// The local catalogue: the folders a visitor has handed over, a thumbnail of
// every frame found in them, and the recipe each frame was last graded with.
// Nothing here leaves the machine — no upload, no server, no account.
//
// The three stores are what the folder is made of: `folders` keeps the directory
// handle so the catalogue survives a reload without a second trip through the
// picker, `photos` keeps a file handle (the RAW itself is read only when a frame
// is opened) plus the small JPEG the grid paints, and `edits` keeps the recipe a
// frame was left at, keyed by the same id the grid uses.
//
// ponytail: Dexie would wrap this in three lines, but IndexedDB stores a
// FileSystemHandle by itself and the whole surface is seven calls — a dependency
// does not pay for itself here. Revisit if the catalogue grows queries (tags,
// smart collections) that hand-rolled cursors would make ugly.
import type { Recipe } from '../../shared/types';
import { readCapturedAt } from './imageOps';
import { isRawName, rawThumbnail } from './rawDevelop';
import { openAt, walkPass, type Walk, type WalkFile } from './rollWalk';

const DB_NAME = 'recipescam-library';
const FOLDERS = 'folders';
const PHOTOS = 'photos';
const EDITS = 'edits';

// The grid cell is ~220px wide and a retina display doubles it: 512 on the long
// edge is the largest a tile ever shows, and it costs ~30KB per frame.
const THUMB_MAX = 512;
const THUMB_QUALITY = 0.75;
// Frames go down in batches, so a 3000-file folder is 60 transactions rather
// than 3000 of them.
const BATCH = 50;
// Frames read at once. Reading a frame is nearly all waiting — the bytes come off
// the disk, the decode runs on a thread of its own — and a wait that is not spent
// on the next frame is time the roll does not get back. Past a few, the disk and
// the decoder are the limit and the page has less room to breathe.
const LANES = 4;
// How often the walk's position is written down, in the same seconds as the
// catalogue is read back on: the position is the whole of what the walk has found
// and not read, and writing it is a string of a few megabytes on a long roll.
const WALK_MS = 2000;

// One LibRaw open at a time, whoever asks for it. A RAW is opened inside a worker
// the library builds with a quarter of a gigabyte of linear memory of its own
// (libraw-wasm instantiates emscripten's shared memory at 256MB), and the open
// copies the frame into it whole: the four lanes are what a folder of RAW would
// otherwise read at once, and measured on a roll of 48 22.6MB frames that is a
// 1335MB page against a 565MB one before the roll is picked — past what a page is
// given, and the page goes with it, which is the crash this answers. One at a time
// the same roll peaks at 1180MB: the frames around the one being opened still have
// their bytes read, their decodes run and their tiles encoded side by side, because
// only the open queues. A folder of JPEGs never touches this.
let rawTurn: Promise<unknown> = Promise.resolve();
function oneRawAtATime<T>(fn: () => Promise<T>): Promise<T> {
  const turn = rawTurn.then(fn, fn);
  rawTurn = turn.catch(() => undefined);
  return turn;
}

export interface LibraryFolder {
  // The directory's own name: the store's key, and the prefix of every frame id
  // found in it. A rename never touches this.
  name: string;
  // What the tree paints instead, when the visitor has renamed the folder.
  label?: string;
  handle: FileSystemDirectoryHandle;
}

export interface LibraryPhoto {
  // `${folder}/${dir}/${name}` — one frame appears once in the catalogue, under
  // the folder it was found in.
  id: string;
  folder: string;
  // The subfolder it sits in, relative to the picked folder — '' at the root.
  // This is what the tree column reads.
  dir: string;
  name: string;
  handle: FileSystemFileHandle;
  thumb: Blob | null;
  // When the shutter fired, per EXIF, else the file's own lastModified.
  taken: number;
  size: number;
  // When the file itself was last written, as the browser reports it. This — with
  // the size — is what says a frame has not moved since the last scan and does not
  // have to be read again; `taken` is the shutter's own time and says nothing
  // about the file. A frame stored before this field existed has none, and is
  // read one last time.
  mtime?: number;
  // The reader's own score for the frame, 0 or absent when they have not given
  // it one. It belongs to the catalogue and not to the disk — nothing about the
  // file changes — so a rescan carries it over, the way it carries `addedAt`.
  star?: number;
  addedAt: number;
}

// A folder a scan has walked into, whether or not a frame has been read out of
// it yet: the column draws a roll from these before its frames arrive. They are
// the scan's own report and live no longer than it does — a reload draws the
// folders that hold frames, and the next scan names the rest again.
export interface LibraryDir {
  // `${folder}/${rel}` — the tree key, the same way a frame id is spelled.
  id: string;
  folder: string;
  // The path under the picked folder, no leading or trailing slash.
  rel: string;
}

export interface LibraryEdit {
  photoId: string;
  recipe: Recipe;
  updatedAt: number;
}

export const SUPPORTED = /\.(jpe?g|png|webp|heic|heif|tiff?|avif)$/i;

export function isSupportedPhoto(name: string): boolean {
  return isRawName(name) || SUPPORTED.test(name);
}

export function photoId(folder: string, name: string): string {
  return `${folder}/${name}`;
}

