/**
 * Checks the images users upload and serves them back, scaled down to the
 * width a request names. The avatar, playlist cover, dump thumbnail and
 * attachment routes call it.
 *
 * @module
 */

import type { Context } from "@oak/oak";
import { APIException } from "./api-exception.ts";
import { serveFile } from "./serve-file.ts";
import { dirname } from "node:path";
import { MAX_IMAGE_SIZE_BYTES } from "../config.ts";
import { type ImageWidth, parseImageWidth } from "../../shared/image.ts";
import {
  ensureGenerated,
  type ScaledFormat,
  scaledImagePath,
  scaleImage,
} from "./ffmpeg.ts";

/**
 * Returns the MIME type of a JPEG, PNG, GIF or WebP image from its magic
 * bytes, or null for anything else. These four are the only image types
 * accepted anywhere. The type the browser declared is never
 * trusted, since the file is later served with the type found here.
 */
export function detectImageMime(data: Uint8Array): string | null {
  if (data[0] === 0xFF && data[1] === 0xD8 && data[2] === 0xFF) {
    return "image/jpeg";
  }
  if (
    data[0] === 0x89 && data[1] === 0x50 && data[2] === 0x4E && data[3] === 0x47
  ) return "image/png";
  if (
    data[0] === 0x47 && data[1] === 0x49 && data[2] === 0x46 && data[3] === 0x38
  ) return "image/gif";
  // RIFF....WEBP
  if (
    data[0] === 0x52 && data[1] === 0x49 && data[2] === 0x46 &&
    data[3] === 0x46 &&
    data[8] === 0x57 && data[9] === 0x45 && data[10] === 0x42 &&
    data[11] === 0x50
  ) return "image/webp";
  return null;
}

/**
 * Checks an uploaded image's size and content, and returns its MIME type,
 * which the route stores to serve the image with.
 *
 * @throws {APIException} 400 when it exceeds {@link MAX_IMAGE_SIZE_BYTES} or
 * isn't a JPEG, PNG, GIF or WebP image.
 */
export function validateImageUpload(data: Uint8Array): string {
  if (data.length > MAX_IMAGE_SIZE_BYTES) {
    throw new APIException(
      400,
      `File too large (max ${MAX_IMAGE_SIZE_BYTES / (1024 * 1024)} MB)`,
    );
  }
  const mime = detectImageMime(data);
  if (!mime) {
    throw new APIException(
      400,
      "File content is not a recognised image (JPEG, PNG, GIF, WebP)",
    );
  }
  return mime;
}

/**
 * Returns the EXIF orientation of a JPEG, 1 to 8, from the start of its
 * bytes, and 1, upright, when it states none. A phone stores a photo as its
 * sensor saw it and says here how to turn it.
 */
function jpegOrientation(data: Uint8Array): number {
  const view = new DataView(data.buffer, data.byteOffset, data.byteLength);
  // The segments follow the two-byte SOI, each a marker and its length.
  let at = 2;
  while (at + 4 <= data.length && data[at] === 0xFF) {
    const marker = data[at + 1];
    const length = view.getUint16(at + 2);
    // APP1 holding "Exif\0\0", then a TIFF header and its first directory.
    if (marker === 0xE1 && view.getUint32(at + 4) === 0x45786966) {
      const tiff = at + 10;
      if (tiff + 8 > data.length) return 1;
      const little = view.getUint16(tiff) === 0x4949;
      const ifd = tiff + view.getUint32(tiff + 4, little);
      if (ifd + 2 > data.length) return 1;
      const entries = view.getUint16(ifd, little);
      for (let i = 0; i < entries; i++) {
        const entry = ifd + 2 + i * 12;
        if (entry + 12 > data.length) return 1;
        if (view.getUint16(entry, little) === 0x0112) {
          const value = view.getUint16(entry + 8, little);
          return value >= 1 && value <= 8 ? value : 1;
        }
      }
      return 1;
    }
    // The image data starts: no EXIF came before it.
    if (marker === 0xDA) return 1;
    at += 2 + length;
  }
  return 1;
}

/**
 * How much of a file {@link scalableImage} needs: the EXIF segment, which
 * comes first in a JPEG, is 64 KB at most.
 */
const IMAGE_HEAD_BYTES = 64 * 1024 + 32;

/** Returned by {@link scalableImage}, for `scaleImage`. */
export interface ScalableImage {
  /** The EXIF orientation to turn the image by, 1 when it is upright. */
  orientation: number;
  /**
   * The format of its copy for a reader without WebP: a JPEG, or a PNG for
   * an image that may be transparent, which a JPEG would fill with black.
   */
  compat: "jpg" | "png";
}

/**
 * Returns how to scale the image whose first {@link IMAGE_HEAD_BYTES} are
 * `head`, or null when it is to be served as it is: an animated WebP, which
 * ffmpeg can't read, or anything but a JPEG, PNG, GIF, WebP or BMP.
 */
export function scalableImage(head: Uint8Array): ScalableImage | null {
  const mime = detectImageMime(head);
  // A file dump may be a BMP ("BM"), which no upload route here accepts.
  if (!mime) {
    return head[0] === 0x42 && head[1] === 0x4D
      ? { orientation: 1, compat: "jpg" }
      : null;
  }
  // "RIFF....WEBPVP8X", whose flags then say whether frames follow.
  const animated = mime === "image/webp" && head[15] === 0x58 &&
    (head[20] & 0x02) !== 0;
  if (animated) return null;
  return mime === "image/jpeg"
    ? { orientation: jpegOrientation(head), compat: "jpg" }
    : { orientation: 1, compat: "png" };
}

