//! Identifiers (spec §9.4): ULIDs, 26 characters of Crockford base32, time

//! ordered and globally unique, so an id made on one machine merges into a

//! hub without renaming.

use rand::RngCore;

const CROCKFORD: &[u8; 32] = b"0123456789ABCDEFGHJKMNPQRSTVWXYZ";

/// A new ULID for the current time.

pub fn ulid() -> String {

    let mut r = [0u8; 16];

    rand::rngs::OsRng.fill_bytes(&mut r);

    ulid_from(now_ms(), u128::from_be_bytes(r))

}

/// A ULID from a millisecond time and 80 bits taken from `random`.

pub fn ulid_from(ms: u64, random: u128) -> String {

    let v: u128 = ((ms as u128 & ((1 << 48) - 1)) << 80) | (random & ((1u128 << 80) - 1));

    (0..26).map(|i| CROCKFORD[((v >> (125 - 5 * i)) & 31) as usize] as char).collect()

}

/// Milliseconds since the Unix epoch.

pub fn now_ms() -> u64 {

    std::time::SystemTime::now().duration_since(std::time::UNIX_EPOCH).map(|d| d.as_millis() as u64).unwrap_or(0)

}

#[cfg(test)]

mod tests {

    use super::*;

    #[test]

    fn ulids_are_26_chars_and_sort_by_time() {

        let a = ulid_from(1_000, 5);

        let b = ulid_from(2_000, 0);

        assert_eq!(a.len(), 26);

        assert!(a < b);

        assert_eq!(ulid_from(0, 0), "00000000000000000000000000");

        assert_ne!(ulid(), ulid());

    }

}