// The picker is Chromium-only (Chrome, Edge, Opera, Brave): Firefox and Safari
// have no way to hand a page a folder that outlives the visit, and the caller
// says so instead of showing a button that cannot work.
export function canBrowseFolders(): boolean {
  return typeof window !== 'undefined' && 'showDirectoryPicker' in window;
}

let dbPromise: Promise<IDBDatabase> | null = null;

function openDb(): Promise<IDBDatabase> {
  dbPromise ??= new Promise<IDBDatabase>((resolve, reject) => {
    // No version on purpose. A version the browser has to reach for is a version
    // another tab can block: a tab still running the previous build holds the
    // catalogue open, the upgrade waits on it, and this one never gets an answer
    // — the screen comes up empty on a catalogue that is all there. Opening
    // without one takes the catalogue as it is and creates it when it is missing.
    const req = indexedDB.open(DB_NAME);
    req.onupgradeneeded = () => {
      const db = req.result;
      if (!db.objectStoreNames.contains(FOLDERS)) db.createObjectStore(FOLDERS, { keyPath: 'name' });
      if (!db.objectStoreNames.contains(PHOTOS)) {
        const photos = db.createObjectStore(PHOTOS, { keyPath: 'id' });
        photos.createIndex('folder', 'folder');
        photos.createIndex('taken', 'taken');
      }
      if (!db.objectStoreNames.contains(EDITS)) db.createObjectStore(EDITS, { keyPath: 'photoId' });
    };
    req.onsuccess = () => resolve(req.result);
    req.onerror = () => reject(req.error);
  });
  return dbPromise;
}

// One request per call: the transaction is the store's own, which is all a
// keyed get/put/delete and a getAll ever need. A cursor-based scan (the folder
// walk) goes through `walk` below instead, because it holds one transaction
// across many requests.
function ask<T>(store: string, mode: IDBTransactionMode, run: (s: IDBObjectStore) => IDBRequest): Promise<T> {
  return openDb().then(
    (db) =>
      new Promise<T>((resolve, reject) => {
        const req = run(db.transaction(store, mode).objectStore(store));
        req.onsuccess = () => resolve(req.result as T);
        req.onerror = () => reject(req.error);
      })
  );
}

// --- folders ---------------------------------------------------------------

export async function listFolders(): Promise<LibraryFolder[]> {
  try {
    const rows = await ask<LibraryFolder[]>(FOLDERS, 'readonly', (s) => s.getAll());
    return rows.sort((a, b) => a.name.localeCompare(b.name));
  } catch {
    return [];
  }
}

// The picker's own dialog; `id` makes the browser reopen at the folder this page
// was last pointed at, which is what makes a second visit one click.
export async function pickFolder(): Promise<LibraryFolder | null> {
  if (!canBrowseFolders()) throw new Error('no-folder-picker');
  const handle = await (
    window as unknown as { showDirectoryPicker: (o?: unknown) => Promise<FileSystemDirectoryHandle> }
  ).showDirectoryPicker({ mode: 'readwrite', id: 'recipescam-library' });
  const folder: LibraryFolder = { name: handle.name, handle };
  await ask(FOLDERS, 'readwrite', (s) => s.put(folder));
  return folder;
}

// A rename is a label over the folder, not a new folder: everything downstream —
// the frame ids, the thumbnail store, the recipe each frame was left at — hangs
// off `name`, which stays the directory's own.
export async function renameFolder(folder: LibraryFolder, label: string): Promise<void> {
  await ask(FOLDERS, 'readwrite', (s) => s.put({ ...folder, label }));
}

export async function removeFolder(name: string): Promise<void> {
  const ids = (await listPhotos(name)).map((p) => p.id);
  const db = await openDb();
  await new Promise<void>((resolve, reject) => {
    const tx = db.transaction([FOLDERS, PHOTOS, EDITS], 'readwrite');
    tx.objectStore(FOLDERS).delete(name);
    const photos = tx.objectStore(PHOTOS);
    for (const id of ids) {
      photos.delete(id);
      tx.objectStore(EDITS).delete(id);
    }
    tx.oncomplete = () => resolve();
    tx.onerror = () => reject(tx.error);
  });
  // The reading's position goes with the folder: a folder that is gone is not
  // one to carry on reading, and a folder picked again starts at the top.
  await clearWalk(name);
}

// A handle kept in IndexedDB comes back without its permission: the browser
// dropped it when the tab closed, and only the visitor can hand it back. Asking
// costs one quiet prompt, which is why this runs at start-up rather than the
// moment a frame is clicked. `requestPermission` needs the user's gesture in
// some builds, so a refusal is not fatal — the folder is listed as needing a
// click.
export async function ensurePermission(handle: FileSystemHandle, write = false): Promise<boolean> {
  // `FileSystemPermissionMode` is not in the DOM lib this project compiles
  // against, and the two values are the whole type.
  type PermissionMode = 'read' | 'readwrite';
  const mode: PermissionMode = write ? 'readwrite' : 'read';
  const askable = handle as FileSystemHandle & {
    queryPermission?: (o: { mode: PermissionMode }) => Promise<PermissionState>;
    requestPermission?: (o: { mode: PermissionMode }) => Promise<PermissionState>;
  };
  try {
    if (!askable.queryPermission) return false;
    if ((await askable.queryPermission({ mode })) === 'granted') return true;
    return (await askable.requestPermission?.({ mode })) === 'granted';
  } catch {
    return false;
  }
}

// --- thumbnails ------------------------------------------------------------

// EXIF is one JPEG segment, and a segment is 64KB at most — but only a JPEG keeps
// it in the first one, so the slice that is handed to the parser is a few hundred
// kilobytes rather than the whole frame. Reading 8MB through a JS array to find a
// date in the first kilobyte of it is most of what a scan of JPEGs costs.
export const HEAD_BYTES = 256 * 1024;
// The probe is decoded at this width, purely to learn which edge of the frame is
// the long one; the decoder scales the real one off that. A JPEG does not need
// it: its own header says how big the frame is, in the first few hundred bytes.
const PROBE_PX = 8;

