/**
	Internationalization/translation support for the web interface module.

	Copyright: © 2014-2015 RejectedSoftware e.K.
	License: Subject to the terms of the MIT license, as written in the included LICENSE.txt file.
	Authors: Sönke Ludwig
*/
module vibe.web.i18n;

import vibe.http.server : HTTPServerRequest;

import std.algorithm : canFind, min, startsWith;
import std.range.primitives : isForwardRange;
import std.range : only;

/**
	Annotates an interface method or class with translation information.

	The translation context contains information about supported languages
	and the translated strings. Any translations will be automatically
	applied to Diet templates, as well as strings passed to
	$(D vibe.web.web.trWeb).

	By default, the "Accept-Language" header of the incoming request will be
	used to determine the language used. To override this behavior, add a
	static method $(D determineLanguage) to the translation context, which
	takes the request and returns a language string (see also the second
	example).
*/
@property TranslationContextAttribute!CONTEXT translationContext(CONTEXT)() { return TranslationContextAttribute!CONTEXT.init; }

///
unittest {
	import vibe.http.router : URLRouter;
	import vibe.web.web : registerWebInterface;

	struct TranslationContext {
		import std.typetuple;
		alias languages = TypeTuple!("en_US", "de_DE", "fr_FR");
		//mixin translationModule!"app";
		//mixin translationModule!"somelib";
	}

	@translationContext!TranslationContext
	class MyWebInterface {
		void getHome()
		{
			//render!("home.dt")
		}
	}

	void test(URLRouter router)
	{
		router.registerWebInterface(new MyWebInterface);
	}
}

/// Defining a custom function for determining the language.
unittest {
	import vibe.http.router : URLRouter;
	import vibe.http.server;
	import vibe.web.web : registerWebInterface;

	struct TranslationContext {
		import std.typetuple;
		// A language can be in the form en_US, en-US or en. Put the languages you want to prioritize first.
		alias languages = TypeTuple!("en_US", "de_DE", "fr_FR");
		//mixin translationModule!"app";
		//mixin translationModule!"somelib";

		// use language settings from the session instead of using the
		// "Accept-Language" header
		static string determineLanguage(scope HTTPServerRequest req)
		{
			if (!req.session) return req.determineLanguageByHeader(languages); // default behaviour using "Accept-Language" header
			return req.session.get("language", "");
		}
	}

	@translationContext!TranslationContext
	class MyWebInterface {
		void getHome()
		{
			//render!("home.dt")
		}
	}

	void test(URLRouter router)
	{
		router.registerWebInterface(new MyWebInterface);
	}
}

@safe unittest {
	import vibe.http.router : URLRouter;
	import vibe.http.server : HTTPServerRequest;
	import vibe.web.web : registerWebInterface;

	struct TranslationContext {
		import std.typetuple;
		alias languages = TypeTuple!("en_US", "de_DE", "fr_FR");
		static string determineLanguage(scope HTTPServerRequest req) { return "en_US"; }
	}

	@translationContext!TranslationContext
	class MyWebInterface { void getHome() @safe {} }

	auto router = new URLRouter;
	router.registerWebInterface(new MyWebInterface);
}


struct TranslationContextAttribute(CONTEXT) {
	alias Context = CONTEXT;
}

/*
doctype 5
html
	body
		p& Hello, World!
		p& This is a translated version of #{appname}.
	html
		p(class="Sasdasd")&.
			This is a complete paragraph of translated text.
*/

/** Makes a set of PO files available to a web interface class.

	This mixin template needs to be mixed in at the class scope. It will parse all
	translation files with the specified file name prefix and make their
	translations available.

	Params:
		FILENAME = Base name of the set of PO files to mix in. A file with the
			name "<FILENAME>.<LANGUAGE>.po" must be available as a string import
			for each language defined in the translation context.

	Bugs:
		`FILENAME` should not contain (back)slash characters, as string imports
		from sub directories will currently fail on Windows. See
		$(LINK https://issues.dlang.org/show_bug.cgi?id=14349).

