import type { Config } from '../config.ts';
import type {
	BoardContext,
	BoardSkeleton,
	CardResponse,
	ClassItem,
	CourseBoardResponse,
	CourseMetadata,
	DashboardResponse,
	FileParentType,
	FileRecord,
	GroupItem,
	LegacyCourse,
	LegacyUser,
	LessonResponse,
	MeResponse,
	NewsResponse,
	Paginated,
	ParentFileStats,
	PreviewWidth,
	SubmissionStatus,
	LessonLinkedTask,
	RoomApplicant,
	RoomBoardItem,
	RoomDetails,
	RoomInvitationLink,
	RoomItem,
	RoomMember,
	TaskContent,
} from './types.ts';

/**
 * Maximum repeated query parameters the API's parser will still treat as an
 * array — the `qs` default. See `getCards` for what happens above it.
 */
export const MAX_IDS_PER_QUERY = 20;

/** The only output format the preview endpoint accepts; anything else is a 400. */
const PREVIEW_OUTPUT_FORMAT = 'image/webp';

/** Statuses worth retrying: transient by definition, and every call here is a GET. */
const RETRYABLE = new Set([429, 500, 502, 503, 504]);
const MAX_RETRIES = 3;

/** Exponential backoff with jitter, so parallel workers do not retry in lockstep. */
function backoffMs(attempt: number): number {
	const base = 400 * 2 ** (attempt - 1);
	return Math.round(base + Math.random() * base * 0.5);
}

function delay(ms: number): Promise<void> {
	return new Promise((resolve) => setTimeout(resolve, ms));
}

/** An API response outside the 2xx range, carrying the status for callers to branch on. */
export class SchulcloudApiError extends Error {
	readonly status: number;
	readonly path: string;
	readonly body: string;

	constructor(status: number, path: string, body: string) {
		super(`Schulcloud API ${status} for ${path}${body ? `: ${truncate(body, 400)}` : ''}`);
		this.name = 'SchulcloudApiError';
		this.status = status;
		this.path = path;
		this.body = body;
	}

	/** True when the instance rejected our JWT — the one error the user must act on. */
	get isAuthFailure(): boolean {
		return this.status === 401;
	}
}

function truncate(value: string, max: number): string {
	return value.length > max ? `${value.slice(0, max)}…` : value;
}

export interface DownloadedFile {
	bytes: Buffer;
	mimeType: string;
	fileName: string;
	/** True when the file was longer than `maxDownloadBytes` and got cut short. */
	truncated: boolean;
}

/**
 * HTTP client for a Schulcloud instance. Read-only apart from `extendSession`,
 * which touches only the caller's own session — see its doc comment.
 *
 * Two services sit behind the same origin and both accept the same bearer
 * token: the main server under `/api/v3/*`, and the files-storage service
 * under `/api/v3/file/*`. The JWT from the browser's `jwt` cookie works
 * verbatim as `Authorization: Bearer` — no cookie jar or session refresh is
 * involved, and the token is valid for 30 days (see docs/AUTH.md).
 *
 * Every method that touches user data is a GET. Keeping the client incapable of
 * writing is the main safety property of this server: whoever reaches the MCP
 * endpoint can read this account's data but cannot act as the user inside
 * Schulcloud. `extendSession` is the single exception and is not exposed as a
 * tool, so no model-driven call can ever be a POST.
 */
export class SchulcloudClient {
	private readonly config: Config;

	constructor(config: Config) {
		this.config = config;
	}

	// --- transport -------------------------------------------------------

	private url(path: string, query?: Record<string, string | number | string[] | undefined>): URL {
		const url = new URL(`${this.config.baseUrl}${path}`);
		for (const [key, value] of Object.entries(query ?? {})) {
			if (value === undefined) continue;
			if (Array.isArray(value)) for (const v of value) url.searchParams.append(key, v);
			else url.searchParams.set(key, String(value));
		}
		return url;
	}