// A JPEG's frame header — the SOF segment — carries the frame's size, and it sits
// well inside the bytes already read off the disk for the date. Reading it costs a
// walk over a few hundred bytes where the probe costs a second decode of every
// pixel of the file.
function jpegSize(bytes: Uint8Array): { width: number; height: number } | null {
  if (bytes[0] !== 0xff || bytes[1] !== 0xd8) return null;
  for (let i = 2; i + 8 < bytes.length; ) {
    if (bytes[i] !== 0xff) {
      i++;
      continue;
    }
    const marker = bytes[i + 1];
    // SOF0–SOF15 minus the ones that are not frames: DHT, JPG, DAC.
    if (marker >= 0xc0 && marker <= 0xcf && marker !== 0xc4 && marker !== 0xc8 && marker !== 0xcc) {
      return { height: (bytes[i + 5] << 8) | bytes[i + 6], width: (bytes[i + 7] << 8) | bytes[i + 8] };
    }
    const length = (bytes[i + 2] << 8) | bytes[i + 3];
    if (length < 2) return null;
    i += 2 + length;
  }
  return null;
}

// What a frame whose header this code cannot read is measured with: one decode
// eight pixels wide. Only the aspect comes out of it — the two numbers that say
// which edge is the long one — and the frame is decoded once more for real.
async function sizeOf(blob: Blob): Promise<{ width: number; height: number } | null> {
  try {
    const probe = await createImageBitmap(blob, { resizeWidth: PROBE_PX });
    const size = { width: probe.width, height: probe.height };
    probe.close();
    return size;
  } catch {
    return null;
  }
}

// The tile, read at the size the tile is painted at. Asking the decoder for 512
// on the long edge scales on the way out of the decoder — a quarter of the pixels
// of a 24MP frame ever exist — where decoding the frame whole and drawing it
// small pays for every one of them. The probe says which edge that is, and the
// canvas is only here to re-encode to JPEG.
async function tile(blob: Blob, size?: { width: number; height: number } | null): Promise<Blob | null> {
  try {
    const known = size ?? (await sizeOf(blob));
    if (!known) return null;
    const wide = known.width >= known.height;
    const bitmap = await createImageBitmap(blob, wide ? { resizeWidth: THUMB_MAX } : { resizeHeight: THUMB_MAX });
    try {
      const canvas = document.createElement('canvas');
      canvas.width = bitmap.width;
      canvas.height = bitmap.height;
      const ctx = canvas.getContext('2d');
      if (!ctx) return null;
      ctx.drawImage(bitmap, 0, 0);
      return await new Promise<Blob | null>((resolve) => canvas.toBlob((out) => resolve(out), 'image/jpeg', THUMB_QUALITY));
    } finally {
      bitmap.close();
    }
  } catch {
    return null;
  }
}

// --- scanning --------------------------------------------------------------

export interface ScanProgress {
  folder: string;
  total: number;
  done: number;
  added: number;
  // How many frames have reached the catalogue. `done` counts the frames the
  // reading has got to, which is nearly all of them long before they are stored,
  // so a screen that read the catalogue back on `done` would read it again every
  // frame — and reading it back means every thumbnail in it again. This moves a
  // batch at a time, which is exactly when there is something new to read.
  written: number;
  // How many frames the scan has read out of each row of the tree, keyed the way
  // the tree spells a row (`folder` for the picked folder, `folder/sub/dir` for
  // the rest) and counted the way the rows count: a frame sits on its own row and
  // on every row above it. A fresh object every frame, so a screen drawing the
  // column sees a change; the frames themselves only reach the catalogue in one
  // batch at the end, which would leave every row reading zero until then.
  counts: Record<string, number>;
  // Every folder the walk has been into so far, a fresh list each time: the
  // column draws them while the frames under them are still being read. Not
  // filed away — a folder name is worth nothing once the scan it came from ends.
  dirs: LibraryDir[];
}

// Walk the folder, keep what is new or changed, and leave the rest alone: a
// second scan of a 2000-file folder only reads the frames that moved. A scan
// that was cut off — the tab closed, the reader stopped it — is finished by the
// next one: the frames it got through are skipped on their size and their time,
// so asking again on the way in is the whole of resuming.
//
// The walk itself lives in `rollWalk`: layer by layer from the picked folder
// down, one bounded pass at a time, with the folder the reader just clicked
// pulled to the front of the queue. `jump` is that click.
//
// ponytail: the lanes run on the main thread. The reading itself is done
// elsewhere (disk, the decoder's own threads), so what is left here is small —
// but a 5000-frame folder still makes the page lumpy. Move the walk into a
// worker (LibRaw is happy in one) if a catalogue that large is ever real.

// Where the reading had got to. A reload in the middle of a few thousand frames
// takes the page with it, and what it would take from the reading is the
// position: the folders the walk had been through, the ones still in its queue,
// and the frames it had found and not read. None of that is in the catalogue —
// the catalogue only holds the frames that were read — so the reading would
// start again at the top of the roll and walk every folder a second time.
//
// The position is held for the whole origin, which is the point: the tab that
// reloads carries on from the frame it stopped at, and so does the next context
// to open the app — the installed one beside the browser, where session storage,
// being the tab's own, handed it nothing and the walk started at the top of the
// roll a second time, every folder listed again and every frame it already held
// read again only to be skipped. The file goes when the reading finishes.
//
// It is a file in the origin private file system and not a key in local storage,
// which is where it was. The position is one path per frame the walk has found
// and not read — about twenty-eight characters each, measured over a folder of
// three thousand — and local storage on this origin takes 5,000,000 characters
// and then throws, also measured. That is a hundred and seventy-five thousand
// frames: past it the first write of a long roll fails, the position is nowhere,
// and the next visit walks the whole roll from the top — the reader's own
// complaint, and one a roll of RAW reaches in a morning. OPFS takes the same text
// as a file, has room for a catalogue's worth of them, and is the origin's the
// same way. An unwritable position is still a reading that starts at the top:
// private mode lands there, as it always did.
const WALK_DIR = 'walk';
const WALK_FILE = (folder: string) => `${encodeURIComponent(folder)}.json`;

