### Collect Entropy Data for NIST SP 800-90B

Source: https://github.com/rust-random/rngs/blob/master/rand_jitter/README.md

This example demonstrates how to collect entropy data using `JitterRng::new_with_timer` and `timer_stats` for use with the NIST SP 800-90B Entropy Estimation Suite. It collects a specified number of timer deltas and saves them to binary files.

```rust
use rand_jitter::JitterRng;

use std::error::Error;
use std::fs::File;
use std::io::Write;

fn get_nstime() -> u64 {
    use std::time::{SystemTime, UNIX_EPOCH};

    let dur = SystemTime::now().duration_since(UNIX_EPOCH).unwrap();
    // The correct way to calculate the current time is
    // `dur.as_secs() * 1_000_000_000 + dur.subsec_nanos() as u64`
    // But this is faster, and the difference in terms of entropy is
    // negligible (log2(10^9) == 29.9).
    dur.as_secs() << 30 | dur.subsec_nanos() as u64
}

fn main() -> Result<(), Box<dyn Error>> {
    let mut rng = JitterRng::new_with_timer(get_nstime);

    // 1_000_000 results are required for the
    // NIST SP 800-90B Entropy Estimation Suite
    const ROUNDS: usize = 1_000_000;
    let mut deltas_variable: Vec<u8> = Vec::with_capacity(ROUNDS);
    let mut deltas_minimal: Vec<u8> = Vec::with_capacity(ROUNDS);

    for _ in 0..ROUNDS {
        deltas_variable.push(rng.timer_stats(true) as u8);
        deltas_minimal.push(rng.timer_stats(false) as u8);
    }

    // Write out after the statistics collection loop, to not disturb the
    // test results.
    File::create("jitter_rng_var.bin")?.write(&deltas_variable)?;
    File::create("jitter_rng_min.bin")?.write(&deltas_minimal)?;
    Ok(())
}
```

--------------------------------

### Estimate Entropy with NIST SP 800-90B (Shell)

Source: https://github.com/rust-random/rngs/blob/master/rand_jitter/README.md

These shell commands demonstrate how to use the NIST SP 800-90B Entropy Estimation Suite with the collected jitter RNG data. They cover estimating entropy with variable noise sources, and with minimal noise sources.

```sh
python noniid_main.py -v jitter_rng_var.bin 8
restart.py -v jitter_rng_var.bin 8 <min-entropy>
```

```sh
python noniid_main.py -v -u 4 jitter_rng_var.bin 4
restart.py -v -u 4 jitter_rng_var.bin 4 <min-entropy>
```

```sh
python noniid_main.py -v -u 4 jitter_rng_min.bin 4
restart.py -v -u 4 jitter_rng_min.bin 4 <min-entropy>
```

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