	/**
	 * One upstream GET, with retries for the transient failures.
	 *
	 * `auth` picks how the session travels. The v3 API takes it as a bearer
	 * token; the legacy client's pages take it only as the `jwt` cookie; and a
	 * pre-signed storage URL must get **nothing** — it lives on another host, and
	 * the session token has no business leaving this instance. Anything but the
	 * bearer form is fetched with redirects off, so a login bounce or a hop to a
	 * third host is seen rather than silently followed with credentials attached.
	 */
	private async request(
		url: URL,
		accept: string,
		auth: 'bearer' | 'cookie' | 'none' = 'bearer',
		options: { idleTimeout?: boolean; token?: string } = {},
	): Promise<Response> {
		let lastError: unknown;
		const headers: Record<string, string> = { Accept: accept };
		// Read at the moment of use, never earlier: the token can be replaced
		// while the server runs (core/session-token.ts).
		const jwt = options.token ?? this.config.jwt;
		if (auth === 'bearer') headers.Authorization = `Bearer ${jwt}`;
		if (auth === 'cookie') headers.Cookie = `jwt=${jwt}`;

		for (let attempt = 0; attempt <= MAX_RETRIES; attempt++) {
			if (attempt > 0) await delay(backoffMs(attempt));

			let response: Response;
			const deadline = options.idleTimeout ? idleDeadline(this.config.requestTimeoutMs) : undefined;
			try {
				response = await fetch(url, {
					headers,
					signal: deadline?.signal ?? AbortSignal.timeout(this.config.requestTimeoutMs),
					redirect: auth === 'bearer' ? 'follow' : 'manual',
				});
			} catch (error) {
				deadline?.stop();
				// Connection reset or timeout: worth one more try, since every call
				// here is an idempotent GET.
				lastError = error;
				if (attempt === MAX_RETRIES) throw error;
				continue;
			}

			if (response.ok) return deadline ? deadline.watch(response) : response;
			deadline?.stop();

			const body = await response.text().catch(() => '');
			// A pre-signed URL's query string is its credential, so it never goes
			// into an error message; nor does the storage host's error body.
			const error =
				auth === 'none'
					? new SchulcloudApiError(response.status, `${url.host} (pre-signed download)`, '')
					: new SchulcloudApiError(response.status, url.pathname + url.search, body);

			// A crawl issues hundreds of requests and the instance answers some of
			// them with a 503 front-page when it decides we are going too fast.
			// Observed live: 4 of 26 course pages failed that way in one crawl, and
			// all of them succeeded on a retry. 429 and the other gateway errors
			// are the same kind of "come back shortly".
			if (!RETRYABLE.has(response.status) || attempt === MAX_RETRIES) throw error;

			const retryAfter = Number(response.headers.get('retry-after'));
			if (Number.isFinite(retryAfter) && retryAfter > 0) await delay(retryAfter * 1000);
			lastError = error;
		}

		throw lastError instanceof Error ? lastError : new Error('request failed');
	}

	/** Authenticated GET returning JSON. `path` is absolute, e.g. `/api/v3/courses`. */
	async getJson<T>(path: string, query?: Record<string, string | number | string[] | undefined>): Promise<T> {
		const response = await this.request(this.url(path, query), 'application/json');
		return (await response.json()) as T;
	}

	/**
	 * Authenticated GET returning bytes, capped at `maxDownloadBytes`.
	 *
	 * The cap is enforced while streaming rather than via Content-Length, so a
	 * mis-declared or chunked response still can't exhaust memory.
	 */
	async getBytes(path: string, fallbackName: string): Promise<DownloadedFile> {
		const url = this.url(path);
		const response = await this.request(url, '*/*', 'bearer', { idleTimeout: true });
		return this.readCapped(response, fallbackName);
	}