async function walkFile(folder: string, create: boolean): Promise<FileSystemFileHandle | null> {
  const root = await navigator.storage?.getDirectory?.();
  if (!root) return null;
  const dir = await root.getDirectoryHandle(WALK_DIR, { create });
  return dir.getFileHandle(WALK_FILE(folder), { create });
}

interface WalkSaved {
  // The folders the walk is through, and the ones it has not read yet, in the
  // order it was going to read them.
  walked: string[];
  pending: string[];
  // The frames the walk has found and the reading has not got to. The front of
  // this is the frame a reloaded reading picks up at.
  frames: string[];
  // How many of the front of `frames` were already counted as read by the reading
  // that wrote this down: the rows are in hand and not in the catalogue yet, so
  // the next reading reads them again — and counts none of them twice.
  restored: number;
  progress: ScanProgress;
}

async function loadWalk(folder: string): Promise<WalkSaved | null> {
  // One reading at a time: a peer that has announced this folder is walking it
  // right now, and what it has written down is a step of a walk still in
  // progress. Taken up here, two readings would write the same roll down at
  // once, each undoing the other's queue. In the tab's own storage this could not
  // come up; now that the position is the origin's, it can.
  if (peer?.folder === folder) return null;
  try {
    const fh = await walkFile(folder, false);
    if (!fh) return null;
    const saved = JSON.parse(await (await fh.getFile()).text()) as WalkSaved;
    return Array.isArray(saved?.walked) &&
      Array.isArray(saved?.pending) &&
      Array.isArray(saved?.frames) &&
      Number.isInteger(saved?.restored) &&
      saved.progress
      ? saved
      : null;
  } catch {
    return null;
  }
}

// A position that cannot be written down is a reading that starts from the top,
// which is what this did before — private mode lands here, as it always did.
async function saveWalk(
  folder: string,
  walked: string[],
  pending: string[],
  frames: string[],
  restored: number,
  progress: ScanProgress
): Promise<void> {
  try {
    const fh = await walkFile(folder, true);
    if (!fh) return;
    const w = await fh.createWritable();
    await w.write(JSON.stringify({ walked, pending, frames, restored, progress } satisfies WalkSaved));
    await w.close();
  } catch {
    // Nothing to write down: the reading starts from the top, as it did before.
  }
}

async function clearWalk(folder: string): Promise<void> {
  try {
    const root = await navigator.storage?.getDirectory?.();
    const dir = await root?.getDirectoryHandle(WALK_DIR, { create: false });
    await dir?.removeEntry(WALK_FILE(folder));
  } catch {
    // Absent is the desired state.
  }
}

// The frames a written-down position names, asked of the root again: a frame is
// spelled by its path, and a handle is not a thing that can be written down. The
// folder part is asked once and kept, because a reading that stopped in a folder
// of a few thousand frames has nearly all of them in the same folder.
async function framesAt(root: FileSystemDirectoryHandle, rels: string[]): Promise<WalkFile[]> {
  const dirs = new Map<string, FileSystemDirectoryHandle>();
  const out: WalkFile[] = [];
  for (const rel of rels) {
    const cut = rel.lastIndexOf('/');
    const at = cut < 0 ? '' : rel.slice(0, cut);
    let dir = dirs.get(at);
    if (!dir) {
      const opened = await openAt(root, at);
      // A folder that will not open takes its frames with it; they are read
      // again when the walk reaches it, if it opens at all.
      if (!opened) continue;
      dirs.set(at, opened);
      dir = opened;
    }
    try {
      out.push({ handle: await dir.getFileHandle(rel.slice(cut + 1)), rel });
    } catch {
      // A frame that is gone is not a frame to read.
    }
  }
  return out;
}

