lgtm

server-rs/src/language.rs

@@ -0,0 +1,101 @@
+pub const DEFAULT_LANGUAGE: &str = "en";
+
+const SUPPORTED_LANGUAGES: &[&str] = &["en", "fr", "de", "zh", "hi", "hu"];
+
+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"
+        );
+    }
+}