base_n_benchmark.rs

use criterion::{criterion_group, criterion_main, Criterion};

use std::str;

pub const MAX_BASE: usize = 64;
pub const ALPHANUMERIC_ONLY: usize = 62;
pub const CASE_INSENSITIVE: usize = 36;

const BASE_64: &[u8; MAX_BASE] =
    b"0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ@$";

#[inline]
pub fn push_str(mut n: u128, base: usize, output: &mut String) {
    debug_assert!(base >= 2 && base <= MAX_BASE);
    let mut s = [0u8; 128];
    let mut index = 0;

    let base = base as u128;

    loop {
        s[index] = BASE_64[(n % base) as usize];
        index += 1;
        n /= base;

        if n == 0 {
            break;
        }
    }
    s[0..index].reverse();

    output.push_str(str::from_utf8(&s[0..index]).unwrap());
}

#[inline]
pub fn push_str2(mut n: u128, base: usize, output: &mut String) {
    debug_assert!(base >= 2 && base <= MAX_BASE);
    let mut s = [0u8; 128];
    let mut index = s.len();

    let base = base as u128;

    loop {
        index -= 1;
        s[index] = BASE_64[(n % base) as usize];
        n /= base;

        if n == 0 {
            break;
        }
    }

    output.push_str(unsafe {
        // SAFETY: `s` is populated using only valid utf8 characters from `BASE_64`
        str::from_utf8_unchecked(&s[index..])
    });
}

fn bench_push_str(c: &mut Criterion) {
    let mut group = c.benchmark_group("push_str");
    let xs = [0, 1, 35, 36, 37, u64::MAX as u128, u128::MAX];
    group.bench_function("old", |b| {
        b.iter(|| {
            let mut total = 0;
            let mut s = String::new();
            for base in 2..37 {
                for n in xs {
                    s.clear();
                    push_str(n, base, &mut s);
                    total += s.len();
                }
            }
            total
        })
    });
    group.bench_function("new", |b| {
        b.iter(|| {
            let mut total = 0;
            let mut s = String::new();
            for base in 2..37 {
                for n in xs {
                    s.clear();
                    push_str2(n, base, &mut s);
                    total += s.len();
                }
            }
            total
        })
    });
    group.finish();
}

criterion_group!(benches, bench_push_str);
criterion_main!(benches);