const IMAGE_TYPES: [(&str, &str); 4] = [
    ("image/png", "png"),
    ("image/jpeg", "jpg"),
    ("image/gif", "gif"),
    ("image/webp", "webp"),
];

/// The extension to store `media_type` under, or `None` if it isn't an
/// image type this server handles.
pub fn extension_for(media_type: &str) -> Option<&'static str> {
    IMAGE_TYPES
        .iter()
        .find(|(known, _)| *known == media_type)
        .map(|(_, extension)| *extension)
}

/// The media type of a stored file, from its extension. Names are
/// server-generated (`<hex>.<extension>`, always lowercase), so no case
/// folding is needed; `None` for anything else.
pub fn media_type_for(name: &str) -> Option<&'static str> {
    let (_, extension) = name.rsplit_once('.')?;
    IMAGE_TYPES
        .iter()
        .find(|(_, known)| *known == extension)
        .map(|(media_type, _)| *media_type)
}

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

    #[test]
    fn the_two_directions_agree() {
        for (media_type, extension) in IMAGE_TYPES {
            assert_eq!(extension_for(media_type), Some(extension));
            assert_eq!(
                media_type_for(&format!("abc123.{extension}")),
                Some(media_type)
            );
        }
    }

    #[test]
    fn unknown_types_are_the_callers_problem() {
        assert_eq!(extension_for("application/pdf"), None);
        assert_eq!(media_type_for("abc123.pdf"), None);
        assert_eq!(media_type_for("abc123"), None);
    }
}