import type { CheerioAPI } from 'cheerio';
import { resolvePageHtmlUrl } from '../utils/pagePaths.js';

export interface ResourceHintResult {
  readonly added: number;
  readonly candidates: string[];
  readonly missingHead: boolean;
}

export function injectResourceHints(
  document: CheerioAPI,
  currentPage: string,
  pagesUrlPrefix: string,
  useRootIndex: boolean,
): ResourceHintResult {
  const head = document('head').first();
  const pages = [...collectInternalPages(document, currentPage, pagesUrlPrefix)];

  if (head.length === 0) {
    return {
      added: 0,
      candidates: pages,
      missingHead: pages.length > 0,
    };
  }

  if (pages.length === 0) {
    return { added: 0, candidates: [], missingHead: false };
  }

  for (const page of pages) {
    const href = resolvePageHtmlUrl(pagesUrlPrefix, page, useRootIndex);
    head.append(`\n<link rel="prefetch" href="${href}" as="document">`);
  }

  return { added: pages.length, candidates: pages, missingHead: false };
}

function collectInternalPages(
  document: CheerioAPI,
  currentPage: string,
  pagesUrlPrefix: string,
): Set<string> {
  const pages = new Set<string>();
  document('a[href]').each((_index, element) => {
    const href = document(element).attr('href');
    const pageName = normalizePageName(href, pagesUrlPrefix);
    if (!pageName || pageName === currentPage) {
      return;
    }
    pages.add(pageName);
  });
  return pages;
}

function normalizePageName(href: string | undefined, pagesUrlPrefix: string): string | null {
  if (!href || href.length === 0) {
    return null;
  }

  const lower = href.toLowerCase();
  if (
    lower.startsWith('http://') ||
    lower.startsWith('https://') ||
    lower.startsWith('mailto:') ||
    lower.startsWith('#')
  ) {
    return null;
  }

  let path = href.split('#')[0]?.split('?')[0] ?? '';
  if (path.length === 0) {
    return null;
  }

  if (path.startsWith('/')) {
    path = path.slice(1);
  }

  const prefix = trimSlashes(pagesUrlPrefix);
  if (prefix && path.startsWith(`${prefix}/`)) {
    path = path.slice(prefix.length + 1);
  }

  if (path.endsWith('/')) {
    path = path.slice(0, -1);
  }

  const segments = path.split('/');
  const candidate = segments[0];
  if (!candidate) {
    return null;
  }

  return candidate;
}

function trimSlashes(value: string): string {
  return value.replace(/^\/+|\/+$/g, '');
}