	See_Also: `translationContext`
*/
mixin template translationModule(string FILENAME)
{
	import std.string : tr;
	enum NAME = FILENAME.tr(`/.-\`, "____");
	private mixin template file_mixin(size_t i) {
		static if (i < languages.length) {
			enum components = extractDeclStrings(import(FILENAME~"."~languages[i]~".po"));
			mixin("enum "~languages[i]~"_"~NAME~" = components;");
			//mixin decls_mixin!(languages[i], 0);
			mixin file_mixin!(i+1);
		}
	}

	mixin file_mixin!0;
}

/**
	Performs the string translation for a statically given language.

	The second overload takes a plural form and a number to select from a set
	of translations based on the plural forms of the target language.
*/
template tr(CTX, string LANG)
{
	string tr(string key, string context = null)
	{
		return tr!(CTX, LANG)(key, null, 0, context);
	}

	string tr(string key, string key_plural, int n, string context = null)
	{
		static assert([CTX.languages].canFind(LANG), "Unknown language: "~LANG);

		foreach (i, mname; __traits(allMembers, CTX)) {
			static if (mname.startsWith(LANG~"_")) {
				enum langComponents = __traits(getMember, CTX, mname);
				foreach (entry; langComponents.messages) {
					if ((context is null) == (entry.context is null)) {
						if (context is null || entry.context == context) {
							if (entry.key == key) {
								if (key_plural !is null) {
									if (entry.pluralKey !is null && entry.pluralKey == key_plural) {
										static if (langComponents.nplurals_expr !is null && langComponents.plural_func_expr !is null) {
											mixin("int nplurals = "~langComponents.nplurals_expr~";");
											if (nplurals > 0) {
												mixin("int index = "~langComponents.plural_func_expr~";");
												return entry.pluralValues[index];
											}
											return entry.value;
										}
										assert(false, "Plural translations are not supported when the po file does not contain an entry for Plural-Forms.");
									}
								} else {
									return entry.value;
								}
							}
						}
					}
				}
			}
		}

		static if (is(typeof(CTX.enforceExistingKeys)) && CTX.enforceExistingKeys) {
			if (key_plural !is null) {
				if (context is null) {
					assert(false, "Missing translation keys for "~LANG~": "~key~"&"~key_plural);
				}
				assert(false, "Missing translation key for "~LANG~"; "~context~": "~key~"&"~key_plural);
			}

			if (context is null) {
				assert(false, "Missing translation key for "~LANG~": "~key);
			}
			assert(false, "Missing translation key for "~LANG~"; "~context~": "~key);
		} else {
			return n == 1 || !key_plural.length ? key : key_plural;
		}
	}
}

/// Determines a language code from the value of a header string.
/// Returns: The best match from the Accept-Language header for a language. `null` if there is no supported language.
public string determineLanguageByHeader(T)(string accept_language, T allowed_languages) @safe pure @nogc
	if (isForwardRange!T)
{
	import std.algorithm : splitter, countUntil;
	import std.string : indexOf;

	// TODO: verify that allowed_languages doesn't contain a mix of languages with and without extra specifier for the same lanaguage (but only if one without specifier comes before those with specifier)
	// Implementing that feature should try to give a compile time warning and not change the behaviour of this function.

	if (!accept_language.length)
		return null;

	string fallback = null;
	foreach (accept; accept_language.splitter(",")) {
		auto sidx = accept.indexOf(';');
		if (sidx >= 0)
			accept = accept[0 .. sidx];

		string alang, aextra;
		auto asep = accept.countUntil!(a => a == '_' || a == '-');
		if (asep < 0)
			alang = accept;
		else {
			alang = accept[0 .. asep];
			aextra = accept[asep + 1 .. $];
		}

		foreach (lang; allowed_languages) {
			string lcode, lextra;
			sidx = lang.countUntil!(a => a == '_' || a == '-');
			if (sidx < 0)
				lcode = lang;
			else {
				lcode = lang[0 .. sidx];
				lextra = lang[sidx + 1 .. $];
			}
			// request en_US == serve en_US
			if (lcode == alang && lextra == aextra)
				return lang;
			// request en_* == serve en
			if (lcode == alang && !lextra.length)
				return lang;
			// request en* == serve en_* && be first occurence
			if (lcode == alang && lextra.length && !fallback.length)
				fallback = lang;
		}
	}

	return fallback;
}

/// ditto
public string determineLanguageByHeader(Tuple...)(string accept_language, Tuple allowed_languages) @safe pure @nogc
{
	return determineLanguageByHeader(accept_language, only(allowed_languages));
}