export async function scanFolder(
  folder: LibraryFolder,
  onProgress?: (p: ScanProgress) => void,
  shouldStop?: () => boolean,
  jump?: () => string | null
): Promise<ScanProgress> {
  const known = new Map((await listPhotos(folder.name)).map((p) => [p.id, p]));
  const saved = await loadWalk(folder.name);
  const tree: Walk = { root: folder.handle, pending: [], walked: new Set(saved?.walked ?? []) };
  // A queue comes back as paths, so the folders are asked for again; a reading
  // with nothing written down starts at the picked folder.
  if (saved) {
    for (const rel of saved.pending) {
      const dir = await openAt(folder.handle, rel);
      if (dir) tree.pending.push({ dir, rel });
    }
  } else {
    tree.pending.push({ dir: folder.handle, rel: '' });
  }
  // The frames this reading has found and not read: the walk fills it, the lanes
  // empty it, and a frame taken off the front is one a reload will not read
  // again. Also where a reading that was cut off picks up.
  const entries: WalkFile[] = saved ? await framesAt(folder.handle, saved.frames) : [];
  // What the passes found: the folders this scan walked into, and the names they
  // put in the column. `names` is cleared a pass at a time; `found` keeps the
  // whole scan so a folder that has gone from the disk goes from the column too.
  const names: string[] = [];
  const progress: ScanProgress = saved?.progress ?? {
    folder: folder.name,
    total: 0,
    done: 0,
    added: 0,
    written: 0,
    dirs: [],
    counts: {},
  };
  progress.folder = folder.name;
  // A position written down by an older visit has no count to carry on from.
  progress.written ??= 0;
  const found = new Set(progress.dirs.map((d) => d.id));
  const counts: Record<string, number> = progress.counts;
  // Count the frame the moment the scan gets to it, before it knows whether the
  // frame is new: the rows say how far the reading has come, not what it kept.
  const count = (rel: string) => {
    const cut = rel.lastIndexOf('/');
    counts[folder.name] = (counts[folder.name] ?? 0) + 1;
    let path = '';
    for (const part of (cut < 0 ? '' : rel.slice(0, cut)).split('/')) {
      if (!part) continue;
      path = path ? `${path}/${part}` : part;
      const key = `${folder.name}/${path}`;
      counts[key] = (counts[key] ?? 0) + 1;
    }
    progress.counts = { ...counts };
  };
  // The folders this scan has walked into, kept for the whole of it: the column
  // is redrawn from this list, so it only ever grows. A reading that was cut off
  // hands the list on with the rest of its position.
  const dirs: LibraryDir[] = progress.dirs;
  let batch: LibraryPhoto[] = [];
  // The frames in hand: the ones a lane has been handed, and the ones whose row is
  // in `batch` and not yet in the catalogue. They are on neither side of the line a
  // position is written on — the catalogue does not hold them and the walk will not
  // find them again — so a reload has to read them again, and they are also the
  // frames the written-down count has already counted.
  let inHand: string[] = [];
  const pathOf = (row: LibraryPhoto) => (row.dir ? `${row.dir}/${row.name}` : row.name);
  // The position goes down on a clock, not on every batch. The whole of it is
  // written each time — a copy of every path the walk has found and not read —
  // and on a roll of a hundred thousand that is a few megabytes turned into a
  // string per batch, which is the page held still for the whole roll. A position
  // a step behind is a frame or two read again on the next visit, and a frame read
  // again is skipped on its size and its time.
  let wroteAt = 0;
  const write = async () => {
    const at = Date.now();
    if (at - wroteAt < WALK_MS) return;
    wroteAt = at;
    const held = [...inHand, ...batch.map(pathOf)];
    await saveWalk(
      folder.name,
      [...tree.walked],
      tree.pending.map((p) => p.rel),
      [...held, ...entries.map((e) => e.rel)],
      held.length,
      progress
    );
  };
  // One transaction per batch, a put per row: a store with `keyPath: 'id'` takes
  // a record, not an array of them. The position is written with it — a batch is
  // the beat the reading and the stored position are kept in step at, so at most
  // one batch is read again after a reload, and a frame that is read again is
  // skipped on its size and its time.
  const flush = async () => {
    if (!batch.length) return;
    const rows = batch;
    batch = [];
    const db = await openDb();
    await new Promise<void>((resolve, reject) => {
      const tx = db.transaction(PHOTOS, 'readwrite');
      const photos = tx.objectStore(PHOTOS);
      for (const row of rows) photos.put(row);
      tx.oncomplete = () => resolve();
      tx.onerror = () => reject(tx.error);
    });
    progress.written += rows.length;
    await write();
  };
  // One frame, end to end: its bytes, its tile, its shutter time, and the row the
  // catalogue keeps. A frame that has not moved is dropped on its size and its
  // time before a byte of it is read, which is what makes a second scan of the
  // same folder cheap; a frame that will not read at all is skipped by the lane
  // that ran it — one bad file in a folder is not a failed folder.
  const readOne = async (handle: FileSystemFileHandle, rel: string): Promise<void> => {
    const file = await handle.getFile();
    const id = photoId(folder.name, rel);
    const seen = known.get(id);
    // Unchanged means the same size and the same write time: the frame is left
    // exactly as the catalogue already holds it. This is what makes a second scan
    // of the same folder cheap, and what finishes a roll whose first scan was cut
    // short — the frames it got through are skipped, the rest are read.
    if (seen && seen.size === file.size && seen.mtime === file.lastModified && seen.thumb) {
      // A row restored from a backup has no handle: a handle is not a thing that
      // can be written to a file, so the row comes back with everything but the
      // one thing that opens the frame, and the walk is where it gets it back.
      // The row is put again — the bytes of the frame are still never read.
      if (!seen.handle) {
        batch.push({ ...seen, handle });
        if (batch.length >= BATCH) await flush();
      }
      return;
    }
    // A RAW is read whole because that is the only way LibRaw can seek to the
    // preview the camera wrote inside it; a JPEG is handed to the decoder as it
    // is, and only its header is read for the date. Both tiles come off what the
    // camera wrote — a RAW's preview is a copy, where a develop is every pixel.
    const raw = isRawName(file.name);
    const bytes = new Uint8Array(await (raw ? file.arrayBuffer() : file.slice(0, HEAD_BYTES).arrayBuffer()));
    const preview = raw ? await rawThumbnail(bytes) : null;
    // The tile is the preview the camera wrote inside a RAW, or the frame itself.
    // Both carry their own size in their first few hundred bytes — the same bytes
    // the date comes out of — so the decoder is handed the size instead of being
    // asked with a second decode of the whole frame to learn which edge is long.
    const src = preview ? new Blob([preview as BlobPart], { type: 'image/jpeg' }) : raw ? null : file;
    const size = preview ? jpegSize(preview) : raw ? null : jpegSize(bytes);
    const thumb = src ? await tile(src, size) : null;
    const taken = (await readCapturedAt(bytes)) ?? file.lastModified;
    const cut = rel.lastIndexOf('/');
    batch.push({
      id,
      folder: folder.name,
      dir: cut < 0 ? '' : rel.slice(0, cut),
      name: handle.name,
      handle,
      thumb,
      taken,
      size: file.size,
      mtime: file.lastModified,
      // The reader's own score is not the file's: a frame that is read again
      // keeps the stars it was given.
      star: seen?.star,
      addedAt: seen?.addedAt ?? Date.now(),
    });
    progress.added++;
    inHand = inHand.filter((r) => r !== rel);
    if (batch.length >= BATCH) await flush();
  };

