import { readdir, readFile, stat, mkdir, writeFile, appendFile } from 'node:fs/promises';
import { dirname } from 'node:path';
import { schoolToday } from './dates.ts';
import { resolveWithin, safeComponent } from './paths.ts';

/**
 * The user's own lesson notes: a directory of Markdown files.
 *
 * This is the one store here that Schulcloud and WebUntis know nothing about —
 * what the person in the room wrote down. It exists because the two upstreams
 * between them still do not answer "what did the teacher actually say", and a
 * note taken in the lesson is often the only record of it.
 *
 * **Plain files, not a table.** The notes have to be writable from a phone in a
 * classroom and readable when Postgres is down, so the files are the truth and
 * the index is only a view of them — the same split as `file_texts` and the
 * mirror. It also makes migrating in a pile of exported Apple Notes a matter of
 * writing files, and makes the whole store greppable, diffable and syncable by
 * anything the user already runs.
 *
 * Frontmatter is a deliberately small YAML subset (scalars and inline lists),
 * parsed here rather than by a dependency: notes are hand-written, so a strict
 * parser that rejects a file is worse than a lax one that keeps the body. A
 * file with no frontmatter at all is a valid note.
 */

/** Extensions treated as notes. Anything else in the directory is ignored. */
const NOTE_EXTENSIONS = ['.md', '.markdown', '.txt'];

/**
 * Caps. A notes directory is user-controlled, but it may also be a synced
 * folder that has just acquired somebody's 400 MB export, and a crawl must not
 * turn that into an out-of-memory.
 */
const MAX_NOTE_BYTES = 512 * 1024;
const MAX_NOTES = 5_000;
const MAX_DEPTH = 8;

export interface NoteDoc {
	/**
	 * Path relative to the notes root — `Deutsch/2026-09-15 Erörterung.md`.
	 * This is the note's id: there is no other, and it is what `get_note` takes.
	 */
	path: string;
	title: string;
	/** The school day the note belongs to, `YYYY-MM-DD`, when it could be determined. */
	date?: string;
	/** Free text as the user writes it — "Deutsch", "LF07". Not a Schulcloud id. */
	subject?: string;
	/** A Schulcloud course id, when the note names one, so search can group by course. */
	courseId?: string;
	tags: string[];
	/** Where the note came from: `apple-notes`, `add_note`, or absent for a hand-written file. */
	source?: string;
	/** The body, without the frontmatter block. */
	text: string;
	/** Last write to the file, ISO. Not the lesson date — see `date` for that. */
	modifiedAt: string;
	bytes: number;
}

export interface NoteFrontmatter {
	title?: string;
	date?: string;
	subject?: string;
	courseId?: string;
	tags?: string[];
	source?: string;
	/** Anything else the file carried, preserved so a round trip loses nothing. */
	extra?: Record<string, string>;
}

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

/**
 * Every note under `root`, newest lesson first.
 *
 * Never throws for a missing root: a notes directory that has not been created
 * yet is an empty one, and the tools say so far better than a crawl that dies.
 */
export async function readNotes(root: string): Promise<NoteDoc[]> {
	const paths = await listNotePaths(root);
	const notes: NoteDoc[] = [];
	for (const relative of paths) {
		const note = await readNoteAt(root, relative).catch(() => undefined);
		if (note) notes.push(note);
	}
	return notes.sort(byNewest);
}

/** Relative paths of the note files under `root`, sorted for a stable order. */
export async function listNotePaths(root: string): Promise<string[]> {
	const found: string[] = [];

	const walk = async (relative: string, depth: number): Promise<void> => {
		if (depth > MAX_DEPTH || found.length >= MAX_NOTES) return;
		const absolute = relative ? resolveWithin(root, relative) : root;
		let entries;
		try {
			entries = await readdir(absolute, { withFileTypes: true });
		} catch {
			// A root that does not exist yet, or a folder we may not read: an
			// unreadable corner must not cost the notes that are readable.
			return;
		}
		for (const entry of entries.sort((a, b) => a.name.localeCompare(b.name))) {
			// Dotfiles are the sync tools' own business (.obsidian, .git, .stfolder)
			// and never a note.
			if (entry.name.startsWith('.')) continue;
			const child = relative ? `${relative}/${entry.name}` : entry.name;
			if (entry.isDirectory()) await walk(child, depth + 1);
			else if (isNoteFile(entry.name) && found.length < MAX_NOTES) found.push(child);
		}
	};

