Quelle  with_capacity.rs

use criterion::Criterion;
use hashbrown::HashMap;
use std::hint::black_box;

type Map<K, V> = HashMap<K, V>;

macro_rules! bench_with_capacity {
    ($c:ident, $name:ident, $cap:expr) => {
        $c.bench_function(stringify!($name), |b| {
            b.iter(|| {
                // Construct a new empty map with a given capacity and return it to avoid
                // being optimized away. Dropping it measures allocation + minimal setup.
                let m: Map<usize, usize> = Map::with_capacity($cap);
                black_box(m)
            });
        });
    };
}

pub(crate) fn register_benches(c: &mut Criterion) {
    bench_with_capacity!(c, with_capacity_000000, 0);
    bench_with_capacity!(c, with_capacity_000001, 1);
    bench_with_capacity!(c, with_capacity_000003, 3);
    bench_with_capacity!(c, with_capacity_000007, 7);
    bench_with_capacity!(c, with_capacity_000008, 8);
    bench_with_capacity!(c, with_capacity_000016, 16);
    bench_with_capacity!(c, with_capacity_000032, 32);
    bench_with_capacity!(c, with_capacity_000064, 64);
    bench_with_capacity!(c, with_capacity_000128, 128);
    bench_with_capacity!(c, with_capacity_000256, 256);
    bench_with_capacity!(c, with_capacity_000512, 512);
    bench_with_capacity!(c, with_capacity_001024, 1024);
    bench_with_capacity!(c, with_capacity_004096, 4096);
    bench_with_capacity!(c, with_capacity_016384, 16384);
    bench_with_capacity!(c, with_capacity_065536, 65536);
}