perf: Optimize clone (#443)

Introduce the notion of CompiledPolicy to hold stuff that
remains immutable during evaluation - e.g. rules, function,
schedules etc

Cloning takes about 60 nano seconds for an engine loaded with
ACI policies. Earlier it used to take 40 microseconds.
Thus there is easily more than 100x speedup.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

benches/regorus_benchmark.rs

@@ -1,6 +1,6 @@
 use std::hint::black_box;
 
-use regorus::Engine;
+use regorus::{Engine, Value};
 
 use criterion::{criterion_group, criterion_main, BenchmarkId, Criterion};
 use serde_json::json;
@@ -93,9 +93,59 @@ fn allow_with_simple_membership(c: &mut Criterion) {
     group.finish();
 }
 
+fn clone(c: &mut Criterion) {
+    // Use Arc<BtreeMap> as a reference. Clone will only increment
+    // the reference count.
+    let mut m = std::collections::BTreeMap::default();
+    m.insert(1, 2);
+    let m = std::sync::Arc::new(m);
+
+    c.bench_function("clone: Arc<BTreeMap>", |b| {
+        b.iter(|| {
+            let _ = m.clone();
+        })
+    });
+
+    let mut engine = Engine::new();
+    engine.set_rego_v0(true);
+    engine
+        .add_policy_from_file("tests/aci/framework.rego")
+        .unwrap();
+    engine.add_policy_from_file("tests/aci/api.rego").unwrap();
+    engine
+        .add_policy_from_file("tests/aci/policy.rego")
+        .unwrap();
+    engine
+        .add_data(Value::from_json_file("tests/aci/data.json").expect("failed to load data.json"))
+        .expect("failed to add data");
+    engine.set_input(
+        Value::from_json_file("tests/aci/input.json").expect("failed to load input.json"),
+    );
+
+    // An engine without preparation will not have processed fields populated.
+    c.bench_function("clone: engine with aci policies", |b| {
+        b.iter(|| {
+            let _ = engine.clone();
+        })
+    });
+
+    // Trigger engine preparation.
+    let _ = engine.eval_query("data.framework.mount_overlay".to_string(), false);
+
+    // Prepared engine will have many more fields populated. But the fields are
+    // immutable after preparation and will be shared between clones.
+    c.bench_function("clone: prepared engine with aci policies", |b| {
+        b.iter(|| {
+            let _ = engine.clone();
+        })
+    });
+}
+
 criterion_group!(
     benches,
     allow_with_simple_equality,
-    allow_with_simple_membership
+    allow_with_simple_membership,
+    clone
 );
+
 criterion_main!(benches);