ocserve-core 0.1.0

//! Upstream-shaped resource IDs: `<prefix>_<26 chars>` where the first 10 hex
//! chars encode ms-timestamp LE (matches observed `msg_0fa1e3f770014jHIfQNh0Lhiu3`).

use sha2::{Digest, Sha256};

fn encode(ms: u64, rand_tail: &[u8]) -> String {
    let mut h = Sha256::new();
    h.update(ms.to_le_bytes());
    h.update(rand_tail);
    let hex = hex::encode(h.finalize());
    // upstream body = 26 chars: 13-char time hex + 13-char hash slice
    // (observed `msg_0fa1e3f770014jHIfQNh0Lhiu3`)
    let time_hex = format!("{ms:013x}"); // pads; current ms fits 13
    format!("{time_hex}{}", &hex[..13])
}

pub fn msg_id() -> String {
    let ms = now_ms();
    let nonce: Vec<u8> = std::iter::repeat_with(rand_byte).take(8).collect();
    format!("msg_{}", encode(ms, &nonce[..8]))
}

pub fn prt_id() -> String {
    let ms = now_ms();
    let nonce: Vec<u8> = std::iter::repeat_with(rand_byte).take(8).collect();
    format!("prt_{}", encode(ms, &nonce[..8]))
}

pub fn ses_id() -> String {
    let ms = now_ms();
    let nonce: Vec<u8> = std::iter::repeat_with(rand_byte).take(8).collect();
    format!("ses_{}", encode(ms, &nonce[..8]))
}

/// Question request id (v1 QuestionID: `que_` +26 chars, ascending).
pub fn que_id() -> String {
    let ms = now_ms();
    let nonce: Vec<u8> = std::iter::repeat_with(rand_byte).take(8).collect();
    format!("que_{}", encode(ms, &nonce[..8]))
}

pub fn evt_id() -> String {
    let ms = now_ms();
    let nonce: Vec<u8> = std::iter::repeat_with(rand_byte).take(8).collect();
    format!("evt_{}", encode(ms, &nonce[..8]))
}

fn now_ms() -> u64 {
    std::time::SystemTime::now()
        .duration_since(std::time::UNIX_EPOCH)
        .map(|d| d.as_millis() as u64)
        .unwrap_or(0)
}

fn rand_byte() -> u8 {
    use std::collections::hash_map::RandomState;
    use std::hash::{BuildHasher, Hasher};
    let mut h = RandomState::new().build_hasher();
    h.write_u64(now_ms());
    h.finish() as u8
}

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

    #[test]
    fn ids_have_upstream_shape() {
        for id in [msg_id(), prt_id(), ses_id(), evt_id()] {
            let (prefix, body) = id.split_once('_').unwrap();
            assert!(matches!(prefix, "msg" | "prt" | "ses" | "evt"));
            assert_eq!(body.len(), 26, "upstream ids are prefix+26: {id}");
            assert!(body.chars().all(|c| c.is_ascii_hexdigit()), "{id}");
        }
    }

    #[test]
    fn ids_unique_enough() {
        let a = msg_id();
        let b = msg_id();
        assert_ne!(a, b, "collision within a millisecond: {a}");
    }
}