	/** Reads a response body up to `maxDownloadBytes`, flagging anything cut off. */
	private async readCapped(response: Response, fallbackName: string): Promise<DownloadedFile> {
		const limit = this.config.maxDownloadBytes;

		const chunks: Buffer[] = [];
		let total = 0;
		let truncated = false;

		if (response.body) {
			const reader = response.body.getReader();
			try {
				while (total < limit) {
					const { done, value } = await reader.read();
					if (done) break;
					const chunk = Buffer.from(value);
					const room = limit - total;
					if (chunk.length > room) {
						chunks.push(chunk.subarray(0, room));
						total = limit;
						truncated = true;
						break;
					}
					chunks.push(chunk);
					total += chunk.length;
				}
				if (total >= limit) {
					// Anything still queued is beyond the cap; drop the rest.
					const { done } = await reader.read();
					if (!done) truncated = true;
				}
			} finally {
				await reader.cancel().catch(() => {});
			}
		}

		return {
			bytes: Buffer.concat(chunks),
			mimeType: response.headers.get('content-type')?.split(';')[0]?.trim() || 'application/octet-stream',
			fileName: filenameFromDisposition(response.headers.get('content-disposition')) ?? fallbackName,
			truncated,
		};
	}

	// --- identity --------------------------------------------------------

	me(): Promise<MeResponse> {
		return this.getJson<MeResponse>('/api/v3/me');
	}

	/**
	 * `/me` as another token sees it, leaving the token in use untouched — how a
	 * replacement is checked before it is swapped in.
	 */
	async meAs(token: string): Promise<MeResponse> {
		const response = await this.request(this.url('/api/v3/me'), 'application/json', 'bearer', { token });
		return (await response.json()) as MeResponse;
	}

	// --- session ---------------------------------------------------------

	/**
	 * Extends the current session and reports its remaining budget.
	 *
	 * **The only non-GET request in this server, and deliberately so.** It is
	 * what the web UI's "Sitzung verlängern" button calls. It takes no body,
	 * touches nothing but the caller's own session, and cannot read or change
	 * any user data — so it does not weaken the property that matters: nobody
	 * reaching this server can act as the user inside Schulcloud. No MCP tool
	 * exposes it, so Claude can never cause a POST; only the keepalive calls it.
	 *
	 * Using a plain GET here was tried and does not work — see docs/AUTH.md for
	 * the endurance test that ruled it out.
	 */
	async extendSession(): Promise<{ expiresInSeconds: number }> {
		const url = this.url('/api/v3/authentication/refresh-session');
		const response = await fetch(url, {
			method: 'POST',
			headers: {
				Authorization: `Bearer ${this.config.jwt}`,
				Accept: 'application/json',
				'Content-Length': '0',
			},
			signal: AbortSignal.timeout(this.config.requestTimeoutMs),
		});
		if (!response.ok) {
			const body = await response.text().catch(() => '');
			throw new SchulcloudApiError(response.status, url.pathname, body);
		}
		return (await response.json()) as { expiresInSeconds: number };
	}

	// --- courses and the classic course board ----------------------------

	listCourses(params: { skip?: number; limit?: number } = {}): Promise<Paginated<CourseMetadata>> {
		return this.getJson<Paginated<CourseMetadata>>('/api/v3/courses', {
			skip: params.skip,
			limit: clampPageSize(params.limit),
		});
	}

	/** Every course the account can see, paging past the API's per-page ceiling. */
	listAllCourses(max = 500): Promise<CourseMetadata[]> {
		return collectPages((skip, limit) => this.listCourses({ skip, limit }), max);
	}

	/**
	 * The contents of one course, as the course page shows them: lessons,
	 * tasks and column boards interleaved. The route is `course-rooms`, and
	 * its `:roomId` is the *course* id.
	 */
	getCourseBoard(courseId: string): Promise<CourseBoardResponse> {
		return this.getJson<CourseBoardResponse>(`/api/v3/course-rooms/${encodeURIComponent(courseId)}/board`);
	}

	getDashboard(): Promise<DashboardResponse> {
		return this.getJson<DashboardResponse>('/api/v3/dashboard');
	}