/// ditto
public string determineLanguageByHeader(T)(HTTPServerRequest req, T allowed_languages) @safe pure
	if (isForwardRange!T)
{
	return determineLanguageByHeader(req.headers.get("Accept-Language", null), allowed_languages);
}

/// ditto
public string determineLanguageByHeader(Tuple...)(HTTPServerRequest req, Tuple allowed_languages) @safe pure
{
	return determineLanguageByHeader(req.headers.get("Accept-Language", null), only(allowed_languages));
}

@safe unittest {
	assert(determineLanguageByHeader("de,de-DE;q=0.8,en;q=0.6,en-US;q=0.4", ["en-US", "de_DE", "de_CH"]) == "de_DE");
	assert(determineLanguageByHeader("de,de-CH;q=0.8,en;q=0.6,en-US;q=0.4", ["en_US", "de_DE", "de-CH"]) == "de-CH");
	assert(determineLanguageByHeader("en_CA,en_US", ["ja_JP", "en"]) == "en");
	assert(determineLanguageByHeader("en", ["ja_JP", "en"]) == "en");
	assert(determineLanguageByHeader("en", ["ja_JP", "en_US"]) == "en_US");
	assert(determineLanguageByHeader("en_US", ["ja-JP", "en"]) == "en");
	assert(determineLanguageByHeader("de,de-DE;q=0.8,en;q=0.6,en-US;q=0.4", ["ja_JP"]) is null);
	assert(determineLanguageByHeader("de, de-DE ;q=0.8 , en ;q=0.6 , en-US;q=0.4", ["de-DE"]) == "de-DE");
	assert(determineLanguageByHeader("en_GB", ["en_US"]) == "en_US");
	assert(determineLanguageByHeader("de_DE", ["en_US"]) is null);
	assert(determineLanguageByHeader("en_US,enCA", ["en_GB"]) == "en_GB");
	assert(determineLanguageByHeader("en_US,enCA", ["en_GB", "en"]) == "en");
	assert(determineLanguageByHeader("en_US,enCA", ["en", "en_GB"]) == "en");
	// TODO from above (should be invalid input having a more generic language first in the list!)
	//assert(determineLanguageByHeader("en_US,enCA", ["en", "en_US"]) == "en_US");
}

package string determineLanguage(alias METHOD)(scope HTTPServerRequest req)
{
	alias CTX = GetTranslationContext!METHOD;

	static if (!is(CTX == void)) {
		static if (is(typeof(CTX.determineLanguage(req)))) {
			static assert(is(typeof(CTX.determineLanguage(req)) == string),
				"determineLanguage in a translation context must return a language string.");
			return CTX.determineLanguage(req);
		} else {
			return determineLanguageByHeader(req, CTX.languages);
		}
	} else return null;
}

unittest { // make sure that the custom determineLanguage is called
	static struct CTX {
		static string determineLanguage(Object a) { return "test"; }
	}
	@translationContext!CTX
	static class Test {
		void test()
		{
		}
	}
	auto test = new Test;
	assert(determineLanguage!(test.test)(null) == "test");
}

package template GetTranslationContext(alias METHOD)
{
	import vibe.internal.meta.uda;

	alias PARENT = typeof(__traits(parent, METHOD).init);
	enum FUNCTRANS = findFirstUDA!(TranslationContextAttribute, METHOD);
	enum PARENTTRANS = findFirstUDA!(TranslationContextAttribute, PARENT);
	static if (FUNCTRANS.found) alias GetTranslationContext = FUNCTRANS.value.Context;
	else static if (PARENTTRANS.found) alias GetTranslationContext = PARENTTRANS.value.Context;
	else alias GetTranslationContext = void;
}


private struct DeclString {
	string context;
	string key;
	string pluralKey;
	string value;
	string[] pluralValues;
}

private struct LangComponents {
	DeclString[] messages;
	string nplurals_expr;
	string plural_func_expr;
}

