/++
    Module singlang - Lightweight internationalization (i18n) support for D programs.

    Description:
    This module integrates GNU Gettext for translations in D applications, using a singleton
    `Singlang` class and a convenient `_()` function for message translation. Errors are
    reported to stderr for proper error handling.
+/
module singlang;

import core.stdc.locale : setlocale, LC_ALL;
import std.string : toStringz;
import std.conv : to;
import std.stdio : writeln, stderr;

extern(C) {
    char* bindtextdomain(const char* domainname, const char* dirname);
    char* textdomain(const char* domainname);
    char* gettext(const char* msgid);
}

/++
    Class Singlang - Singleton manager for translation configuration.

    Description:
    The `Singlang` class encapsulates the setup and management of the GNU Gettext-based
    translation system. It ensures that the translation environment is initialized only once
    and provides a centralized configuration point for the domain and translation directory.

    Usage Example:
    ---
    import singlang;

    void main() {
        if (Singlang.set("myapp", "/usr/share/locale")) {
            writeln("Translation system ready!");
        }
    }
    ---
+/
class Singlang {
private:
    /++ Translation domain name used by GNU Gettext +/
    string domain;

    /++ Directory path containing translation files (.mo) +/
    string localeDir;

    /++ Singleton instance of the Singlang class +/
    static Singlang instance;

    /++
        Private constructor for initializing a Singlang instance.

        Params:
        domain    = The translation domain name
        localeDir = Path to the directory with translation files

        Note:
        Accessible only internally; use `set` to create an instance.
    +/
    this(string domain, string localeDir) {
        this.domain = domain;
        this.localeDir = localeDir;
    }

public:
    /++
        Initializes the translation system with a domain and directory.

        Description:
        Configures the GNU Gettext environment by setting the translation directory and domain.
        This method must be called before using the `_()` function for translations. Errors
        are reported to stderr.

        Params:
        domain    = Name of the translation domain (e.g., application name)
        localeDir = Path to the directory with translation files (.mo)

        Returns:
        `true` on successful initialization, `false` if an error occurs

        Notes:
        - Attempting to initialize an already initialized instance will fail with an error message to stderr.
        - Errors during directory or domain setup will reset the instance to null and log messages to stderr.

        Example:
        ---
        if (Singlang.set("myapp", "/path/to/translations")) {
            writeln("Ready to translate!");
        } else {
            writeln("Setup failed, check stderr for details!");
        }
        ---
    +/
    static bool set(string domain, string localeDir) {
        if (instance !is null) {
            stderr.writeln("Error: Singlang is already initialized");
            return false;
        }

        setlocale(LC_ALL, "");

        instance = new Singlang(domain, localeDir);
        
        if (bindtextdomain(domain.toStringz, localeDir.toStringz) is null) {
            stderr.writeln("Error: Failed to set translations directory");
            instance = null;
            return false;
        }

        if (textdomain(domain.toStringz) is null) {
            stderr.writeln("Error: Failed to set translations domain");
            instance = null;
            return false;
        }
        
        return true;
    }
}

/++
    Retrieves the translated version of a given message.

    Description:
    Translates a message using the configured `Singlang` instance. If the system is not
    initialized, it returns the original message and logs an error to stderr.

    Params:
    msg = The message to translate (UTF-8 encoded)

    Returns:
    The translated string, or the original message if translation fails or system is uninitialized

    Example:
    ---
    writeln(_("Error: File not found")); // Outputs translated text or original if unavailable
    ---
+/
string _(string msg) {
    if (Singlang.instance is null) {
        stderr.writeln("Error: Singlang module is not initialized");
        return msg;
    }
    auto result = gettext(msg.toStringz);
    return result !is null ? result.to!string : msg;
}