Partial implementation of the MockMainPod (#33)

* wip

* wip

* feat: handle public statements

* fix: remove old backend module

src/middleware.rs

@@ -171,6 +171,7 @@ pub struct Params {
     pub max_signed_pod_values: usize,
     pub max_public_statements: usize,
     pub max_statement_args: usize,
+    pub max_operation_args: usize,
 }
 
 impl Params {
@@ -188,6 +189,7 @@ impl Default for Params {
             max_signed_pod_values: 8,
             max_public_statements: 10,
             max_statement_args: 5,
+            max_operation_args: 5,
         }
     }
 }
@@ -220,6 +222,29 @@ pub trait SignedPod: fmt::Debug + DynClone {
 // impl Clone for Box<dyn SignedPod>
 dyn_clone::clone_trait_object!(SignedPod);
 
+/// This is a filler type that fulfills the SignedPod trait and always verifies.  It's empty.  This
+/// can be used to simulate padding in a circuit.
+#[derive(Debug, Clone)]
+pub struct NoneSignedPod {}
+
+impl SignedPod for NoneSignedPod {
+    fn verify(&self) -> bool {
+        true
+    }
+    fn id(&self) -> PodId {
+        PodId(NULL)
+    }
+    fn kvs(&self) -> HashMap<Hash, Value> {
+        HashMap::new()
+    }
+    fn pub_statements(&self) -> Vec<Statement> {
+        Vec::new()
+    }
+    fn into_any(self: Box<Self>) -> Box<dyn Any> {
+        self
+    }
+}
+
 pub trait PodSigner {
     fn sign(&mut self, params: &Params, kvs: &HashMap<Hash, Value>) -> Result<Box<dyn SignedPod>>;
 }
@@ -305,12 +330,38 @@ pub trait MainPod: fmt::Debug + DynClone {
 // impl Clone for Box<dyn SignedPod>
 dyn_clone::clone_trait_object!(MainPod);
 
+/// This is a filler type that fulfills the MainPod trait and always verifies.  It's empty.  This
+/// can be used to simulate padding in a circuit.
+#[derive(Debug, Clone)]
+pub struct NoneMainPod {}
+
+impl MainPod for NoneMainPod {
+    fn verify(&self) -> bool {
+        true
+    }
+    fn id(&self) -> PodId {
+        PodId(NULL)
+    }
+    fn pub_statements(&self) -> Vec<Statement> {
+        Vec::new()
+    }
+    fn into_any(self: Box<Self>) -> Box<dyn Any> {
+        self
+    }
+}
+
+// TODO: Figure out a way to signal which signed_pods entries and which main_pods statements need
+// to be made public.  Idea: introduce an operation called reveal, which the backend translates to
+// CopyOf but moves copies that statement to a public slot?
 #[derive(Debug)]
 pub struct MainPodInputs<'a> {
-    pub signed_pods: &'a [&'a dyn SignedPod],
-    pub main_pods: &'a [&'a dyn MainPod],
+    pub signed_pods: &'a [&'a Box<dyn SignedPod>],
+    pub main_pods: &'a [&'a Box<dyn MainPod>],
     pub statements: &'a [Statement],
     pub operations: &'a [Operation],
+    /// Statements that need to be made public (they can come from input pods or input
+    /// statements)
+    pub public_statements: &'a [Statement],
 }
 
 pub trait PodProver {