/// Safely truncate a string at character boundaries, not byte boundaries
///
/// This function ensures that multi-byte UTF-8 characters (like Japanese, emoji, etc.)
/// are not split in the middle, which would cause a panic.
///
/// # Arguments
/// * `s` - The string to truncate
/// * `max_chars` - Maximum number of characters to keep
///
/// # Returns
/// A truncated string with "..." appended if truncation occurred
pub fn safe_truncate(s: &str, max_chars: usize) -> String {
    if s.chars().count() <= max_chars {
        s.to_string()
    } else {
        let truncated: String = s.chars().take(max_chars.saturating_sub(3)).collect();
        format!("{}...", truncated)
    }
}

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

    #[test]
    fn test_safe_truncate_ascii() {
        assert_eq!(safe_truncate("hello world", 20), "hello world");
        assert_eq!(safe_truncate("hello world", 8), "hello...");
        assert_eq!(safe_truncate("hello", 5), "hello");
        assert_eq!(safe_truncate("hello", 3), "...");
    }

    #[test]
    fn test_safe_truncate_japanese() {
        // Japanese characters: "こんにちは世界" (Hello World)
        let japanese = "こんにちは世界";
        assert_eq!(safe_truncate(japanese, 10), japanese);
        assert_eq!(safe_truncate(japanese, 5), "こん...");
        assert_eq!(safe_truncate(japanese, 7), japanese);
    }

    #[test]
    fn test_safe_truncate_mixed() {
        // Mixed ASCII and Japanese
        let mixed = "Hello こんにちは";
        assert_eq!(safe_truncate(mixed, 20), mixed);
        assert_eq!(safe_truncate(mixed, 8), "Hello...");
    }
}