	// --- tasks -----------------------------------------------------------

	listTasks(params: { skip?: number; limit?: number } = {}): Promise<Paginated<TaskContent>> {
		return this.getJson<Paginated<TaskContent>>('/api/v3/tasks', {
			skip: params.skip,
			limit: clampPageSize(params.limit),
		});
	}

	listFinishedTasks(params: { skip?: number; limit?: number } = {}): Promise<Paginated<TaskContent>> {
		return this.getJson<Paginated<TaskContent>>('/api/v3/tasks/finished', {
			skip: params.skip,
			limit: clampPageSize(params.limit),
		});
	}

	// --- submissions -----------------------------------------------------

	/**
	 * Submission statuses for one task.
	 *
	 * The only way to obtain a submission id: there is no `GET /submissions` and
	 * no `GET /submissions/{id}`. What comes back depends on the account — a
	 * student sees their own submission, a teacher sees the whole class's.
	 */
	async listSubmissionStatuses(taskId: string): Promise<SubmissionStatus[]> {
		const page = await this.getJson<{ data: SubmissionStatus[] }>(
			`/api/v3/submissions/status/task/${encodeURIComponent(taskId)}`,
		);
		return page.data ?? [];
	}

	// --- lessons ---------------------------------------------------------

	getLesson(lessonId: string): Promise<LessonResponse> {
		return this.getJson<LessonResponse>(`/api/v3/lessons/${encodeURIComponent(lessonId)}`);
	}

	/**
	 * A lesson's tasks.
	 *
	 * Returns a bare array, not the `{data, total}` envelope every other list
	 * endpoint uses — checked against both the live instance and a local 33.40.
	 * Typing it as `Paginated` made `.data` undefined, which silently dropped
	 * every task attached to a topic: they vanished from get_lesson, get_task
	 * reported them as non-existent, and their submissions — grades included —
	 * could not be reached at all. The envelope branch is kept in case the
	 * endpoint is ever normalised to match its siblings.
	 */
	async getLessonTasks(lessonId: string): Promise<LessonLinkedTask[]> {
		const body = await this.getJson<LessonLinkedTask[] | Paginated<LessonLinkedTask>>(
			`/api/v3/lessons/${encodeURIComponent(lessonId)}/tasks`,
		);
		return Array.isArray(body) ? body : (body.data ?? []);
	}

	// --- rooms ------------------------------------------------------------

	/**
	 * The rooms this account belongs to.
	 *
	 * Returns `{data}` with no `total` — not the usual paginated envelope. The
	 * server derives the list from actual room memberships, so an empty result
	 * means exactly that, and a room a teacher revoked access to simply stops
	 * appearing.
	 */
	async listRooms(): Promise<RoomItem[]> {
		const body = await this.getJson<{ data?: RoomItem[] }>('/api/v3/rooms');
		return body.data ?? [];
	}

	getRoom(roomId: string): Promise<RoomDetails> {
		return this.getJson<RoomDetails>(`/api/v3/rooms/${encodeURIComponent(roomId)}`);
	}

	async listRoomBoards(roomId: string): Promise<RoomBoardItem[]> {
		const body = await this.getJson<Paginated<RoomBoardItem>>(
			`/api/v3/rooms/${encodeURIComponent(roomId)}/boards`,
		);
		return body.data ?? [];
	}

	/**
	 * A room's members. Needs no special permission for a member to see who else
	 * is in the room, but it can still be refused — callers treat that as "not
	 * available" rather than as an error worth surfacing.
	 */
	async listRoomMembers(roomId: string): Promise<RoomMember[]> {
		const body = await this.getJson<{ data?: RoomMember[] }>(
			`/api/v3/rooms/${encodeURIComponent(roomId)}/members`,
		);
		return body.data ?? [];
	}

	/**
	 * People waiting to be let into a room, and the room's invitation links.
	 *
	 * Both are room-admin surface: a viewer gets 403, which is ordinary rather
	 * than exceptional. `allowedOperations` on the room says which of these the
	 * account may ask for, so callers can skip the ones that would be refused.
	 */
	async listRoomApplicants(roomId: string): Promise<RoomApplicant[]> {
		const body = await this.getJson<{ data?: RoomApplicant[] }>(
			`/api/v3/rooms/${encodeURIComponent(roomId)}/applicants`,
		);
		return body.data ?? [];
	}

	async listRoomInvitationLinks(roomId: string): Promise<RoomInvitationLink[]> {
		const body = await this.getJson<{ data?: RoomInvitationLink[] }>(
			`/api/v3/rooms/${encodeURIComponent(roomId)}/room-invitation-links`,
		);
		return body.data ?? [];
	}

