Add worst case AI benchmark

The worst case AI update time is where a new game with an empty board
must be evaluated.

See issue #5.

Cargo.toml

@@ -12,3 +12,10 @@ edition = "2018"
 
 [dependencies]
 rand = "0.7.2"
+
+[dev-dependencies]
+criterion = "0.3.0"
+
+[[bench]]
+name = "benchmarks"
+harness = false

benches/benchmarks.rs

@@ -0,0 +1,30 @@
+#[macro_use]
+extern crate criterion;
+
+use criterion::Criterion;
+
+use open_ttt_lib::ai;
+use open_ttt_lib::game;
+
+fn flawless_ai_moves_benchmarks(c: &mut Criterion) {
+    let game = game::Game::new();
+    let mistake_probability = 0.0;
+    let ai_opponent = ai::Opponent::new(mistake_probability);
+
+    c.bench_function("New game with flawless AI", |b| {
+        b.iter(|| ai_opponent.get_move(&game))
+    });
+}
+
+fn build_benchmark_configuration() -> Criterion {
+    // At the moment the AI takes a LONG time to update so reduce
+    // the sample size so the test completes in this decade.
+    Criterion::default().sample_size(10)
+}
+
+criterion_group!(
+    name = benches; 
+    config = build_benchmark_configuration(); 
+    targets = flawless_ai_moves_benchmarks);
+
+criterion_main!(benches);