// Example po header
/*
 * # Translation of kstars.po into Spanish.
 * # This file is distributed under the same license as the kdeedu package.
 * # Pablo de Vicente <pablo@foo.com>, 2005, 2006, 2007, 2008.
 * # Eloy Cuadra <eloy@bar.net>, 2007, 2008.
 * msgid ""
 * msgstr ""
 * "Project-Id-Version: kstars\n"
 * "Report-Msgid-Bugs-To: http://bugs.kde.org\n"
 * "POT-Creation-Date: 2008-09-01 09:37+0200\n"
 * "PO-Revision-Date: 2008-07-22 18:13+0200\n"
 * "Last-Translator: Eloy Cuadra <eloy@bar.net>\n"
 * "Language-Team: Spanish <kde-l10n-es@kde.org>\n"
 * "MIME-Version: 1.0\n"
 * "Content-Type: text/plain; charset=UTF-8\n"
 * "Content-Transfer-Encoding: 8bit\n"
 * "Plural-Forms: nplurals=2; plural=n != 1;\n"
 */

// PO format notes
/*
 * # - Translator comment
 * #: - source reference
 * #. - extracted comments
 * #, - flags such as "c-format" to indicate what king of substitutions may be present
 * #| msgid - previous string comment
 * #~ - obsolete message
 * msgctxt - disabmbiguating context, like variable scope (optional, defaults to null)
 * msgid - key to translate from (required)
 * msgid_plural - plural form of the msg id (optional)
 * msgstr - value to translate to (required)
 * msgstr[0] - indexed translation for handling the various plural forms
 * msgstr[1] - ditto
 * msgstr[2] - ditto and etc...
 */

LangComponents extractDeclStrings(string text)
{
	DeclString[] declStrings;
	string nplurals_expr;
	string plural_func_expr;

	size_t i = 0;
	while (true) {
		i = skipToDirective(i, text);
		if (i >= text.length) break;

		string context = null;

		// msgctxt is an optional field
		if (text.length - i >= 7 && text[i .. i+7] == "msgctxt") {
			i = skipWhitespace(i+7, text);

			auto icntxt = skipString(i, text);
			context = dstringUnescape(wrapText(text[i+1 .. icntxt-1]));
			i = skipToDirective(icntxt, text);
		}

		// msgid is a required field
		assert(text.length - i >= 5 && text[i .. i+5] == "msgid", "Expected 'msgid', got '"~text[i .. min(i+10, $)]~"'.");
		i += 5;

		i = skipWhitespace(i, text);

		auto iknext = skipString(i, text);
		auto key = dstringUnescape(wrapText(text[i+1 .. iknext-1]));
		i = iknext;

		i = skipToDirective(i, text);

		// msgid_plural is an optional field
		string key_plural = null;
		if (text.length - i >= 12 && text[i .. i+12] == "msgid_plural") {
			i = skipWhitespace(i+12, text);
			auto iprl = skipString(i, text);
			key_plural = dstringUnescape(wrapText(text[i+1 .. iprl-1]));
			i = skipToDirective(iprl, text);
		}

		// msgstr is a required field
		assert(text.length - i >= 6 && text[i .. i+6] == "msgstr", "Expected 'msgstr', got '"~text[i .. min(i+10, $)]~"'.");
		i += 6;

		i = skipWhitespace(i, text);
		auto ivnext = skipString(i, text);
		auto value = dstringUnescape(wrapText(text[i+1 .. ivnext-1]));
		i = ivnext;
		i = skipToDirective(i, text);