	// --- column boards ---------------------------------------------------

	getBoardSkeleton(boardId: string): Promise<BoardSkeleton> {
		return this.getJson<BoardSkeleton>(`/api/v3/boards/${encodeURIComponent(boardId)}`);
	}

	getBoardContext(boardId: string): Promise<BoardContext> {
		return this.getJson<BoardContext>(`/api/v3/boards/${encodeURIComponent(boardId)}/context`);
	}

	/**
	 * The content behind an H5P element: a quiz with all of its questions.
	 *
	 * `params` is what the player loads before it renders anything, so one GET
	 * returns the whole exercise — every question, every answer option and which
	 * of them is correct — even though the player then shows one question at a
	 * time. There is nothing to step through and no page to scrape.
	 *
	 * The shape belongs to whichever H5P library the content uses, so it stays
	 * `unknown` here and `core/h5p.ts` interprets it.
	 */
	getH5pParams(contentId: string): Promise<unknown> {
		return this.getJson<unknown>(`/api/v3/h5p-editor/params/${encodeURIComponent(contentId)}`);
	}

	/**
	 * Card bodies for the given ids.
	 *
	 * **Never request more than 20 at once.** Express/NestJS parse the query
	 * string with `qs`, whose default `arrayLimit` is 20: past that, repeated
	 * `ids=` params stop becoming an array and become an object keyed `"0"`,
	 * `"1"`, … `@IsMongoId({ each: true })` then iterates something that is not
	 * an array and rejects every value, so the API answers
	 * `"each value in ids must be a mongodb id"` — blaming the ids when the real
	 * problem is how many there are. Verified against the live instance: 20 ids
	 * return 200, 21 return 400, with identical ids.
	 *
	 * Nothing in the controller or its DTO says this; the limit lives in the
	 * query parser underneath them.
	 */
	async getCards(cardIds: string[]): Promise<CardResponse[]> {
		const CHUNK = MAX_IDS_PER_QUERY;
		const out: CardResponse[] = [];
		for (let i = 0; i < cardIds.length; i += CHUNK) {
			const chunk = cardIds.slice(i, i + CHUNK);
			const page = await this.getJson<{ data: CardResponse[] }>('/api/v3/cards', { ids: chunk });
			out.push(...page.data);
		}
		return out;
	}

	// --- files -----------------------------------------------------------

	/**
	 * Files attached to one parent entity.
	 *
	 * `storageLocationId` is the school id for `storageLocation: 'school'`,
	 * which is what every parent type in normal use resolves to. Board file
	 * elements are addressed with `parentType: 'boardnodes'` and the *element*
	 * id as `parentId`.
	 */
	listFiles(args: {
		storageLocationId: string;
		parentType: FileParentType;
		parentId: string;
		storageLocation?: 'school' | 'instance';
	}): Promise<Paginated<FileRecord>> {
		const location = args.storageLocation ?? 'school';
		const path =
			`/api/v3/file/list/${location}/${encodeURIComponent(args.storageLocationId)}` +
			`/${args.parentType}/${encodeURIComponent(args.parentId)}`;
		return this.getJson<Paginated<FileRecord>>(path);
	}

	getFileRecord(fileRecordId: string): Promise<FileRecord> {
		return this.getJson<FileRecord>(`/api/v3/file/${encodeURIComponent(fileRecordId)}`);
	}

	downloadFile(record: Pick<FileRecord, 'id' | 'name'>): Promise<DownloadedFile> {
		const path = `/api/v3/file/download/${encodeURIComponent(record.id)}/${encodeURIComponent(record.name)}`;
		return this.getBytes(path, record.name);
	}

