Implement benchmarks for usernames API

Cargo.lock

@@ -2014,6 +2014,7 @@ checksum = "99c0ec316ab08201476c032feb2f94a5c8ece5b209765c1fbc4430dd6e931ad6"
 name = "usernames"
 version = "0.1.0"
 dependencies = [
+ "criterion",
  "curve25519-dalek",
  "displaydoc",
  "lazy_static",

rust/usernames/Cargo.toml

@@ -28,3 +28,8 @@ branch = "lizard2"
 [dev-dependencies]
 zkgroup = { path = "../zkgroup" }
 proptest = "1.0"
+criterion = "0.4"
+
+[[bench]]
+name = "usernames_benchmarks"
+harness = false

rust/usernames/benches/usernames_benchmarks.rs

@@ -0,0 +1,53 @@
+//
+// Copyright 2023 Signal Messenger, LLC.
+// SPDX-License-Identifier: AGPL-3.0-only
+//
+use criterion::{criterion_group, criterion_main, Criterion};
+use rand::rngs::OsRng;
+
+extern crate usernames;
+use usernames::{NicknameLimits, Username, UsernameError};
+
+pub fn username_hash(username: &str) -> Result<[u8; 32], UsernameError> {
+    Username::new(username).map(|un| un.hash())
+}
+
+pub fn username_proof(username: &str, randomness: &[u8]) -> Result<Vec<u8>, UsernameError> {
+    Username::new(username)?.proof(randomness)
+}
+
+// Username validation is inseparable from the hash/proof calculations and therefore its costs are
+// included in the benchmarks for both.
+fn bench_usernames(c: &mut Criterion) {
+    let mut rng = OsRng;
+    let usernames =
+        Username::candidates_from(&mut rng, "signal", NicknameLimits::default()).unwrap();
+
+    let mut infinite_usernames = usernames.iter().cycle();
+    c.bench_function("username_hash", |b| {
+        b.iter(|| username_hash(infinite_usernames.next().unwrap()))
+    });
+    let randomness: Vec<u8> = (0..32).collect();
+    c.bench_function("username_proof", |b| {
+        b.iter(|| username_proof(infinite_usernames.next().unwrap(), &randomness))
+    });
+
+    let mut infinite_input = usernames
+        .iter()
+        .map(|name| {
+            let username = Username::new(name).unwrap();
+            let proof = username.proof(&randomness).unwrap();
+            (username, proof)
+        })
+        .cycle();
+
+    c.bench_function("username_verify_proof", |b| {
+        b.iter(|| {
+            let (username, proof) = infinite_input.next().unwrap();
+            Username::verify_proof(&proof, username.hash()).unwrap()
+        })
+    });
+}
+
+criterion_group!(benches, bench_usernames);
+criterion_main!(benches);