRef, { toLevel: 1 })
+    );
+
+    expect(result.current.length).toBe(1);
+    expect(result.current[0].label).toBe(labels.h1);
+    expect(result.current[0].children).toStrictEqual([]);
+  });
+
+  it('throws an error if the options are invalid', () => {
+    const wrapperRef = { current: null };
+
+    expect(() =>
+      useHeadingsTree(wrapperRef, { fromLevel: 2, toLevel: 1 })
+    ).toThrowError(
+      'Invalid options: `fromLevel` must be lower or equal to `toLevel`.'
+    );
+  });
+});
diff --git a/src/utils/hooks/use-headings-tree/use-headings-tree.ts b/src/utils/hooks/use-headings-tree/use-headings-tree.ts
new file mode 100644
index 0000000..6a081e7
--- /dev/null
+++ b/src/utils/hooks/use-headings-tree/use-headings-tree.ts
@@ -0,0 +1,148 @@
+import { useEffect, useState, type RefObject } from 'react';
+import type { HeadingLevel } from '../../../components';
+
+export type HeadingsTreeNode = {
+  /**
+   * The heading children.
+   */
+  children: HeadingsTreeNode[];
+  /**
+   * The heading depth.
+   */
+  depth: number;
+  /**
+   * The heading id.
+   */
+  id: string;
+  /**
+   * The heading label.
+   */
+  label: string;
+};
+
+const headingTags = ['h1', 'h2', 'h3', 'h4', 'h5', 'h6'] as const;
+
+type HeadingTagNames = (typeof headingTags)[number];
+
+export type UseHeadingsTreeOptions = {
+  /**
+   * Look for headings starting from this level (1 = `h1`, ...).
+   *
+   * @default undefined
+   */
+  fromLevel?: HeadingLevel;
+  /**
+   * Look for headings ending with this level (1 = `h1`, ...).
+   *
+   * @default undefined
+   */
+  toLevel?: HeadingLevel;
+};
+
+/**
+ * Retrieve a list of heading tags.
+ *
+ * @param {UseHeadingsTreeOptions} options - An options object.
+ * @returns {HeadingTagNames[]} The heading tags list.
+ */
+const getHeadingTagsList = (
+  options?: UseHeadingsTreeOptions
+): HeadingTagNames[] => {
+  const tagsList = headingTags.slice(0);
+
+  if (options?.toLevel) tagsList.length = options.toLevel;
+  if (options?.fromLevel) tagsList.splice(0, options.fromLevel - 1);
+
+  return tagsList;
+};
+
+type HeadingsTreeNodeWithParentIndex = HeadingsTreeNode & {
+  parentIndex: number;
+};
+
+/**
+ * Convert a node list of heading elements to an array of indexed nodes.
+ *
+ * @param {NodeListOf<HTMLHeadingElement>} nodes - The heading elements list.
+ * @returns {HeadingsTreeNodeWithParentIndex[]} The headings nodes.
+ */
+const getHeadingNodesFrom = (
+  nodes: NodeListOf<HTMLHeadingElement>
+): HeadingsTreeNodeWithParentIndex[] => {
+  const depthLastIndexes = Array.from({ length: headingTags.length }, () => -1);
+
+  return Array.from(nodes).map(
+    (node, index): HeadingsTreeNodeWithParentIndex => {
+      const depth = headingTags.findIndex((tag) => tag === node.localName);
+      const parentDepthIndexes = depthLastIndexes.slice(0, depth);
+
+      depthLastIndexes[depth] = index;
+
+      return {
+        children: [],
+        depth,
+        id: node.id,
+        label: node.textContent ?? '',
+        parentIndex: Math.max(...parentDepthIndexes),
+      };
+    }
+  );
+};
+
+/**
+ * Build an headings tree from a list of heading elements.
+ *
+ * @param {NodeListOf<HTMLHeadingElement>} nodes - The heading nodes.
+ * @returns {HeadingsTreeNode[]} The headings tree.
+ */
+const buildHeadingsTreeFrom = (
+  nodes: NodeListOf<HTMLHeadingElement>
+): HeadingsTreeNode[] => {
+  const headings = getHeadingNodesFrom(nodes);
+  const treeNodes: HeadingsTreeNode[] = [];
+
+  for (const heading of headings) {
+    const { parentIndex, ...node } = heading;
+
+    if (parentIndex >= 0) headings[parentIndex].children.push(node);
+    else treeNodes.push(node);
+  }
+
+  return treeNodes;
+};
+
+/**
+ * React hook to retrieve the headings tree in a document or in a given wrapper.
+ *
+ * @param {RefObject<T>} ref - A ref to the element where to look for headings.
+ * @param {UseHeadingsTreeOptions} options - The headings tree config.
+ * @returns {HeadingsTreeNode[]} The headings tree.
+ */
+export const useHeadingsTree = <T extends HTMLElement = HTMLElement>(
+  ref: RefObject<T>,
+  options?: UseHeadingsTreeOptions
+): HeadingsTreeNode[] => {
+  if (
+    options?.fromLevel &&
+    options.toLevel &&
+    options.fromLevel > options.toLevel
+  )
+    throw new Error(
+      'Invalid options: `fromLevel` must be lower or equal to `toLevel`.'
+    );
+
+  const [tree, setTree] = useState<HeadingsTreeNode[]>([]);
+  const requestedHeadingTags = getHeadingTagsList(options);
+  const query = requestedHeadingTags.join(', ');
+
+  useEffect(() => {
+    if (typeof window === 'undefined') return;
+
+    const headingNodes =
+      ref.current?.querySelectorAll<HTMLHeadingElement>(query);
+
+    if (headingNodes) setTree(buildHeadingsTreeFrom(headingNodes));
+  }, [query, ref]);
+
+  return tree;
+};