		// msgstr[n] is a required field when msgid_plural is not null, and ignored otherwise
		string[] value_plural;
		if (key_plural !is null) {
			while (text.length - i >= 6 && text[i .. i+6] == "msgstr") {
				i = skipIndex(i+6, text);
				i = skipWhitespace(i, text);
				auto ims = skipString(i, text);

				string plural = dstringUnescape(wrapText(text[i+1 .. ims-1]));
				i = skipLine(ims, text);

				// Is it safe to assume that the entries are always sequential?
				value_plural ~= plural;
			}
		}

		// Add the translation for the current language
		if (key == "") {
			nplurals_expr = parse_nplurals(value);
			plural_func_expr = parse_plural_expression(value);
		}

		declStrings ~= DeclString(context, key, key_plural, value, value_plural);
	}

	return LangComponents(declStrings, nplurals_expr, plural_func_expr);
}

// Verify that two simple messages can be read and parsed correctly
unittest {
	auto str = `
# first string
msgid "ordinal.1"
msgstr "first"

# second string
msgid "ordinal.2"
msgstr "second"`;

	auto components = extractDeclStrings(str);
	auto ds = components.messages;
	assert(2 == ds.length, "Not enough DeclStrings have been processed");
	assert(ds[0].key == "ordinal.1", "The first key is not right.");
	assert(ds[0].value == "first", "The first value is not right.");
	assert(ds[1].key == "ordinal.2", "The second key is not right.");
	assert(ds[1].value == "second", "The second value is not right.");
}

// Verify that the fields cannot be defined out of order
unittest {
	import core.exception : AssertError;
	import std.exception : assertThrown;

	auto str1 = `
# unexpected field ahead
msgstr "world"
msgid "hello"`;

	assertThrown!AssertError(extractDeclStrings(str1));
}

// Verify that string wrapping is handled correctly
unittest {
	auto str = `
# The following text is wrapped
msgid ""
"This is an example of text that "
"has been wrapped on two lines."
msgstr ""
"It should not matter where it takes place, "
"the strings should all be concatenated properly."`;

	auto ds = extractDeclStrings(str).messages;
	assert(1 == ds.length, "Expected one DeclString to have been processed.");
	assert(ds[0].key == "This is an example of text that has been wrapped on two lines.", "Failed to properly wrap the key");
	assert(ds[0].value == "It should not matter where it takes place, the strings should all be concatenated properly.", "Failed to properly wrap the key");
}

// Verify that string wrapping and unescaping is handled correctly on example of PO headers
unittest {
	auto str = `
# English translations for ThermoWebUI package.
# This file is put in the public domain.
# Automatically generated, 2015.
#
msgid ""
msgstr ""
"Project-Id-Version: PROJECT VERSION\n"
"Report-Msgid-Bugs-To: developer@example.com\n"
"POT-Creation-Date: 2015-04-13 17:55+0600\n"
"PO-Revision-Date: 2015-04-13 14:13+0600\n"
"Last-Translator: Automatically generated\n"
"Language-Team: none\n"
"Language: en\n"
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=UTF-8\n"
"Content-Transfer-Encoding: 8bit\n"
"Plural-Forms: nplurals=2; plural=(n != 1);\n"
`;
	auto expected = `Project-Id-Version: PROJECT VERSION
Report-Msgid-Bugs-To: developer@example.com
POT-Creation-Date: 2015-04-13 17:55+0600
PO-Revision-Date: 2015-04-13 14:13+0600
Last-Translator: Automatically generated
Language-Team: none
Language: en
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Plural-Forms: nplurals=2; plural=(n != 1);
`;

	auto ds = extractDeclStrings(str).messages;
	assert(1 == ds.length, "Expected one DeclString to have been processed.");
	assert(ds[0].key == "", "Failed to properly wrap or unescape the key");
	assert(ds[0].value == expected, "Failed to properly wrap or unescape the value");
}

