fn request_id_hash(sequence: u64) -> String {
    // Same thread-local CSPRNG as before, now with no per-request SHA-256.
    // XORing an increasing sequence into independent uniformly random bytes
    // preserves the full 256-bit unpredictability and 64-hex-character shape.
    // The sequence is still mixed in; this is not a predictable counter ID.
    let mut token: [u8; 32] = rand::random();
    for (byte, sequence_byte) in token[..8].iter_mut().zip(sequence.to_be_bytes()) {
        *byte ^= sequence_byte;
    }
    request_id_hex(&token)
}

fn request_id_hex(digest: &[u8]) -> String {
    const HEX: &[u8; 16] = b"0123456789abcdef";
    // Encode two ASCII bytes per digest byte in one allocation; avoid 64
    // separate String::push bounds checks for the 32-byte SHA-256 digest.
    let mut encoded = vec![0u8; digest.len() * 2];
    for (i, &byte) in digest.iter().enumerate() {
        encoded[2 * i] = HEX[(byte >> 4) as usize];
        encoded[2 * i + 1] = HEX[(byte & 0x0f) as usize];
    }
    String::from_utf8(encoded).expect("hex digits are always valid UTF-8")
}

#[cfg(test)]
include!("request_id/tests.rs");