Factor the crdt test code

fuzz/src/lib.rs

@@ -0,0 +1,45 @@
+use garage_table::crdt::Crdt;
+use std::fmt::Debug;
+
+pub fn check_crdt_laws<T>(a: T, b: T, c: T)
+where
+	T: Crdt + PartialEq + Clone + Debug,
+{
+	// Idempotency: merge(a, a) == a
+	{
+		let mut a2 = a.clone();
+		a2.merge(&a.clone());
+		assert_eq!(a2, a, "merge is not idempotent: {a2:#?} != {a:#?}");
+	}
+
+	// Commutativity: merge(a, b) == merge(b, a)
+	let ab = {
+		let mut t = a.clone();
+		t.merge(&b);
+		t
+	};
+	let ba = {
+		let mut t = b.clone();
+		t.merge(&a);
+		t
+	};
+	assert_eq!(ab, ba, "merge is not commutative: {ab:#?} != {ba:#?}");
+
+	// Associativity: merge(merge(a, b), c) == merge(a, merge(b, c))
+	let ab_c = {
+		let mut t = ab.clone();
+		t.merge(&c);
+		t
+	};
+	let bc = {
+		let mut t = b.clone();
+		t.merge(&c);
+		t
+	};
+	let a_bc = {
+		let mut t = a.clone();
+		t.merge(&bc);
+		t
+	};
+	assert_eq!(ab_c, a_bc, "merge is not associative: {ab_c:#?} != {a_bc:#?}");
+}