	// --- misc ------------------------------------------------------------

	listNews(params: { skip?: number; limit?: number } = {}): Promise<Paginated<NewsResponse>> {
		return this.getJson<Paginated<NewsResponse>>('/api/v3/news', {
			skip: params.skip,
			limit: clampPageSize(params.limit),
		});
	}

	/** File count and total bytes under one parent, without listing the records. */
	getParentFileStats(parentType: FileParentType, parentId: string): Promise<ParentFileStats> {
		return this.getJson<ParentFileStats>(
			`/api/v3/file/stats/${parentType}/${encodeURIComponent(parentId)}`,
		);
	}

	/**
	 * A rasterised preview of one file.
	 *
	 * The reason this exists: many course PDFs are image-only scans, so text
	 * extraction legitimately yields nothing and their contents are otherwise
	 * unreadable. A preview is a picture of the page, which Claude can read
	 * directly. Only meaningful when the record's `previewStatus` is
	 * `preview_possible`; anything else 404s or returns the placeholder.
	 */
	async getFilePreview(
		record: Pick<FileRecord, 'id' | 'name'>,
		width?: PreviewWidth,
	): Promise<DownloadedFile> {
		// Two traps here, both of which answer with a 400 that names the value but
		// not the permitted set:
		//  - `width` is an enum (50 | 150 | 500), not a free number;
		//  - `outputFormat` accepts only `image/webp`. Omitting it is worse than
		//    wrong: the preview is then rendered in the *source* format, so a PDF
		//    comes back as a PDF and the whole point — a picture of the page — is
		//    lost.
		const query = new URLSearchParams({ outputFormat: PREVIEW_OUTPUT_FORMAT });
		if (width) query.set('width', String(width));
		const path =
			`/api/v3/file/preview/${encodeURIComponent(record.id)}/${encodeURIComponent(record.name)}` +
			`?${query.toString()}`;
		const file = await this.getBytes(path, record.name);

		// The response labels itself `webp` rather than `image/webp`, which no
		// image consumer would accept. We asked for the format, so we know it.
		return file.mimeType.startsWith('image/') ? file : { ...file, mimeType: PREVIEW_OUTPUT_FORMAT };
	}

	// --- legacy /api/v1 ---------------------------------------------------
	//
	// Exactly three legacy routes survive in the deployment's ingress table
	// (dof_app_deploy .../all/x_ingress.yml): courses, users and classes. They
	// are production surface, not a leftover — the table even notes why each
	// one is still needed. Everything else under /api/v1 is unrouted and 404s,
	// so do not reach for it.

	/** One course with the fields v3 drops: description, members, timetable. */
	getLegacyCourse(courseId: string): Promise<LegacyCourse> {
		return this.getJson<LegacyCourse>(`/api/v1/courses/${encodeURIComponent(courseId)}`);
	}

	/**
	 * One user's name.
	 *
	 * The only id-to-name mapping available: submission `submitters`, file
	 * `creatorId` and course `teacherIds` are all bare ids, and no v3 route
	 * resolves them for a non-admin.
	 */
	getLegacyUser(userId: string): Promise<LegacyUser> {
		return this.getJson<LegacyUser>(`/api/v1/users/${encodeURIComponent(userId)}`);
	}

	// --- groups and classes ------------------------------------------------

	/** Classes ("Klassen") this account belongs to, with teacher names. */
	async listClasses(): Promise<ClassItem[]> {
		const body = await this.getJson<Paginated<ClassItem>>('/api/v3/groups/class', { limit: MAX_PAGE_SIZE });
		return body.data ?? [];
	}

	/** Groups this account belongs to — room membership groups, classes, courses. */
	async listGroups(): Promise<GroupItem[]> {
		const body = await this.getJson<Paginated<GroupItem>>('/api/v3/groups', { limit: MAX_PAGE_SIZE });
		return body.data ?? [];
	}

	// --- the "Dateien" file manager ------------------------------------------
	//
	// Persönliche Dateien, Kurs-Dateien, Team-Dateien and Geteilte Dateien live in
	// the legacy file system, a different store from files-storage: listing a
	// course through /api/v3/file answers 0 files for a course holding dozens.
	// Its Feathers service is not in the public ingress, so the only way in is
	// the legacy client — HTML pages for listings, one JSON route for downloads.
	// See core/legacy-files.ts for the parsing and the path model.