	await walk('', 0);
	return found;
}

/** One note by its relative path. Throws `NoteNotFound` when there is none. */
export async function readNoteAt(root: string, relative: string): Promise<NoteDoc> {
	const absolute = resolveWithin(root, normalizeRelative(relative));
	let info;
	try {
		info = await stat(absolute);
	} catch {
		throw new NoteNotFound(relative);
	}
	if (!info.isFile()) throw new NoteNotFound(relative);
	if (info.size > MAX_NOTE_BYTES) {
		throw new Error(
			`Note ${relative} is ${Math.round(info.size / 1024)} KB, past the ${MAX_NOTE_BYTES / 1024} KB limit for a note.`,
		);
	}
	const raw = await readFile(absolute, 'utf8');
	return parseNote(normalizeRelative(relative), raw, { modifiedAt: info.mtime.toISOString(), bytes: info.size });
}

export class NoteNotFound extends Error {
	readonly path: string;

	constructor(path: string) {
		super(`No note at "${path}".`);
		this.name = 'NoteNotFound';
		this.path = path;
	}
}

/**
 * A file's text as a note.
 *
 * Pure, so the whole frontmatter/title/date story is testable without a disk.
 */
export function parseNote(
	relative: string,
	raw: string,
	stamp: { modifiedAt: string; bytes: number },
): NoteDoc {
	const { front, body } = splitFrontmatter(raw);
	const fileName = relative.split('/').pop() ?? relative;
	return {
		path: relative,
		title: front.title || headingTitle(body) || titleFromFileName(fileName),
		// Frontmatter first, then a date the filename starts with. Never the
		// file's mtime: an import writes every note today, and dating a year of
		// lessons "today" would make the whole store useless for "what did we do
		// before the test".
		...pick('date', front.date ?? dateFromFileName(fileName)),
		...pick('subject', front.subject ?? subjectFromPath(relative)),
		...pick('courseId', front.courseId),
		...pick('source', front.source),
		tags: front.tags ?? [],
		text: body.trim(),
		modifiedAt: stamp.modifiedAt,
		bytes: stamp.bytes,
	};
}

/**
 * Splits `---\nkey: value\n---\n` off the front.
 *
 * Only a leading block counts, and only when it closes: a note that happens to
 * begin with a horizontal rule keeps its text rather than losing half of it.
 */
export function splitFrontmatter(raw: string): { front: NoteFrontmatter; body: string } {
	const text = raw.replace(/^\ufeff/, '');
	const open = /^---[ \t]*\r?\n/.exec(text);
	if (!open) return { front: {}, body: text };
	const close = /\r?\n---[ \t]*(\r?\n|$)/.exec(text.slice(open[0].length - 1));
	if (!close) return { front: {}, body: text };

	const end = open[0].length - 1 + close.index;
	const block = text.slice(open[0].length, end);
	const rest = text.slice(end + close[0].length);

	const front: NoteFrontmatter = {};
	const extra: Record<string, string> = {};
	for (const line of block.split(/\r?\n/)) {
		const match = /^([A-Za-z][A-Za-z0-9_-]*)\s*:\s*(.*)$/.exec(line.trim());
		if (!match) continue;
		const key = match[1]!.toLowerCase();
		const value = unquote(match[2]!.trim());
		if (!value) continue;
		switch (key) {
			case 'title':
				front.title = value;
				break;
			case 'date':
				front.date = normalizeDate(value);
				break;
			case 'subject':
			case 'fach':
				front.subject = value;
				break;
			case 'courseid':
			case 'course':
				front.courseId = value;
				break;
			case 'source':
				front.source = value;
				break;
			case 'tags':
				front.tags = parseList(value);
				break;
			default:
				extra[key] = value;
		}
	}
	if (Object.keys(extra).length > 0) front.extra = extra;
	return { front, body: rest };
}

// --- writing -------------------------------------------------------------

export interface NoteInput {
	title: string;
	text: string;
	/** The lesson's day. Defaults to today in the school's timezone. */
	date?: string;
	subject?: string;
	courseId?: string;
	tags?: string[];
	source?: string;
	/** Write here instead of deriving a path from subject, date and title. */
	path?: string;
	/**
	 * Add to the note at that path if it already exists, rather than creating a
	 * second one. This is what makes a lesson's notes accumulate in one file as
	 * they are taken, which is how anyone actually takes them.
	 */
	append?: boolean;
}