  // The frames in hand, read a few at a time: reading a frame is almost all
  // waiting — the bytes come off the disk, the decode runs on a thread of its own
  // — and the wait of one frame is spent on the next one instead of sitting
  // still. Lanes are why the same roll lands in a fraction of the time; past a
  // few the disk and the decoder are the limit. A frame leaves the queue the
  // moment its lane is handed it, so the position on disk trails the reading by
  // no more than one batch.
  const drain = async () => {
    const lanes = new Set<Promise<void>>();
    while (entries.length) {
      if (shouldStop?.()) {
        stop = true;
        break;
      }
      const { handle, rel } = entries.shift()!;
      inHand.push(rel);
      // A frame a reload handed back that was counted already is read without
      // being counted twice; the ones behind it are frames that reading found and
      // never reached, and they are counted here as they are reached.
      if (restored) restored--;
      else {
        progress.done++;
        count(rel);
      }
      // A RAW takes the one open there is before its lane does anything, so a
      // folder of them is read a frame at a time whatever the lanes say.
      const read = isRawName(handle.name) ? oneRawAtATime(() => readOne(handle, rel)) : readOne(handle, rel);
      const lane = read
        .catch(() => undefined)
        .finally(() => lanes.delete(lane));
      lanes.add(lane);
      if (lanes.size >= LANES) await Promise.race(lanes);
      onProgress?.(progress);
    }
    await Promise.all(lanes);
  };

  // How many of the frames in hand were counted by the reading that handed them
  // back: the rows it had read and not yet stored. The rest of the queue is frames
  // it found and never got to, and this reading counts those as it reaches them —
  // the rows and the counter both say how far the reading has come, and a reload
  // that stopped at a frame must not count it twice.
  let restored = Math.min(saved?.restored ?? 0, entries.length);

  let stop = false;
  let walked = false;
  // One bounded pass at a time, and the pass yields between frames, so a folder
  // clicked while the scan runs is read on the next jump rather than after the
  // whole tree. Nothing clicked: the picked folder and its frames, then each
  // layer below with theirs, deepest last — the order `pending` is filled in.
  do {
    names.length = 0;
    // The queue a reading that was cut off left behind goes first, and the walk
    // is asked for nothing until it is through: the frames already found are
    // what that reading was in the middle of.
    if (!entries.length) {
      walked = await walkPass(tree, entries, names, isSupportedPhoto, jump ?? (() => null));
      // The names come in before the frames they hold: the column grows one pass
      // ahead of the strip, which is the whole point of reading layer by layer.
      for (const rel of names) {
        const id = `${folder.name}/${rel.slice(0, -1)}`;
        if (found.has(id)) continue;
        found.add(id);
        dirs.push({ id, folder: folder.name, rel: rel.slice(0, -1) });
      }
      // The column is handed the names the moment the pass is through, before a
      // single frame under them has been read. A copy, so the redraw has
      // something new to look at rather than the list growing under it, and a
      // reading that is cut off hands the same list on with its position.
      progress.dirs = dirs.slice();
      // Every frame found is a frame the reading has or will reach, so what is
      // still in the queue is what is left of the total.
      progress.total = progress.done + entries.length;
      // The position before a frame of this pass is looked at: a reload here
      // reads the pass's frames again, and a frame already in the catalogue is
      // skipped anyway — where the other order would leave the last pass's
      // frames out of the catalogue until the next visit.
      await write();
      onProgress?.(progress);
    }
    await drain();
  } while (!walked && !stop);
  await flush();
  // A reading that came to its end has no position worth keeping: the next one
  // walks the roll from the top and skips what has not moved.
  if (!stop) await clearWalk(folder.name);
  return progress;
}

// --- the scan in flight ----------------------------------------------------

// A scan belongs to the tab, not to the screen it was started from. A roll takes
// minutes, and the reader who started it has every reason to go to the studio and
// work on a frame while it runs — so the screen that started it may well be gone
// before the scan is. What that screen would have held lives here instead: one
// scan at a time, watched by whichever screen is up.
export interface ScanSession {
  // The folder being read, by name.
  folder: string;
  progress: ScanProgress;
  // What the reader has asked for since: stop, or read this folder next (the path
  // as `rollWalk` spells it — `''` for the picked folder itself).
  stop: boolean;
  jump: string | null;
}

let live: ScanSession | null = null;
const watchers = new Set<() => void>();

const tell = () => {
  for (const fn of watchers) fn();
};

