/**
 * Renders the Markdown that `TextEditor` writes, wherever a dump's note, a
 * comment, a chat message or a description is shown.
 *
 * @module
 */

import { memo } from "react";
import { Link } from "react-router";
import ReactMarkdown from "react-markdown";
import remarkGfm from "remark-gfm";
import { useAuth } from "../../hooks/auth/useAuth.ts";
import { userPath } from "../../../shared/paths.ts";
import { EVERYONE } from "../../../shared/mention.ts";
import { HANDLE_SOURCE } from "../../../shared/validation.ts";
import { isAttachmentUrl } from "../../../shared/attachment.ts";

/** Passed to {@link Markdown} by any component that shows a user's text. */
export interface MarkdownProps {
  children: string;
  className?: string;
  /**
   * Drops the images, for a line-clamped teaser in a card, where an image
   * would break the layout. The full text is one click away.
   */
  inline?: boolean;
  /**
   * Shows `@everyone` as a mention, from a chat message's
   * `mentionsEveryone`. Without it, `@everyone` is plain text: anyone may type
   * it, and it reaches no one.
   */
  everyone?: boolean;
}

/**
 * The rule of shared/mention.ts, which the server notifies by, applied to
 * parsed text: link syntax is already parsed away here, so only a preceding
 * word character (an email address) rules a mention out.
 */
const MENTION_RE = new RegExp(`(?<!\\w)@(${HANDLE_SOURCE})`, "g");

/** The part of a Markdown syntax tree node that the plugins below read. */
interface MdNode {
  type: string;
  value?: string;
  url?: string;
  children?: MdNode[];
  /** Read by mdast-util-to-hast: the element and attributes to render. */
  data?: { hName: string; hProperties: Record<string, string> };
}

/**
 * Returns `children` with each `@name@host` that GFM took for an email address
 * (text ending in `@`, then a `mailto:` link of `name@host`) back in one text
 * node, which {@link remarkMentions} then reads as a remote user's handle.
 */
function unlinkHandles(children: MdNode[]): MdNode[] {
  const out: MdNode[] = [];
  for (const child of children) {
    const prev = out.at(-1);
    const only = child.children?.length === 1 ? child.children[0] : undefined;
    const address = child.type === "link" && only?.type === "text"
      ? only.value
      : undefined;
    if (
      prev?.type === "text" &&
      ((address && child.url === `mailto:${address}` &&
        /(?<!\w)@$/.test(prev.value ?? "")) ||
        child.type === "text")
    ) {
      prev.value = `${prev.value}${address ?? child.value}`;
      continue;
    }
    out.push(child);
  }
  return out;
}

/**
 * Turns a bare `@username`, or a remote user's `@name@host`, into a link to
 * that user's profile. It works on the parsed tree rather than the source, so
 * code keeps its `@`s and a URL that contains one
 * (`https://mastodon.social/@bob`) isn't split in two.
 * `@everyone` names no profile: it is left as text, or with `everyone` set,
 * becomes a `span.md-everyone`.
 */
function remarkMentions({ everyone = false }: { everyone?: boolean } = {}) {
  const walk = (node: MdNode) => {
    if (!node.children) return;
    // Link text is already a link, whatever it says.
    if (node.type === "link" || node.type === "linkReference") return;
    node.children = unlinkHandles(node.children).flatMap((child) => {
      if (child.type !== "text" || !child.value?.includes("@")) {
        walk(child);
        return [child];
      }
      const out: MdNode[] = [];
      let last = 0;
      for (const m of child.value.matchAll(MENTION_RE)) {
        const isEveryone = m[1].toLowerCase() === EVERYONE;
        if (isEveryone && !everyone) continue;
        if (m.index > last) {
          out.push({ type: "text", value: child.value.slice(last, m.index) });
        }
        out.push(
          isEveryone
            ? {
              type: "emphasis",
              data: {
                hName: "span",
                hProperties: { className: "md-everyone" },
              },
              children: [{ type: "text", value: m[0] }],
            }
            : {
              type: "link",
              url: userPath(m[1]),
              children: [{ type: "text", value: m[0] }],
            },
        );
        last = m.index + m[0].length;
      }
      if (last === 0) return [child];
      if (last < child.value.length) {
        out.push({ type: "text", value: child.value.slice(last) });
      }
      return out;
    });
  };
  return (tree: MdNode) => walk(tree);
}