	/**
	 * One file-manager page, as HTML.
	 *
	 * **Only the listing routes are reachable here, by construction.** Several of
	 * the legacy client's GET routes write: `GET /files/share/` mints a share
	 * token when the file has none, and `GET /files/file?share=…` grants the
	 * caller a permission on someone else's file. A GET-only client is therefore
	 * not read-only against this surface by itself; the allowlist is what makes
	 * the invariant hold.
	 */
	async getFileManagerPage(path: string): Promise<string> {
		if (!FILE_MANAGER_PAGE.test(path)) {
			throw new Error(`refusing file-manager path outside the listing routes: ${path}`);
		}
		const response = await this.legacyRequest(path, 'text/html');
		return response.text();
	}

	/**
	 * A pre-signed download URL for one legacy file.
	 *
	 * `name` only sets the download's filename; the server checks read access on
	 * the id. The route is `/files/signedurl` rather than `/files/file`, which
	 * answers the same thing as a redirect but also accepts `share`, the
	 * parameter that writes.
	 */
	async getFileManagerSignedUrl(fileId: string, name: string): Promise<string> {
		if (!/^[0-9a-f]{24}$/i.test(fileId)) throw new Error(`not a file id: ${fileId}`);
		const query = new URLSearchParams({ file: fileId, name: name || fileId });
		const response = await this.legacyRequest(`/files/signedurl?${query.toString()}`, 'application/json');
		// The server's error path *returns* its Forbidden rather than throwing it,
		// so a refused file arrives as a 200 whose body has no url.
		const body = (await response.json().catch(() => ({}))) as { url?: unknown; message?: unknown };
		if (typeof body.url !== 'string' || !body.url) {
			throw new SchulcloudApiError(403, '/files/signedurl', typeof body.message === 'string' ? body.message : 'no download url');
		}
		return body.url;
	}

	/** Downloads one legacy file: signed URL, then the bytes, capped like every download. */
	async downloadFileManagerFile(fileId: string, name: string): Promise<DownloadedFile> {
		const signed = await this.getFileManagerSignedUrl(fileId, name);
		const response = await this.openSignedUrl(signed);
		return this.readCapped(response, name);
	}

	/**
	 * Opens a pre-signed storage URL — with no credentials at all.
	 *
	 * The URL names another host (live: an S3 endpoint at the storage provider),
	 * so neither the bearer nor the cookie may go with it. It must also be
	 * https whenever the instance is, which keeps a URL the server hands back from
	 * pointing this process at a plaintext service on its own network.
	 */
	async openSignedUrl(signedUrl: string): Promise<Response> {
		const target = checkSignedUrl(signedUrl, this.config.baseUrl);
		return this.request(target, '*/*', 'none', { idleTimeout: true });
	}

	private async legacyRequest(path: string, accept: string): Promise<Response> {
		try {
			return await this.request(this.url(path), accept, 'cookie');
		} catch (error) {
			// The legacy client answers a rejected cookie with a redirect to its
			// login page. Report it as what it is, so tools say "token expired"
			// rather than "HTTP 302".
			if (error instanceof SchulcloudApiError && error.status >= 300 && error.status < 400) {
				throw new SchulcloudApiError(401, path, 'redirected to login: the session is not accepted');
			}
			throw error;
		}
	}
}

