pub const DEFAULT_LANGUAGE: &str = "en";

const SUPPORTED_LANGUAGES: &[&str] = &["en", "fr", "de", "zh", "hi", "hu"];

/// The set of language codes the frontend can render, used e.g. to emit
/// `hreflang` alternates for indexable pages.
pub fn supported_languages() -> &'static [&'static str] {
    SUPPORTED_LANGUAGES
}

pub fn supported_language(value: &str) -> Option<&'static str> {
    let value = value.trim().to_ascii_lowercase();
    if value.is_empty() || value == "*" {
        return None;
    }

    let primary = value.split('-').next().unwrap_or("");
    SUPPORTED_LANGUAGES
        .iter()
        .copied()
        .find(|language| value == *language || primary == *language)
}

pub fn language_from_accept_language(header: Option<&str>) -> &'static str {
    let Some(header) = header else {
        return DEFAULT_LANGUAGE;
    };

    let mut best_language = None;
    let mut best_quality = 0.0f32;

    for item in header.split(',') {
        let mut parts = item.split(';');
        let tag = parts.next().unwrap_or("").trim();
        let mut quality = 1.0f32;

        for param in parts {
            let param = param.trim();
            if let Some(value) = param.strip_prefix("q=") {
                quality = value.parse::<f32>().unwrap_or(0.0);
            }
        }

        if quality <= 0.0 {
            continue;
        }

        if let Some(language) = supported_language(tag) {
            if quality > best_quality {
                best_language = Some(language);
                best_quality = quality;
            }
        }
    }

    best_language.unwrap_or(DEFAULT_LANGUAGE)
}

pub fn query_string_with_language(query_string: &str, language: &str) -> String {
    if url::form_urlencoded::parse(query_string.as_bytes()).any(|(key, _)| key == "lang") {
        return query_string.to_string();
    }

    if query_string.is_empty() {
        format!("lang={language}")
    } else {
        format!("{query_string}&lang={language}")
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn maps_browser_language_tags_to_supported_languages() {
        assert_eq!(supported_language("fr-CA"), Some("fr"));
        assert_eq!(supported_language("zh-Hans-CN"), Some("zh"));
        assert_eq!(supported_language("es"), None);
    }

    #[test]
    fn chooses_highest_quality_supported_language() {
        assert_eq!(
            language_from_accept_language(Some("en-US;q=0.6, fr-FR;q=0.9, de;q=0.7")),
            "fr"
        );
        assert_eq!(
            language_from_accept_language(Some("es-ES, hi-IN;q=0.8")),
            "hi"
        );
        assert_eq!(language_from_accept_language(Some("es-ES")), "en");
    }

    #[test]
    fn appends_language_without_overriding_explicit_query_language() {
        assert_eq!(query_string_with_language("", "de"), "lang=de");
        assert_eq!(
            query_string_with_language("lat=51.5&lon=-0.1", "de"),
            "lat=51.5&lon=-0.1&lang=de"
        );
        assert_eq!(
            query_string_with_language("lang=fr&lat=1", "de"),
            "lang=fr&lat=1"
        );
    }
}