/** Reads the first {@link IMAGE_HEAD_BYTES} of the file at `path`. */
async function readHead(path: string): Promise<Uint8Array | null> {
  let file: Deno.FsFile;
  try {
    file = await Deno.open(path, { read: true });
  } catch {
    return null;
  }
  try {
    const head = new Uint8Array(IMAGE_HEAD_BYTES);
    let filled = 0;
    while (filled < head.length) {
      const n = await file.read(head.subarray(filled));
      if (n === null) break;
      filled += n;
    }
    return head.subarray(0, filled);
  } finally {
    file.close();
  }
}

/** The type each format of a scaled copy is served as. */
const SCALED_MIMES: Record<ScaledFormat, string> = {
  webp: "image/webp",
  jpg: "image/jpeg",
  png: "image/png",
};

/**
 * Returns the copy of the image at `path` scaled to `width`, made now unless
 * it was already, as its path and type, or null when there is none to serve:
 * the image can't be scaled ({@link scalableImage}), or ffmpeg is missing or
 * failed on it. The copy is a WebP, or with `compat` the JPEG or PNG a reader
 * without WebP takes.
 */
async function scaledCopy(
  path: string,
  width: ImageWidth,
  compat: boolean,
): Promise<{ path: string; mime: string } | null> {
  // The other format depends on the image, so it is read before the copy is
  // looked for; a WebP's path is known without.
  let how = compat ? await describe(path) : undefined;
  if (how === null) return null;
  const format: ScaledFormat = how ? how.compat : "webp";
  const scaledPath = scaledImagePath(path, width, format);
  const made = await ensureGenerated(scaledPath, async () => {
    how ??= await describe(path);
    if (!how) throw new Error("Not an image that can be scaled");
    await scaleImage(path, scaledPath, width, how.orientation, format);
  }, path);
  return made ? { path: scaledPath, mime: SCALED_MIMES[format] } : null;
}

/** Returns how to scale the image at `path`: see {@link scalableImage}. */
async function describe(path: string): Promise<ScalableImage | null> {
  const head = await readHead(path);
  return head && scalableImage(head);
}

/**
 * Sends the image at `path`, of type `mime`, with `cacheControl`: a WebP copy
 * at most as wide as the request's `?w=` names (`shared/image.ts`), or as
 * `defaultWidth` without one, and the image as it is when the request names
 * no width or the copy can't be made. With `?f=compat`, the copy is a JPEG,
 * or a PNG for an image that may be transparent, which the digest's emails
 * ask for, since not every mail client reads WebP. Every route that answers
 * one of the site's own images ends here, so each takes both. It resolves
 * false, having sent nothing, when the file is missing.
 */
export async function serveImage(
  ctx: Context,
  path: string,
  mime: string,
  cacheControl: string,
  defaultWidth?: ImageWidth,
): Promise<boolean> {
  // The browser must take the checked type, never guess another from the
  // bytes.
  ctx.response.headers.set("X-Content-Type-Options", "nosniff");
  const width = parseImageWidth(ctx.request.url.searchParams.get("w")) ??
    defaultWidth;
  const compat = ctx.request.url.searchParams.get("f") === "compat";
  const scaled = width ? await scaledCopy(path, width, compat) : null;
  if (
    scaled &&
    await serveFile(ctx, scaled.path, { mime: scaled.mime, cacheControl })
  ) return true;
  return await serveFile(ctx, path, { mime, cacheControl });
}

/**
 * Serves an uploaded image inline as `mime`, or scaled down as
 * {@link serveImage} does, cached for `cacheMaxAge` seconds, and only by the
 * browser when `isPrivate`. It answers 404 and resolves false when the file
 * is missing. The caller checks first that the viewer may see it.
 */
export async function serveUploadedFile(
  ctx: Context,
  filePath: string,
  mime: string,
  cacheMaxAge = 3600,
  isPrivate = false,
  defaultWidth?: ImageWidth,
): Promise<boolean> {
  ctx.response.headers.set("Content-Disposition", "inline");
  // A frontend on another origin may still show the image.
  ctx.response.headers.set("Cross-Origin-Resource-Policy", "cross-origin");
  const served = await serveImage(
    ctx,
    filePath,
    mime,
    `${isPrivate ? "private" : "public"}, max-age=${cacheMaxAge}`,
    defaultWidth,
  );
  if (!served) ctx.response.status = 404;
  return served;
}

/**
 * Writes `data` to `filePath`, replacing whatever is there only once the whole
 * file is written, so a failed write leaves the old file whole. The upload
 * routes call it before recording the new file, so that clients told of it
 * never fetch it early.
 */
export async function replaceUploadedFile(
  filePath: string,
  data: Uint8Array,
): Promise<void> {
  const tmpPath = `${filePath}.${crypto.randomUUID()}.tmp`;
  await Deno.mkdir(dirname(filePath), { recursive: true });
  try {
    await Deno.writeFile(tmpPath, data);
    await Deno.rename(tmpPath, filePath);
  } catch (err) {
    await Deno.remove(tmpPath).catch(() => {});
    throw err;
  }
}