// Verify that the message context is properly parsed
unittest {
	auto str1 = `
# "C" is for cookie
msgctxt "food"
msgid "C"
msgstr "C is for cookie, that's good enough for me."`;

	auto ds1 = extractDeclStrings(str1).messages;
	assert(1 == ds1.length, "Expected one DeclString to have been processed.");
	assert(ds1[0].context == "food", "Expected a context of food");
	assert(ds1[0].key == "C", "Expected to find the letter C for the msgid.");
	assert(ds1[0].value == "C is for cookie, that's good enough for me.", "Unexpected value encountered for the msgstr.");

	auto str2 = `
# No context validation
msgid "alpha"
msgstr "First greek letter."`;

	auto ds2 = extractDeclStrings(str2).messages;
	assert(1 == ds2.length, "Expected one DeclString to have been processed.");
	assert(ds2[0].context is null, "Expected the context to be null when it is not defined.");
}

unittest {
	enum str = `
# "C" is for cookie
msgctxt "food"
msgid "C"
msgstr "C is for cookie, that's good enough for me."

# "C" is for language
msgctxt "lang"
msgid "C"
msgstr "Catalan"

# Just "C"
msgid "C"
msgstr "Third letter"
`;

	enum components = extractDeclStrings(str);

	struct TranslationContext {
		import std.typetuple;
		enum enforceExistingKeys = true;
		alias languages = TypeTuple!("en_US");

		// Note that this is normally handled by mixing in an external file.
		enum en_US_unittest = components;
	}

	auto newTr(string msgid, string msgcntxt = null) {
		return tr!(TranslationContext, "en_US")(msgid, msgcntxt);
	}

	assert(newTr("C", "food") == "C is for cookie, that's good enough for me.", "Unexpected translation based on context.");
	assert(newTr("C", "lang") == "Catalan", "Unexpected translation based on context.");
	assert(newTr("C") == "Third letter", "Unexpected translation based on context.");
}

unittest {
	enum str = `msgid ""
msgstr ""
"Project-Id-Version: kstars\\n"
"Plural-Forms: nplurals=2; plural=n != 1;\\n"

msgid "One file was deleted."
msgid_plural "Files were deleted."
msgstr "One file was deleted."
msgstr[0] "1 file was deleted."
msgstr[1] "%d files were deleted."

msgid "One file was created."
msgid_plural "Several files were created."
msgstr "One file was created."
msgstr[0] "1 file was created"
msgstr[1] "%d files were created."
`;

	import std.stdio;
	enum components = extractDeclStrings(str);

	struct TranslationContext {
		import std.typetuple;
		enum enforceExistingKeys = true;
		alias languages = TypeTuple!("en_US");

		// Note that this is normally handled by mixing in an external file.
		enum en_US_unittest2 = components;
	}

	auto newTr(string msgid, string msgid_plural, int count, string msgcntxt = null) {
		return tr!(TranslationContext, "en_US")(msgid, msgid_plural, count, msgcntxt);
	}

	string expected = "1 file was deleted.";
	auto actual = newTr("One file was deleted.", "Files were deleted.", 1);
	assert(expected == actual, "Expected: '"~expected~"' but got '"~actual~"'");

	expected = "%d files were deleted.";
	actual = newTr("One file was deleted.", "Files were deleted.", 42);
	assert(expected == actual, "Expected: '"~expected~"' but got '"~actual~"'");
}

private size_t skipToDirective(size_t i, ref string text)
{
	while (i < text.length) {
		i = skipWhitespace(i, text);
		if (i < text.length && text[i] == '#') i = skipLine(i, text);
		else break;
	}
	return i;
}

private size_t skipWhitespace(size_t i, ref string text)
{
	while (i < text.length && (text[i] == ' ' || text[i] == '\t' || text[i] == '\n' || text[i] == '\r'))
		i++;
	return i;
}

private size_t skipLine(size_t i, ref string text)
{
	while (i < text.length && text[i] != '\r' && text[i] != '\n') i++;
	if (i+1 < text.length && (text[i+1] == '\r' || text[i+1] == '\n') && text[i] != text[i+1]) i++;
	return i+1;
}

