/**
 * Best-effort calendar date guessed from a photo's filename / parent
 * folders, with a confidence label so the UI can warn the user when
 * the day is fabricated.
 *
 * Conventions we support (no false-positive risk):
 *   - Samsung / Android stock    20240226_135421.jpg
 *   - Google Pixel               PXL_20240226_135421.123.jpg
 *   - WhatsApp                   IMG-20240226-WA0001.jpg
 *   - Telegram                   photo_2024-02-26_13-54-21.jpg
 *   - macOS screenshot           Screen Shot 2024-02-26 at 1.54.21 PM.png
 *   - Android screenshot         Screenshot_20240226-135421.png
 *   - Manual dot-format          2024.02.26 - title.jpg
 *   - WeChat                     mmexport1645900000000.jpeg  (13-digit ms)
 *   - Facebook saves             FB_IMG_1583926812.jpg       (10-digit s)
 *   - Path forms                 2024/02/26, 2024-02-26, 2024_02_26,
 *                                2024.02.26, plus Y-M-only 2024/02/
 *
 * Conventions we deliberately do NOT parse (locale-ambiguous or no
 * recoverable signal):
 *   - DD-MM-YYYY / MM-DD-YYYY    26-02-2024.jpg, 02-26-2024.jpg
 *   - 2-digit years              24-02-26.jpg
 *   - Bare sequence numbers      IMG_1234.HEIC, DSC_0123.NEF,
 *                                GOPR0123.JPG, DJI_0123.JPG
 *
 * Used by the EXIF Stripped review tab to surface a date suggestion
 * row in the metadata sidebar and to power the "Accept date & Keep"
 * bulk action.
 */

import { isValidISODate } from '$lib/services/photoprism';

interface Input {
	fileName?: string; // basename, e.g. '20240226_000000_A6D42DF3.jpg'
	originalName?: string; // optional second filename signal (PpPhoto.OriginalName)
	path?: string; // directory portion, e.g. '2024/02'
}

export interface DateGuess {
	iso: string;
	confidence: 'high' | 'medium';
	source:
		| 'filename-agrees-path'
		| 'filename-only'
		| 'unix-timestamp'
		| 'path-ymd'
		| 'path-ym-default-day';
}

interface YMD {
	y: number;
	m: number;
	d: number;
}

interface YM {
	y: number;
	m: number;
}

function pad2(n: number): string {
	return n < 10 ? `0${n}` : String(n);
}

function isoOf(ymd: YMD): string {
	return `${ymd.y}-${pad2(ymd.m)}-${pad2(ymd.d)}`;
}

function tryYMD(y: number, m: number, d: number): YMD | null {
	if (y < 1900 || y > 2100) return null;
	if (m < 1 || m > 12) return null;
	if (d < 1 || d > 31) return null;
	if (!isValidISODate(`${y}-${pad2(m)}-${pad2(d)}`)) return null;
	return { y, m, d };
}

function tryYM(y: number, m: number): YM | null {
	if (y < 1900 || y > 2100) return null;
	if (m < 1 || m > 12) return null;
	return { y, m };
}

// `YYYY[sep]MM[sep]DD` for basenames. sep ∈ {nothing, -, _, ., space}. The
// non-digit lookbehind/ahead keeps a leading prefix like `PXL_` and a
// trailing time like `_135421` from polluting the match.
const BASENAME_YMD = /(?<!\d)(\d{4})[-_. ]?(\d{2})[-_. ]?(\d{2})(?!\d)/;

// Path Y-M-D and Y-M. Includes `/` for directory separators and `.` for
// rare dot-organised libraries (`Photos/2024.02/...`).
const PATH_YMD = /(?<!\d)(\d{4})[-_/.](\d{2})[-_/.](\d{2})(?!\d)/;
const PATH_YM = /(?<!\d)(\d{4})[-_/.](\d{2})(?!\d)/;

// 10- or 13-digit Unix epoch, anchored. Years widened to [1990, current+1]
// to dodge accidental matches on phone numbers, hex hashes containing
// digits, etc. — but 10-digit seconds still has to round-trip into a
// plausible calendar year before we trust it.
const BASENAME_EPOCH = /(?<!\d)(\d{10}|\d{13})(?!\d)/;

function parseFilenameYMD(name: string): YMD | null {
	const m = name.match(BASENAME_YMD);
	if (!m) return null;
	return tryYMD(Number(m[1]), Number(m[2]), Number(m[3]));
}

