/*!
 * @license
 * Copyright Google LLC All Rights Reserved.
 *
 * Use of this source code is governed by an MIT-style license that can be
 * found in the LICENSE file at https://angular.dev/license
 */

import {DOCUMENT} from '@angular/common';
import {inject, signal, DestroyRef, Service} from '@angular/core';

import {TableOfContentsItem, TableOfContentsLevel} from '../interfaces/index';

/**
 * Name of an attribute that is set on an element that should be
 * excluded from the `TableOfContentsLoader` lookup. This is needed
 * to exempt SSR'ed content of the `TableOfContents` component from
 * being inspected and accidentally pulling more content into ToC.
 */
export const TOC_SKIP_CONTENT_MARKER = 'toc-skip-content';

@Service()
export class TableOfContentsLoader {
  // There are some cases when default browser anchor scrolls a little above the
  // heading In that cases wrong item was selected. The value found by trial and
  // error.
  readonly toleranceThreshold = 40;

  readonly tableOfContentItems = signal([] as TableOfContentsItem[]);

  private readonly document = inject(DOCUMENT);

  buildTableOfContent(docElement: Element): void {
    const headings = this.getHeadings(docElement);

    const tocList: TableOfContentsItem[] = headings.map((heading) => ({
      id: heading.id,
      level: heading.tagName.toLowerCase() as TableOfContentsLevel,
      title: this.getHeadingTitle(heading),
    }));

    this.tableOfContentItems.set(tocList);
  }

  private getHeadingTitle(heading: HTMLHeadingElement): string {
    const div: HTMLDivElement = this.document.createElement('div');

    div.innerHTML = heading.innerHTML;
    // Remove any existing copy link buttons to avoid including their text in the title
    div.querySelectorAll('docs-copy-link-button').forEach((el) => el.remove());

    return (div.textContent || '').trim();
  }

  // Get all headings (h2 and h3) with ids, which are not children of the
  // docs-example-viewer component.
  private getHeadings(element: Element): HTMLHeadingElement[] {
    return Array.from(
      element.querySelectorAll<HTMLHeadingElement>(
        `h2[id]:not(docs-example-viewer h2):not([${TOC_SKIP_CONTENT_MARKER}]),` +
          `h3[id]:not(docs-example-viewer h3):not([${TOC_SKIP_CONTENT_MARKER}])`,
      ),
    );
  }

  /**
   * The methods setups several IntersectionObservers to determine when a heading is at the top of
   * the viewport. Using an IntersectionObserver is more efficient than reading DOM position
   * as it won't trigger any reflow.
   */
  setupIntersectionObserver(
    element: Element,
    destroyRef: DestroyRef,
    onActiveId: (id: string) => void,
  ): void {
    // If we're at the top the we need the default active id to be the first heading.
    const headings = this.getHeadings(element);
    onActiveId(headings[0].id);

    headings.forEach((heading) => {
      const ioConfiguration = {
        /**
         * This rootMargin creates a horizontal line at 5% from the top of the viewport
         * that will help trigger an intersection at that the very point.
         */
        rootMargin: '0% 0% -95% 0%',
        /** 0 is the default  */
        threshold: 0,
      };
      const observer = new IntersectionObserver((entries, o) => {
        if (entries[0].isIntersecting) {
          onActiveId(entries[0].target.id);
        }
      }, ioConfiguration);
      observer.observe(heading);
      destroyRef.onDestroy(() => observer.disconnect());
    });
  }
}