/**
 * Creates a note, or appends to one.
 *
 * Every component of the path goes through `safeComponent`: the title and
 * subject arrive from a tool call, so they are untrusted input that becomes a
 * filename, exactly as course titles do in the mirror.
 */
export async function writeNote(root: string, input: NoteInput): Promise<{ note: NoteDoc; appended: boolean }> {
	const date = input.date ?? schoolToday();
	// Appending is about a *lesson*, not about a title: "note this down too" in
	// the middle of Tuesday's German lesson means the note already open for
	// Tuesday and German, whatever it happens to be called. Deriving the path
	// from the new title instead would start a second note every time, which is
	// the one thing append exists to prevent.
	const relative = input.path
		? normalizeRelative(input.path)
		: ((input.append ? await noteForLesson(root, date, input.subject) : undefined) ??
			notePathFor({ date, subject: input.subject, title: input.title }));
	const absolute = resolveWithin(root, relative);

	const existing = await stat(absolute).then(
		() => true,
		() => false,
	);

	if (existing && input.append) {
		// A heading rather than a bare paragraph, so a note built from four
		// appends still reads as four things and not as one run-on.
		await appendFile(absolute, `\n\n## ${input.title}\n\n${input.text.trim()}\n`, 'utf8');
		return { note: await readNoteAt(root, relative), appended: true };
	}

	// Never overwrite: a note is the only copy of what someone wrote down, and a
	// second note with the same title on the same day is a normal thing to have.
	const target = existing ? await freePath(root, relative) : relative;
	await mkdir(dirname(resolveWithin(root, target)), { recursive: true });
	await writeFile(
		resolveWithin(root, target),
		renderNote(
			{
				title: input.title,
				date,
				...pick('subject', input.subject),
				...pick('courseId', input.courseId),
				...pick('source', input.source),
				...(input.tags && input.tags.length > 0 ? { tags: input.tags } : {}),
			},
			input.text,
		),
		'utf8',
	);
	return { note: await readNoteAt(root, target), appended: false };
}

/** A note as it is stored: frontmatter, then the body. */
export function renderNote(front: NoteFrontmatter, body: string): string {
	const lines = [
		front.title !== undefined && `title: ${quote(front.title)}`,
		front.date !== undefined && `date: ${front.date}`,
		front.subject !== undefined && `subject: ${quote(front.subject)}`,
		front.courseId !== undefined && `courseId: ${front.courseId}`,
		front.tags && front.tags.length > 0 && `tags: [${front.tags.map((tag) => quote(tag)).join(', ')}]`,
		front.source !== undefined && `source: ${quote(front.source)}`,
		...Object.entries(front.extra ?? {}).map(([key, value]) => `${key}: ${quote(value)}`),
	].filter((line): line is string => typeof line === 'string');
	return `---\n${lines.join('\n')}\n---\n\n${body.trim()}\n`;
}

/**
 * Where a new note goes: `Deutsch/2026-09-15 Erörterung.md`.
 *
 * Subject-first because that is how anyone looks for a note by hand, and the
 * date leads the filename so a folder sorts chronologically in every file
 * browser there is.
 */
export function notePathFor(input: { date: string; subject?: string; title: string }): string {
	const folder = safeComponent(input.subject ?? 'Allgemein', 'Allgemein');
	const name = safeComponent(`${input.date} ${input.title}`, input.date);
	return `${folder}/${name}.md`;
}

/**
 * The note already written for this day and subject, if there is one.
 *
 * The newest by path, so a day that somehow grew two notes still gets the one
 * a person would reach for.
 */
async function noteForLesson(root: string, date: string, subject: string | undefined): Promise<string | undefined> {
	const wanted = subject?.trim().toLowerCase();
	const candidates = (await readNotes(root)).filter(
		(note) => note.date === date && (note.subject ?? '').toLowerCase() === (wanted ?? ''),
	);
	return candidates[0]?.path;
}

/** `note.md` → `note 2.md`, for the day someone titles two notes the same. */
async function freePath(root: string, relative: string): Promise<string> {
	const dot = relative.lastIndexOf('.');
	const stem = dot > 0 ? relative.slice(0, dot) : relative;
	const ext = dot > 0 ? relative.slice(dot) : '';
	for (let n = 2; n < 100; n++) {
		const candidate = `${stem} ${n}${ext}`;
		const taken = await stat(resolveWithin(root, candidate)).then(
			() => true,
			() => false,
		);
		if (!taken) return candidate;
	}
	throw new Error(`Too many notes named like ${relative}.`);
}