// --- the other windows on this origin --------------------------------------

// An installed app and a browser tab share one origin, one catalogue and one
// roll: without a word between them the second window opens, finds nothing in
// flight here, and reads the same frames again — two readers of one folder, two
// RAW decoders at 256MB apiece, for a progress the first window already has.
// So the window holding the reading says so, on every frame and on a heartbeat,
// and the windows that do not hold it take the reading as their own: they draw
// the same progress, offer the same stop, and above all start nothing.
const CHANNEL = 'recipescam-library';
const SELF = Math.random().toString(36).slice(2);
// A window that has heard nothing for this long has a peer that was closed, or
// one that is wedged — either way the reading is nobody's again, and this window
// may pick it up as it always could.
const PEER_MS = 6000;
const HEART_MS = 2000;
// One round trip, for the window that has just opened: whether another window
// holds the reading is what decides whether this one starts its own.
const ASK_MS = 250;

interface Peer {
  folder: string;
  progress: ScanProgress;
  at: number;
}

let peer: Peer | null = null;
let peerTimer: ReturnType<typeof setTimeout> | null = null;
let heart: ReturnType<typeof setInterval> | null = null;
let askers: ((busy: boolean) => void)[] = [];
// A browser without the channel (an old one, a worker) simply has no peers, and
// everything below is a no-op there.
const channel = typeof BroadcastChannel === 'function' ? new BroadcastChannel(CHANNEL) : null;

const say = (m: Record<string, unknown>): void => {
  channel?.postMessage({ ...m, from: SELF });
};

const dropPeer = (): void => {
  if (peerTimer) {
    clearTimeout(peerTimer);
    peerTimer = null;
  }
  if (!peer) return;
  peer = null;
  tell();
};

const holdPeer = (folder: string, progress: ScanProgress): void => {
  peer = { folder, progress, at: Date.now() };
  for (const fn of askers) fn(true);
  askers = [];
  if (peerTimer) clearTimeout(peerTimer);
  peerTimer = setTimeout(dropPeer, PEER_MS);
  tell();
};

// A peer holds the reading the way this window holds one, as far as a screen is
// concerned: the same shape, read the same way. Whether the reading is stopped
// or jumped from here is answered by relaying it, which is why the window
// watching can offer the buttons the window reading has.
const peerSession = (): ScanSession | null =>
  peer ? { folder: peer.folder, progress: peer.progress, stop: false, jump: null } : null;

const announce = (): void => {
  if (live) say({ k: 'scan', folder: live.folder, progress: live.progress });
};

if (channel) {
  channel.onmessage = (e: MessageEvent) => {
    const m = e.data as { k?: string; from?: string; folder?: string; progress?: ScanProgress; rel?: string | null } | null;
    if (!m || m.from === SELF) return;
    if (m.k === 'scan' && m.folder && m.progress) holdPeer(m.folder, m.progress);
    else if (m.k === 'end') {
      if (peer?.folder === m.folder) dropPeer();
    } else if (m.k === 'who') announce(); // a window just opened and asks who reads
    else if (m.k === 'stop' && live && live.folder === m.folder) {
      live.stop = true;
      tell();
    } else if (m.k === 'jump' && live && live.folder === m.folder) live.jump = m.rel ?? null;
  };
}

// Whether another window is reading right now, asked rather than assumed: the
// window that opens beside a running scan knows nothing until the reading
// answers, and reading the disk twice over is exactly what the answer prevents.
export function scanBusy(): Promise<boolean> {
  if (live) return Promise.resolve(true);
  if (!channel) return Promise.resolve(false);
  const answered = new Promise<boolean>((resolve) => {
    const settle = (busy: boolean): void => {
      askers = askers.filter((fn) => fn !== settle);
      resolve(busy);
    };
    askers.push(settle);
    // Nobody answered: no reading is running, and waiting longer only delays
    // this window's own.
    setTimeout(() => settle(false), ASK_MS);
  });
  say({ k: 'who' });
  return answered;
}

// The scan in flight, or null. The same object for the whole of the scan, with
// `progress` replaced on every pass — watchers are told, so they re-read it.
// A reading another window holds answers the same way: the screens no longer
// know or care which window the reading is in.
export function scanSession(): ScanSession | null {
  return live ?? peerSession();
}

// Watch, and get the unsubscribe back: the shape an effect already has.
export function watchScan(fn: () => void): () => void {
  watchers.add(fn);
  return () => {
    watchers.delete(fn);
  };
}

export function stopScan(): void {
  // A reading another window holds is stopped by telling it so: the button this
  // window drew is the same button, on the window that can act on it.
  if (!live) {
    if (peer) say({ k: 'stop', folder: peer.folder });
    return;
  }
  live.stop = true;
  tell();
}

export function jumpScan(rel: string | null): void {
  if (live) {
    live.jump = rel;
    return;
  }
  if (peer) say({ k: 'jump', folder: peer.folder, rel });
}

// Start reading a roll. Only one at a time: the walk reads one frame at a time on
// this thread, so a second scan would only slow the first one down. The promise
// settles when the scan does — the caller that started it may be long gone.
export function startScan(folder: LibraryFolder): Promise<ScanProgress> {
  if (live) return Promise.reject(new Error('a scan is already running'));
  const session: ScanSession = {
    folder: folder.name,
    progress: { folder: folder.name, total: 0, done: 0, added: 0, written: 0, dirs: [], counts: {} },
    stop: false,
    jump: null,
  };
  live = session;
  tell();
  announce();
  // Between two frames there is nothing to announce, and a window that hears
  // silence long enough would take the reading for abandoned and start its own.
  if (heart) clearInterval(heart);
  heart = setInterval(announce, HEART_MS);
  return scanFolder(
    folder,
    (p) => {
      session.progress = p;
      tell();
      announce();
    },
    () => session.stop,
    () => session.jump
  ).finally(() => {
    if (heart) {
      clearInterval(heart);
      heart = null;
    }
    live = null;
    say({ k: 'end', folder: session.folder });
    tell();
  });
}