function parseUnixTimestampInName(name: string): YMD | null {
	const m = name.match(BASENAME_EPOCH);
	if (!m) return null;
	const digits = m[1];
	const ms = digits.length === 13 ? Number(digits) : Number(digits) * 1000;
	if (!Number.isFinite(ms)) return null;
	const d = new Date(ms);
	if (Number.isNaN(d.getTime())) return null;
	const y = d.getUTCFullYear();
	if (y < 1990 || y > new Date().getUTCFullYear() + 1) return null;
	return tryYMD(y, d.getUTCMonth() + 1, d.getUTCDate());
}

function parsePathYMD(path: string): YMD | null {
	const m = path.match(PATH_YMD);
	if (!m) return null;
	return tryYMD(Number(m[1]), Number(m[2]), Number(m[3]));
}

function parsePathYM(path: string): YM | null {
	const m = path.match(PATH_YM);
	if (!m) return null;
	return tryYM(Number(m[1]), Number(m[2]));
}

function ymdAgreesWithYM(ymd: YMD, ym: YM): boolean {
	return ymd.y === ym.y && ymd.m === ym.m;
}

function ymdEqual(a: YMD, b: YMD): boolean {
	return a.y === b.y && a.m === b.m && a.d === b.d;
}

/** Pick the filename-derived YMD that best aligns with the path. When
 *  both `fileName` and `originalName` yield candidates, prefer the one
 *  that matches the path's year+month; ties fall back to `fileName`. */
function pickFilenameYMD(
	fileName: string,
	originalName: string,
	pathYM: YM | null
): YMD | null {
	const candidates: YMD[] = [];
	const a = parseFilenameYMD(fileName);
	if (a) candidates.push(a);
	if (originalName && originalName !== fileName) {
		const b = parseFilenameYMD(originalName);
		if (b && !candidates.some((c) => ymdEqual(c, b))) candidates.push(b);
	}
	if (candidates.length === 0) return null;
	if (!pathYM) return candidates[0];
	const aligned = candidates.find((c) => ymdAgreesWithYM(c, pathYM));
	return aligned ?? candidates[0];
}

function pickUnixTimestamp(fileName: string, originalName: string): YMD | null {
	return (
		parseUnixTimestampInName(fileName) ??
		(originalName && originalName !== fileName
			? parseUnixTimestampInName(originalName)
			: null)
	);
}

export function suggestDateFromPath(input: Input): DateGuess | null {
	const fileName = (input.fileName ?? '').trim();
	const originalName = (input.originalName ?? '').trim();
	const path = (input.path ?? '').trim();

	const pathYMD = path ? parsePathYMD(path) : null;
	const pathYM = path && !pathYMD ? parsePathYM(path) : null;

	// 1. Filename Y-M-D corroborated by the path.
	const fnYMD = pickFilenameYMD(fileName, originalName, pathYM);
	if (fnYMD) {
		if (pathYMD && ymdEqual(fnYMD, pathYMD)) {
			return { iso: isoOf(fnYMD), confidence: 'high', source: 'filename-agrees-path' };
		}
		if (pathYM && ymdAgreesWithYM(fnYMD, pathYM)) {
			return { iso: isoOf(fnYMD), confidence: 'high', source: 'filename-agrees-path' };
		}
		// 2. Filename Y-M-D with no path signal at all.
		if (!pathYMD && !pathYM) {
			return { iso: isoOf(fnYMD), confidence: 'high', source: 'filename-only' };
		}
		// Filename present but disagrees with path → fall through.
	}

	// 3. Unix epoch in filename, optionally corroborated.
	const epoch = pickUnixTimestamp(fileName, originalName);
	if (epoch) {
		if (!pathYM && !pathYMD) {
			return { iso: isoOf(epoch), confidence: 'high', source: 'unix-timestamp' };
		}
		if (pathYM && ymdAgreesWithYM(epoch, pathYM)) {
			return { iso: isoOf(epoch), confidence: 'high', source: 'unix-timestamp' };
		}
		if (pathYMD && ymdEqual(epoch, pathYMD)) {
			return { iso: isoOf(epoch), confidence: 'high', source: 'unix-timestamp' };
		}
		// disagreement → fall through to path
	}

	// 4. Path Y-M-D standalone.
	if (pathYMD) {
		return { iso: isoOf(pathYMD), confidence: 'high', source: 'path-ymd' };
	}

	// 5. Path Y-M with synthesised day = 01.
	if (pathYM) {
		const ymd = tryYMD(pathYM.y, pathYM.m, 1);
		if (ymd) {
			return { iso: isoOf(ymd), confidence: 'medium', source: 'path-ym-default-day' };
		}
	}

	return null;
}