import { moment } from 'obsidian';

const momentFn = 'default' in moment ? moment.default : moment;

export class DateFormatter {
	private static readonly RFC2822_FORMAT = 'ddd, DD MMM YYYY HH:mm:ss ZZ';

	toFormat: string;
	locale: string;

	constructor() {
		this.toFormat = 'YYYY-MM-DD';
		// get locale of this machine (e.g. en, en-gb, de, fr, etc.)
		this.locale = new Intl.DateTimeFormat().resolvedOptions().locale;
	}

	setFormat(format: string): void {
		this.toFormat = format;
	}

	getPreview(format?: string): string {
		const today = momentFn();

		format ??= this.toFormat;

		return today.locale(this.locale).format(format);
	}

	/**
	 * Tries to format a given date string with the currently set date format.
	 * You can set a date format by calling `setFormat()`.
	 *
	 * @param dateString the date string to be formatted
	 * @param dateFormat the current format of `dateString`. When this is `null` and the actual format of the
	 * given date string is not `C2822` or `ISO` format, this function will try to guess the format by using the native `Date` module.
	 * @param locale the locale of `dateString`. This is needed when `dateString` includes a month or day name and its locale format differs
	 * from the locale of this machine.
	 * @returns formatted date string or null if `dateString` is not a valid date
	 */
	format(dateString: string | null | undefined, dateFormat?: string, locale: string = 'en'): string | null {
		if (!dateString) {
			return null;
		}

		let date: moment.Moment;

		if (!dateFormat) {
			date = this.parseWithoutFormat(dateString);
		} else {
			date = momentFn(dateString, dateFormat, locale);
		}

		// format date (if it is valid)
		return date.isValid() ? date.locale(this.locale).format(this.toFormat) : null;
	}

	private parseWithoutFormat(dateString: string): moment.Moment {
		// reading date formats other than C2822 or ISO with moment is deprecated
		// see https://momentjs.com/docs/#/parsing/string/
		if (this.hasMomentFormat(dateString)) {
			// expect C2822 or ISO format
			return momentFn(dateString);
		}

		// fall back to native Date parsing for unknown formats
		return momentFn(new Date(dateString));
	}

	private hasMomentFormat(dateString: string): boolean {
		const date = momentFn(dateString, [moment.ISO_8601, DateFormatter.RFC2822_FORMAT], true); // strict mode
		return date.isValid();
	}
}