// --- reading ---------------------------------------------------------------

export async function listPhotos(folder?: string): Promise<LibraryPhoto[]> {
  try {
    const rows = await ask<LibraryPhoto[]>(PHOTOS, 'readonly', (s) =>
      folder ? s.index('folder').getAll(folder) : s.getAll()
    );
    // Newest shutter first — a folder of stills reads in the order it was shot.
    return rows.sort((a, b) => b.taken - a.taken);
  } catch {
    return [];
  }
}

export async function getPhoto(id: string): Promise<LibraryPhoto | null> {
  try {
    return (await ask<LibraryPhoto | undefined>(PHOTOS, 'readonly', (s) => s.get(id))) ?? null;
  } catch {
    return null;
  }
}

// Score a frame, or take the score back with a 0. Nothing on the disk changes —
// the stars are the catalogue's own — so this is a read and a put, and a frame
// that is read again on the next scan carries its score over.
export async function setStar(id: string, star: number): Promise<void> {
  const photo = await getPhoto(id);
  if (!photo) return;
  try {
    await ask(PHOTOS, 'readwrite', (s) => s.put({ ...photo, star }));
  } catch {
    // Private mode: the score lives no longer than the visit.
  }
}

// The frame itself, at last: nothing is read from the disk until this runs, so
// the catalogue costs thumbnails and no more.
export async function readPhotoFile(photo: LibraryPhoto): Promise<File> {
  return photo.handle.getFile();
}

// --- edits -----------------------------------------------------------------

export async function saveEdit(id: string, recipe: Recipe): Promise<void> {
  try {
    const edit: LibraryEdit = { photoId: id, recipe, updatedAt: Date.now() };
    await ask(EDITS, 'readwrite', (s) => s.put(edit));
  } catch {
    // Private mode: the frame is still editable, it just forgets the recipe.
  }
}

export async function loadEdit(id: string): Promise<LibraryEdit | null> {
  try {
    return (await ask<LibraryEdit | undefined>(EDITS, 'readonly', (s) => s.get(id))) ?? null;
  } catch {
    return null;
  }
}

export async function listEditedIds(): Promise<Set<string>> {
  try {
    const rows = await ask<LibraryEdit[]>(EDITS, 'readonly', (s) => s.getAll());
    return new Set(rows.map((e) => e.photoId));
  } catch {
    return new Set();
  }
}

export async function listEdits(): Promise<LibraryEdit[]> {
  try {
    return await ask<LibraryEdit[]>(EDITS, 'readonly', (s) => s.getAll());
  } catch {
    return [];
  }
}

// --- restore ---------------------------------------------------------------

// The catalogue as it travels: the rows without the two things that cannot be
// written to a file — a handle, which only the visitor's picker can hand out, and
// the tile itself, which is a blob and would be a second copy of the backup
// folder. Everything else is here, `mtime` and `size` and `star` included, which
// is what lets the walk that follows match every frame and read none of them.
export type CatalogueRow = Omit<LibraryPhoto, 'handle' | 'thumb'>;

// The rows a set of ids stands for, gaps left as gaps — one transaction for the
// lot, where `getPhoto` would be one per frame.
async function rowsAt(ids: string[]): Promise<(LibraryPhoto | undefined)[]> {
  const db = await openDb();
  return new Promise((resolve, reject) => {
    const tx = db.transaction(PHOTOS, 'readonly');
    const store = tx.objectStore(PHOTOS);
    const out: (LibraryPhoto | undefined)[] = [];
    ids.forEach((id, i) => {
      const req = store.get(id);
      req.onsuccess = () => {
        out[i] = req.result as LibraryPhoto | undefined;
      };
    });
    tx.oncomplete = () => resolve(out);
    tx.onerror = () => reject(tx.error);
  });
}

// Put a catalogue back. A frame the catalogue already holds and can open — one
// with a handle — is left exactly as it is: the backup's copy of it would come
// back without one, which is a working catalogue traded for a worse one. Every
// other row goes in as it came out. `thumb` fetches the tile that was backed up
// beside the row, and a tile that will not read leaves the row without one: the
// grid paints a blank cell until the next scan reads the frame.
export async function restoreCatalogue(
  photos: CatalogueRow[],
  edits: LibraryEdit[],
  thumb?: (id: string) => Promise<Blob | null>
): Promise<number> {
  const db = await openDb();
  const size = 200;
  let put = 0;
  for (let at = 0; at < photos.length; at += size) {
    const slice = photos.slice(at, at + size);
    const had = await rowsAt(slice.map((r) => r.id));
    const want = slice.filter((_, i) => !had[i]?.handle);
    if (!want.length) continue;
    const rows = await Promise.all(
      want.map(async (row) => ({ ...row, thumb: thumb ? await thumb(row.id) : null }) as unknown as LibraryPhoto)
    );
    await new Promise<void>((resolve, reject) => {
      const tx = db.transaction(PHOTOS, 'readwrite');
      const store = tx.objectStore(PHOTOS);
      for (const row of rows) store.put(row);
      tx.oncomplete = () => resolve();
      tx.onerror = () => reject(tx.error);
    });
    put += rows.length;
  }
  for (let at = 0; at < edits.length; at += size) {
    const slice = edits.slice(at, at + size);
    await new Promise<void>((resolve, reject) => {
      const tx = db.transaction(EDITS, 'readwrite');
      const store = tx.objectStore(EDITS);
      for (const row of slice) store.put(row);
      tx.oncomplete = () => resolve();
      tx.onerror = () => reject(tx.error);
    });
  }
  return put;
}