Implements Mozilla's localization and plurals site in D for i18n

plurals.d

/// Implements mozilla's plural forms.
/// See_Also: https://developer.mozilla.org/en-US/docs/Mozilla/Localization/Localization_and_Plurals
/// Returns: the index which plural word to use. For each rule from top to bottom.
int resolvePlural(int form, int n)
{
	import std.algorithm : among;

	switch (form)
	{
		// Asian, Persian, Turkic/Altaic, Thai, Lao
	case 0:
		return 0;
		// Germanic, Finno-Ugric, Language isolate, Latin/Greek, Semitic, Romanic, Vietnamese
	case 1:
		return n == 1 ? 0 : 1;
		// Romanic, Lingala
	case 2:
		return n == 0 || n == 1 ? 0 : 1;
		// Baltic
	case 3:
		if (n % 10 == 0)
			return 0;
		else if (n != 11 && n % 10 == 1)
			return 1;
		else
			return 2;
		// Celtic
	case 4:
		if (n == 1 || n == 11)
			return 0;
		else if (n == 2 || n == 12)
			return 1;
		else if ((n >= 3 && n <= 10) || (n >= 13 && n <= 19))
			return 2;
		else
			return 3;
		// Romanic
	case 5:
		if (n == 1)
			return 0;
		else if ((n % 100) >= 0 && (n % 100) <= 19)
			return 1;
		else
			return 2;
		// Baltic
	case 6:
		if (n != 11 && n % 10 == 1)
			return 0;
		else if (n % 10 == 0 || (n % 100 >= 11 && n % 100 <= 19))
			return 1;
		else
			return 2;
		// Belarusian, Russian, Ukrainian
	case 7:
		// Slavic
	case 19:
		if (n != 11 && n % 10 == 1)
			return 0;
		else if (n != 12 && n != 13 && n != 14 && (n % 10 >= 2 && n % 10 <= 4))
			return 1;
		else
			return 2;
		// Slavic
	case 8:
		if (n == 1)
			return 0;
		else if (n >= 2 && n <= 4)
			return 1;
		else
			return 2;
		// Slavic
	case 9:
		if (n == 1)
			return 0;
		else if (n >= 2 && n <= 4 && !(n >= 12 && n <= 14))
			return 1;
		else
			return 2;
		// Slavic
	case 10:
		if (n % 100 == 1)
			return 0;
		else if (n % 100 == 2)
			return 1;
		else if (n % 100 == 3 || n % 100 == 4)
			return 2;
		else
			return 3;
		// Celtic
	case 11:
		if (n == 1)
			return 0;
		else if (n == 2)
			return 1;
		else if (n >= 3 && n <= 6)
			return 2;
		else if (n >= 7 && n <= 10)
			return 3;
		else
			return 4;
		// Semitic
	case 12:
		if (n == 1)
			return 0;
		else if (n == 2)
			return 1;
		else if (n == 0)
			return 5;
		else
		{
			const d = n % 100;
			if (d >= 0 && d <= 2)
				return 4;
			else if (d >= 3 && d <= 10)
				return 2;
			else
				return 3;
		}
		// Semitic
	case 13:
		if (n == 1)
			return 0;
		else
		{
			const d = n % 100;
			if (d >= 1 && d <= 10)
				return 1;
			else if (d >= 11 && d <= 19)
				return 2;
			else
				return 3;
		}
		// unused
	case 14:
		if (n % 10 == 1)
			return 0;
		else if (n % 10 == 2)
			return 1;
		else
			return 2;
		// Icelandic, Macedonian
	case 15:
		if (n != 11 && n % 10 == 1)
			return 0;
		else
			return 1;
		// Celtic
	case 16:
		const a = n % 10;
		const b = n % 100;
		if (a == 1 && b != 11 && b != 71 && b != 91)
			return 0;
		else if (a == 2 && b != 12 && b != 72 && b != 92)
			return 1;
		else if (a.among!(3, 4, 9) && !b.among!(13, 14, 19, 73, 74, 79, 93, 94, 99))
			return 2;
		else if (n % 1_000_000 == 0)
			return 3;
		else
			return 4;
		// Ecuador indigenous languages
	case 17:
		return (n == 0) ? 0 : 1;
		// Welsh
	case 18:
		switch (n)
		{
		case 0:
			return 0;
		case 1:
			return 1;
		case 2:
			return 2;
		case 3:
			return 3;
		case 6:
			return 4;
		default:
			return 5;
		}
	default:
		throw new Exception("Unknown plural form");
	}
}