/**
 * Where downloads go.
 *
 * A web page cannot be told a path. It cannot read "~/Downloads", cannot write
 * to it, and cannot be configured with "C:\Users\…" either — which is also why
 * none of this needs to change when someone runs it on Windows. What a browser
 * grants is a *handle* to a directory the user picked, once, in a dialog. That
 * handle is what this module keeps.
 *
 * Two modes, and the default matters:
 *
 *   auto (default) — write into the granted folder without asking. Downloading
 *     twelve files puts twelve files there. Without a granted folder, the file
 *     goes wherever the browser puts downloads, which on most machines is the
 *     same folder anyway.
 *   ask — a Save As dialog for every file, which is the right answer for one
 *     file and the wrong one for twelve.
 *
 * Firefox and Safari have no File System Access API at all: no folder can be
 * granted there, and both modes fall back to the browser's own download folder.
 * The setting says so rather than offering a choice that does nothing.
 */

const PREF_KEY = 'mb_dl_mode';
const IDB_NAME = 'meshbay-downloads';
const IDB_STORE = 'handles';
const HANDLE_KEY = 'target-dir';

export const SUPPORTED = typeof window !== 'undefined'
  && typeof window.showDirectoryPicker === 'function';

export function getMode() {
  const v = localStorage.getItem(PREF_KEY);
  return v === 'ask' ? 'ask' : 'auto';
}

export function setMode(mode) {
  localStorage.setItem(PREF_KEY, mode === 'ask' ? 'ask' : 'auto');
}

// ── The granted directory ────────────────────────────────────────────────────

function idb() {
  return new Promise((resolve, reject) => {
    const req = indexedDB.open(IDB_NAME, 1);
    req.onupgradeneeded = () => {
      if (!req.result.objectStoreNames.contains(IDB_STORE)) {
        req.result.createObjectStore(IDB_STORE);
      }
    };
    req.onsuccess = () => resolve(req.result);
    req.onerror = () => reject(req.error);
  });
}

async function idbGet(key) {
  try {
    const db = await idb();
    const tx = db.transaction(IDB_STORE, 'readonly');
    const req = tx.objectStore(IDB_STORE).get(key);
    const out = await new Promise((r, j) => {
      req.onsuccess = () => r(req.result); req.onerror = () => j(req.error);
    });
    db.close();
    return out || null;
  } catch { return null; }
}

async function idbPut(key, value) {
  try {
    const db = await idb();
    const tx = db.transaction(IDB_STORE, 'readwrite');
    if (value === null) tx.objectStore(IDB_STORE).delete(key);
    else tx.objectStore(IDB_STORE).put(value, key);
    await new Promise((r, j) => { tx.oncomplete = r; tx.onerror = j; });
    db.close();
  } catch { /* best effort: the setting simply will not stick */ }
}

/** The folder handle, if one was ever granted. Says nothing about permission. */
export async function savedDirectory() {
  return SUPPORTED ? idbGet(HANDLE_KEY) : null;
}

/** Ask for a folder. Must be called from a click — browsers require a gesture. */
export async function chooseDirectory() {
  const handle = await window.showDirectoryPicker({ mode: 'readwrite' });
  await idbPut(HANDLE_KEY, handle);
  return handle;
}

export async function forgetDirectory() {
  await idbPut(HANDLE_KEY, null);
}

/**
 * Permission survives a reload as a *claim*, not as a grant: the handle comes
 * back from IndexedDB in state "prompt" and has to be re-authorized once per
 * session. Doing that quietly during a download click is why this takes no
 * arguments and returns a boolean instead of throwing.
 */
export async function ensurePermission(handle, { prompt = true } = {}) {
  if (!handle) return false;
  try {
    const opts = { mode: 'readwrite' };
    if (await handle.queryPermission(opts) === 'granted') return true;
    if (!prompt) return false;
    return await handle.requestPermission(opts) === 'granted';
  } catch {
    return false;
  }
}

// ── Names ────────────────────────────────────────────────────────────────────

/**
 * A name not already taken, as "clip (2).mp4".
 *
 * Writing into a folder repeatedly is the whole point of the automatic mode, so
 * the second copy of a file must not silently replace the first. `exists` is
 * passed in so this can be tested without a filesystem.
 */
export async function freeName(name, exists) {
  if (!await exists(name)) return name;
  const dot = name.lastIndexOf('.');
  const stem = dot > 0 ? name.slice(0, dot) : name;
  const ext = dot > 0 ? name.slice(dot) : '';
  for (let n = 2; n < 1000; n++) {
    const candidate = `${stem} (${n})${ext}`;
    if (!await exists(candidate)) return candidate;
  }
  return `${stem} (${Date.now()})${ext}`;
}

/**
 * Where this download should be written.
 *
 * Returns a writable stream, or null meaning "there is nowhere to stream to —
 * collect it and hand the browser a blob". The caller opens the picker itself
 * in "ask" mode, because that one has to happen inside the click.
 */
export async function openTarget(filename) {
  if (!SUPPORTED || getMode() === 'ask') return null;

  const dir = await savedDirectory();
  if (!dir || !await ensurePermission(dir)) return null;

  const exists = async (name) => {
    try {
      await dir.getFileHandle(name);
      return true;
    } catch {
      return false;
    }
  };
  const name = await freeName(filename, exists);
  const handle = await dir.getFileHandle(name, { create: true });
  return { writable: await handle.createWritable(), name };
}