// Truncate a &str to a byte budget at a char boundary (prefix)
#[inline]
pub fn take_bytes_at_char_boundary(s: &str, maxb: usize) -> &str {
    if s.len() <= maxb {
        return s;
    }
    let mut last_ok = 0;
    for (i, ch) in s.char_indices() {
        let nb = i + ch.len_utf8();
        if nb > maxb {
            break;
        }
        last_ok = nb;
    }
    &s[..last_ok]
}

// Take a suffix of a &str within a byte budget at a char boundary
#[inline]
pub fn take_last_bytes_at_char_boundary(s: &str, maxb: usize) -> &str {
    if s.len() <= maxb {
        return s;
    }
    let mut start = s.len();
    let mut used = 0usize;
    for (i, ch) in s.char_indices().rev() {
        let nb = ch.len_utf8();
        if used + nb > maxb {
            break;
        }
        start = i;
        used += nb;
        if start == 0 {
            break;
        }
    }
    &s[start..]
}

/// Sanitize a tag value to comply with metric tag validation rules:
/// only ASCII alphanumeric, '.', '_', '-', and '/' are allowed.
pub fn sanitize_metric_tag_value(value: &str) -> String {
    const MAX_LEN: usize = 256;
    let sanitized: String = value
        .chars()
        .map(|ch| {
            if ch.is_ascii_alphanumeric() || matches!(ch, '.' | '_' | '-' | '/') {
                ch
            } else {
                '_'
            }
        })
        .collect();
    let trimmed = sanitized.trim_matches('_');
    if trimmed.is_empty() || trimmed.chars().all(|ch| !ch.is_ascii_alphanumeric()) {
        return "unspecified".to_string();
    }
    if trimmed.len() <= MAX_LEN {
        trimmed.to_string()
    } else {
        trimmed[..MAX_LEN].to_string()
    }
}

#[cfg(test)]
mod tests {
    use super::sanitize_metric_tag_value;
    use pretty_assertions::assert_eq;

    #[test]
    fn sanitize_metric_tag_value_trims_and_fills_unspecified() {
        let msg = "///";
        assert_eq!(sanitize_metric_tag_value(msg), "unspecified");
    }

    #[test]
    fn sanitize_metric_tag_value_replaces_invalid_chars() {
        let msg = "bad value!";
        assert_eq!(sanitize_metric_tag_value(msg), "bad_value");
    }
}