factor::numeric::gcd: Add criterion-powered benchmark

The function had to be made `pub`, this is a [known limitation] of Criterion.

[known limitation]: https://bheisler.github.io/criterion.rs/book/user_guide/known_limitations.html

src/uu/factor/Cargo.toml

@@ -22,8 +22,14 @@ uucore = { version="0.0.4", package="uucore", git="https://github.com/uutils/uuc
 uucore_procs = { version="0.0.4", package="uucore_procs", git="https://github.com/uutils/uucore.git", branch="canary" }
 
 [dev-dependencies]
+criterion = "0.3"
 paste = "0.1.18"
 quickcheck = "0.9.2"
+rand_chacha = "0.2.2"
+
+[[bench]]
+name = "gcd"
+harness = false
 
 [[bin]]
 name = "factor"

src/uu/factor/benches/gcd.rs

@@ -0,0 +1,29 @@
+use criterion::{criterion_group, criterion_main, BenchmarkId, Criterion};
+use uu_factor::numeric;
+
+fn gcd(c: &mut Criterion) {
+    let inputs = {
+        // Deterministic RNG; use an explicitely-named RNG to guarantee stability
+        use rand::{RngCore, SeedableRng};
+        use rand_chacha::ChaCha8Rng;
+        const SEED: u64 = 0xa_b4d_1dea_dead_cafe;
+        let mut rng = ChaCha8Rng::seed_from_u64(SEED);
+
+        std::iter::repeat_with(move || (rng.next_u64(), rng.next_u64()))
+    };
+
+    let mut group = c.benchmark_group("gcd");
+    for (n, m) in inputs.take(10) {
+        group.bench_with_input(
+            BenchmarkId::from_parameter(format!("{}_{}", n, m)),
+            &(n, m),
+            |b, &(n, m)| {
+                b.iter(|| numeric::gcd(n, m));
+            },
+        );
+    }
+    group.finish()
+}
+
+criterion_group!(benches, gcd);
+criterion_main!(benches);

src/uu/factor/src/cli.rs

@@ -16,7 +16,7 @@ mod factor;
 pub(crate) use factor::*;
 
 mod miller_rabin;
-mod numeric;
+pub mod numeric;
 mod rho;
 mod table;
 

src/uu/factor/src/numeric.rs

@@ -17,7 +17,7 @@ use std::mem::swap;
 // This is incorrectly reported as dead code,
 //  presumably when included in build.rs.
 #[allow(dead_code)]
-pub(crate) fn gcd(mut a: u64, mut b: u64) -> u64 {
+pub fn gcd(mut a: u64, mut b: u64) -> u64 {
     while b > 0 {
         a %= b;
         swap(&mut a, &mut b);