private size_t skipString(size_t i, ref string text)
{
	import std.conv : to;
	assert(text[i] == '"', "Expected to encounter the start of a string at position: "~to!string(i));
	i++;
	while (true) {
		assert(i < text.length, "Missing closing '\"' for string: "~text[i .. min($, 10)]);
		if (text[i] == '"') {
			if (i+1 < text.length) {
				auto j = skipWhitespace(i+1, text);
				if (j<text.length && text[j] == '"') return skipString(j, text);
			}
			return i+1;
		}
		if (text[i] == '\\') i += 2;
		else i++;
	}
}

private size_t skipIndex(size_t i, ref string text) {
	import std.conv : to;
	assert(text[i] == '[', "Expected to encounter a plural form of msgstr at position: "~to!string(i));
	for (; i<text.length; ++i) {
		if (text[i] == ']') {
			return i+1;
		}
	}
	assert(false, "Missing a ']' for a msgstr in a translation file.");
}

private string wrapText(string str)
{
	string ret;
	bool wrapped = false;

	for (size_t i=0; i<str.length; ++i) {
		if (str[i] == '\\') {
			assert(i+1 < str.length, "The string ends with the escape char: " ~ str);
			ret ~= str[i..i+2];
			++i;
		} else if (str[i] == '"') {
			wrapped = true;
			size_t j = skipWhitespace(i+1, str);
			if (j < str.length && str[j] == '"') {
				i=j;
			}
		} else ret ~= str[i];
	}

	if (wrapped) return ret;
	return str;
}

private string parse_nplurals(string msgstr)
in { assert(msgstr, "An empty string cannot be parsed for Plural-Forms."); }
body {
	import std.string : indexOf, CaseSensitive;

	auto start = msgstr.indexOf("Plural-Forms:", CaseSensitive.no);
	if (start > -1) {
		auto beg = msgstr.indexOf("nplurals=", start+13, CaseSensitive.no);
		if (beg > -1) {
			auto end = msgstr.indexOf(';', beg+9, CaseSensitive.no);
			if (end > -1) {
				return msgstr[beg+9 .. end];
			}
			return msgstr[beg+9 .. $];
		}
	}

	return null;
}

unittest {
	auto res = parse_nplurals("Plural-Forms: nplurals=2; plural=n != 1;\n");
	assert(res == "2", "Failed to parse the correct number of plural forms for a language.");
}

private string parse_plural_expression(string msgstr)
in { assert(msgstr, "An empty string cannot be parsed for Plural-Forms."); }
body {
	import std.string : indexOf, CaseSensitive;

	auto start = msgstr.indexOf("Plural-Forms:", CaseSensitive.no);
	if (start > -1) {
		auto beg = msgstr.indexOf("plural=", start+13, CaseSensitive.no);
		if (beg > -1) {
			auto end = msgstr.indexOf(';', beg+7, CaseSensitive.no);
			if (end > -1) {
				return msgstr[beg+7 .. end];
			}
			return msgstr[beg+7 .. $];
		}
	}

	return null;
}

unittest {
	auto res = parse_plural_expression("Plural-Forms: nplurals=2; plural=n != 1;\n");
	assert(res == "n != 1", "Failed to parse the plural expression for a language.");
}

private string dstringUnescape(in string str)
{
	string ret;
	size_t i, start = 0;
	for( i = 0; i < str.length; i++ )
		if( str[i] == '\\' ){
			if( i > start ){
				if( start > 0 ) ret ~= str[start .. i];
				else ret = str[0 .. i];
			}
			assert(i+1 < str.length, "The string ends with the escape char: " ~ str);
			switch(str[i+1]){
				default: ret ~= str[i+1]; break;
				case 'r': ret ~= '\r'; break;
				case 'n': ret ~= '\n'; break;
				case 't': ret ~= '\t'; break;
			}
			i++;
			start = i+1;
		}

	if( i > start ){
		if( start == 0 ) return str;
		else ret ~= str[start .. i];
	}
	return ret;
}