/**
 * Turns a single newline into a line break, as chat apps do. Markdown folds it
 * into a space, so a message typed on three lines with Shift+Enter would show
 * as one. Code keeps its newlines, since it has no text nodes.
 */
function remarkBreaks() {
  const walk = (node: MdNode) => {
    if (!node.children) return;
    node.children = node.children.flatMap((child) => {
      if (child.type !== "text" || !child.value?.includes("\n")) {
        walk(child);
        return [child];
      }
      return child.value.split("\n").flatMap((part, i) =>
        i === 0
          ? [{ type: "text", value: part }]
          : [{ type: "break" }, { type: "text", value: part }]
      );
    });
  };
  return (tree: MdNode) => walk(tree);
}

const REMARK_PLUGINS = [remarkGfm, remarkMentions, remarkBreaks];

/** {@link REMARK_PLUGINS} for a text whose `@everyone` is a mention. */
const REMARK_PLUGINS_EVERYONE = [
  remarkGfm,
  [remarkMentions, { everyone: true }],
  remarkBreaks,
] as React.ComponentProps<typeof ReactMarkdown>["remarkPlugins"];

/**
 * Returns the path of a link into this app, relative or absolute to this
 * origin, or null for anywhere else. The API's files and attachments count as
 * elsewhere, since they aren't routes of the app.
 */
function internalPath(href: string | undefined): string | null {
  if (!href) return null;
  if (href.startsWith("/") && !href.startsWith("//")) {
    return href.startsWith("/api/") ? null : href;
  }
  try {
    const url = new URL(href);
    if (url.origin !== globalThis.location.origin) return null;
    if (url.pathname.startsWith("/api/")) return null;
    return url.pathname + url.search + url.hash;
  } catch {
    return null;
  }
}

type Components = React.ComponentProps<typeof ReactMarkdown>["components"];

/**
 * Renders an image of the text. An attachment gets the viewer's media token,
 * which the server asks for when the text linking it is private; any other
 * image is left as written.
 */
function MarkdownImage(
  { src, alt }: { src?: string | Blob; alt?: string },
): React.JSX.Element {
  const { mediaToken } = useAuth();
  let url = typeof src === "string" ? src : undefined;
  if (url && mediaToken && isAttachmentUrl(url)) {
    url = `${url}?token=${encodeURIComponent(mediaToken)}`;
  }
  return <img src={url} alt={alt ?? ""} loading="lazy" decoding="async" />;
}

/**
 * Overrides react-markdown's elements. It is built once, since a new object on
 * each render would re-render every element.
 */
const MARKDOWN_COMPONENTS: Components = {
  // Mentions and links to the app navigate in place, without a reload. The
  // rest open in a new tab.
  a: ({ href, children: linkChildren }) => {
    const path = internalPath(href);
    if (path) return <Link to={path}>{linkChildren}</Link>;
    return (
      <a href={href} target="_blank" rel="noopener noreferrer">
        {linkChildren}
      </a>
    );
  },
  img: ({ src, alt }) => <MarkdownImage src={src} alt={alt} />,
  // A wide GFM table scrolls inside its own box instead of the page.
  table: ({ children: tableChildren }) => (
    <div className="md-table-wrap">
      <table>{tableChildren}</table>
    </div>
  ),
};

/** Overrides react-markdown's elements for an `inline` rendering. */
const INLINE_MARKDOWN_COMPONENTS: Components = {
  ...MARKDOWN_COMPONENTS,
  img: () => null,
};

/**
 * Renders GFM with `@mentions` as profile links and single newlines kept. It
 * is memoized because parsing runs the whole remark pipeline, and the text
 * rarely changes when its parent re-renders.
 */
export const Markdown: React.MemoExoticComponent<
  (props: MarkdownProps) => React.JSX.Element
> = memo(function Markdown(
  { children, className, inline = false, everyone = false }: MarkdownProps,
) {
  return (
    <div
      className={`md${className ? ` ${className}` : ""}${
        inline ? " md--inline" : ""
      }`}
    >
      <ReactMarkdown
        remarkPlugins={everyone ? REMARK_PLUGINS_EVERYONE : REMARK_PLUGINS}
        components={inline ? INLINE_MARKDOWN_COMPONENTS : MARKDOWN_COMPONENTS}
      >
        {children}
      </ReactMarkdown>
    </div>
  );
});