// --- matching ------------------------------------------------------------

/** Everything about a note that search should look at, as one string. */
export function noteSearchText(note: NoteDoc): string {
	return [note.title, note.subject, note.tags.join(' '), note.text].filter(Boolean).join('\n');
}

/** Filters a list the way `list_notes` does. Pure, and shared with the CLI. */
export function filterNotes(
	notes: NoteDoc[],
	filter: { subject?: string; since?: string; until?: string; courseId?: string },
): NoteDoc[] {
	const subject = filter.subject?.trim().toLowerCase();
	return notes.filter((note) => {
		if (subject && !(note.subject ?? '').toLowerCase().includes(subject)) return false;
		if (filter.courseId && note.courseId !== filter.courseId) return false;
		// A note with no date cannot be excluded by a date window without
		// silently hiding it; undated notes always pass.
		if (filter.since && note.date && note.date < filter.since) return false;
		if (filter.until && note.date && note.date > filter.until) return false;
		return true;
	});
}

// --- helpers -------------------------------------------------------------

function isNoteFile(name: string): boolean {
	const lower = name.toLowerCase();
	return NOTE_EXTENSIONS.some((extension) => lower.endsWith(extension));
}

/**
 * A caller's path in the one form the rest of this module uses.
 *
 * Leading slashes and backslashes are accepted and normalised because people
 * paste `/Deutsch/…` from a listing; traversal is not — `resolveWithin` refuses
 * it, and this must not quietly make it look legal first.
 */
export function normalizeRelative(path: string): string {
	return path.replace(/\\/g, '/').replace(/^\/+/, '').replace(/\/+/g, '/').trim();
}

function byNewest(a: NoteDoc, b: NoteDoc): number {
	// Undated notes sort last: they are usually imports that never carried a
	// date, and they should not head a list of "the last few lessons".
	if (a.date && b.date && a.date !== b.date) return b.date.localeCompare(a.date);
	if (a.date && !b.date) return -1;
	if (!a.date && b.date) return 1;
	return b.modifiedAt.localeCompare(a.modifiedAt) || a.path.localeCompare(b.path);
}

function pick<K extends string>(key: K, value: string | undefined): Partial<Record<K, string>> {
	return value ? ({ [key]: value } as Record<K, string>) : {};
}

function headingTitle(body: string): string | undefined {
	const match = /^\s*#\s+(.+)$/m.exec(body);
	return match?.[1]?.trim();
}

function titleFromFileName(fileName: string): string {
	const withoutExtension = fileName.replace(/\.(md|markdown|txt)$/i, '');
	return withoutExtension.replace(/^\d{4}-\d{2}-\d{2}[ _-]*/, '').trim() || withoutExtension;
}

function dateFromFileName(fileName: string): string | undefined {
	const match = /^(\d{4})-(\d{2})-(\d{2})/.exec(fileName);
	return match ? `${match[1]}-${match[2]}-${match[3]}` : undefined;
}

/** The first folder is the subject, by the layout `notePathFor` writes. */
function subjectFromPath(relative: string): string | undefined {
	const parts = relative.split('/');
	return parts.length > 1 ? parts[0] : undefined;
}

/** `15.09.2026` and `2026-09-15T08:00:00Z` both mean the same school day. */
function normalizeDate(value: string): string | undefined {
	const german = /^(\d{1,2})\.(\d{1,2})\.(\d{4})$/.exec(value);
	if (german) return `${german[3]}-${german[2]!.padStart(2, '0')}-${german[1]!.padStart(2, '0')}`;
	const iso = /^(\d{4}-\d{2}-\d{2})/.exec(value);
	return iso ? iso[1] : undefined;
}

function parseList(value: string): string[] {
	const inner = /^\[(.*)\]$/.exec(value)?.[1] ?? value;
	return inner
		.split(',')
		.map((entry) => unquote(entry.trim()))
		.filter(Boolean);
}

function unquote(value: string): string {
	const match = /^(['"])(.*)\1$/.exec(value);
	return match ? match[2]! : value;
}

/** Quotes only when the value would otherwise change meaning on the way back in. */
function quote(value: string): string {
	const clean = value.replace(/[\r\n]+/g, ' ').trim();
	return /^[\w äöüÄÖÜß.,/()+-]+$/.test(clean) && !/^\[/.test(clean) ? clean : JSON.stringify(clean);
}