incr

Simple, fast and self-contained structs for tracking whether a newly observed value (u64) is greater than the largest previously observed value.

Use cases include message sequence numbers, timestamps, and other situations that present a need to quickly assess whether incoming data is "new", i.e. its numbering is larger than any previous value.

All of the structs include an is_new function that returns true if the passed value is a new maximum, while simultaneously storing the new value to check against future values.

Two of the is_new implementations (Incr and Map) require an &mut self signature, while RcIncr and AtomicIncr require only &self due to RcIncr's interior mutability and AtomicIncr's thread safe syncrhonization.

The cost of checking a new value is minimal: 0-2ns for the single-threaded implementations, and ~5-10ns for AtomicIncr, except in cases of pathological contention. In a worst-case, nightmare scenrio benchmark for the AtomicIncr, it's possible to induce delays of hundreds of nanoseconds. A more realistic case of 24 threads contending to increment the atomic but yielding each iteration resulted in checks in the ~5-10ns range.

Enabling the "nightly" feature (on by default) allows the use of AtomicU64 as the backing storage for AtomicIncr (vs. AtomicUsize otherwise). Also, nightly is required to run the benchmarks.

Usage

Add this to your Cargo.toml:

[dependencies]
incr = "0.2"

Examples

Simple usage:

use incr::Incr;
let mut last = Incr::default();
assert_eq!(last.is_new(1), true);
assert_eq!(last.is_new(1), false);
assert_eq!(last.is_new(2), true);
assert_eq!(last.get(), 2);

AtomicIncr offers a threadsafe implementation:

extern crate incr;

use std::thread::{spawn, JoinHandle};
use std::sync::{Arc, Barrier};
use incr::AtomicIncr;

fn main() {
    let last: AtomicIncr = Default::default();
    let barrier = Arc::new(Barrier::new(2));
    let thread: JoinHandle<u64> = {
        let barrier = Arc::clone(&barrier);
        let last = last.clone();
        spawn(move || {
            assert_eq!(last.is_new(2), true);
            assert_eq!(last.is_new(3), true);
            assert_eq!(last.is_new(3), false);
            barrier.wait();
            last.get()
        })
    };
    barrier.wait();
    assert_eq!(last.is_new(3), false);
    assert_eq!(thread.join().unwrap(), 3);
}