/**
 * A timeout that measures silence rather than total time, for downloads.
 *
 * The request timeout is right for an API call and wrong for a file: it bounds
 * the whole transfer, so an 11 MB scan from a slow storage host was cut off at
 * 30 seconds while its bytes were still arriving — four files on the live
 * account, recorded as failures. Here the clock starts over with every chunk,
 * so only a transfer that stalls is abandoned.
 */
function idleDeadline(ms: number) {
	const controller = new AbortController();
	const expire = () => controller.abort(new Error(`no data received for ${Math.round(ms / 1000)}s`));
	let timer = setTimeout(expire, ms);
	// Never the reason a process stays alive: a caller that stops reading early
	// (the download cap) leaves the last timer behind.
	timer.unref();
	const rearm = () => {
		clearTimeout(timer);
		timer = setTimeout(expire, ms);
		timer.unref();
	};
	const stop = () => clearTimeout(timer);
	const watch = (response: Response): Response => {
		if (!response.body) {
			stop();
			return response;
		}
		rearm();
		const body = response.body.pipeThrough(
			new TransformStream<Uint8Array, Uint8Array>({
				transform(chunk, output) {
					rearm();
					output.enqueue(chunk);
				},
				flush() {
					stop();
				},
			}),
		);
		return new Response(body, { status: response.status, statusText: response.statusText, headers: response.headers });
	};
	return { signal: controller.signal, stop, watch };
}

/**
 * The file-manager listing routes, and nothing else.
 *
 * Folders are addressed by id alone — `/files/courses/{course}/{folder}` holds
 * one folder segment however deep the folder is — so every listing fits one of
 * these shapes. `/files/my/{a}/{b}` exists too, but lists `b` exactly as
 * `/files/my/{b}` does, so it is not needed.
 */
const FILE_MANAGER_PAGE =
	/^\/files\/(?:(?:my|courses|teams|shared)\/|my\/[0-9a-f]{24}|(?:courses|teams)\/[0-9a-f]{24}(?:\/[0-9a-f]{24})?)$/i;

/** Validates a pre-signed URL before anything is sent to it. Exported for testing. */
export function checkSignedUrl(signedUrl: string, baseUrl: string): URL {
	let target: URL;
	try {
		target = new URL(signedUrl);
	} catch {
		throw new Error('the download url the server returned is not a url');
	}
	const instanceIsHttps = new URL(baseUrl).protocol === 'https:';
	const allowed = instanceIsHttps ? ['https:'] : ['https:', 'http:'];
	if (!allowed.includes(target.protocol)) {
		throw new Error(`refusing a ${target.protocol} download url from an ${instanceIsHttps ? 'https' : 'http'} instance`);
	}
	if (target.username || target.password) {
		throw new Error('refusing a download url that carries credentials');
	}
	return target;
}

/**
 * The list endpoints reject `limit` above 100 and document a maximum of 99, so
 * page at 99 and let `collectPages` stitch the results back together.
 */
export const MAX_PAGE_SIZE = 99;

function clampPageSize(limit: number | undefined): number | undefined {
	if (limit === undefined) return undefined;
	return Math.min(Math.max(1, Math.trunc(limit)), MAX_PAGE_SIZE);
}

/** Follows `skip`/`limit` paging until `max` items or the server runs out. */
async function collectPages<T>(
	fetchPage: (skip: number, limit: number) => Promise<Paginated<T>>,
	max: number,
): Promise<T[]> {
	const items: T[] = [];
	let skip = 0;
	while (items.length < max) {
		const page = await fetchPage(skip, Math.min(MAX_PAGE_SIZE, max - items.length));
		items.push(...page.data);
		skip += page.data.length;
		// Stop on an empty page too, so a server that ignores `skip` can't loop forever.
		if (page.data.length === 0 || skip >= page.total) break;
	}
	return items.slice(0, max);
}

function filenameFromDisposition(header: string | null): string | undefined {
	if (!header) return undefined;
	// Prefer RFC 5987 `filename*`, which carries the encoding explicitly.
	const extended = /filename\*=(?:UTF-8|utf-8)''([^;]+)/.exec(header);
	if (extended?.[1]) return safeDecode(extended[1].trim());
	const plain = /filename="?([^";]+)"?/.exec(header);
	if (plain?.[1]) return safeDecode(plain[1].trim());
	return undefined;
}

function safeDecode(value: string): string {
	try {
		return decodeURIComponent(value);
	} catch {
		return value;
	}
}