Add some top-level examples (#303)

The examples show:
- Building a Signed Pod with different types of values
- Building a MainPod
- Input SignedPod to MainPod
- Input MainPod to MainPod
- Using MainPod or MockMainPod
- Using custom predicates

examples/main_pod_points.rs

@@ -0,0 +1,159 @@
+//! Example of building main pods that verify signed pods and other main pods using custom
+//! predicates
+//!
+//! The example follows a scenario where a game issues signed pods to players with the points
+//! accumulated after finishing each game level.  Then we build a custom predicate to prove that
+//! the sum of points from level 1 and 2 for a player is over 9000.
+//!
+//! Run in real mode: `cargo run --release --example main_pod_points`
+//! Run in mock mode: `cargo run --release --example main_pod_points -- --mock`
+use std::env;
+
+use pod2::{
+    backends::plonky2::{
+        basetypes::DEFAULT_VD_SET, mainpod::Prover, mock::mainpod::MockProver,
+        primitives::ec::schnorr::SecretKey, signedpod::Signer,
+    },
+    frontend::{MainPodBuilder, SignedPodBuilder},
+    lang::parse,
+    middleware::{Params, PodProver, PodType, VDSet, Value, KEY_SIGNER, KEY_TYPE},
+    op,
+};
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let args: Vec<String> = env::args().collect();
+    let mock = args.get(1).is_some_and(|arg1| arg1 == "--mock");
+    if mock {
+        println!("Using MockMainPod")
+    } else {
+        println!("Using MainPod")
+    }
+
+    let params = Params::default();
+
+    let mock_prover = MockProver {};
+    let real_prover = Prover {};
+    let (vd_set, prover): (_, &dyn PodProver) = if mock {
+        (&VDSet::new(8, &[])?, &mock_prover)
+    } else {
+        println!("Prebuilding circuits to calculate vd_set...");
+        let vd_set = &*DEFAULT_VD_SET;
+        println!("vd_set calculation complete");
+        (vd_set, &real_prover)
+    };
+
+    // Create a schnorr key pair to sign pods
+    let game_sk = SecretKey::new_rand();
+    let game_pk = game_sk.public_key();
+
+    let mut game_signer = Signer(game_sk);
+
+    // Build 2 signed pods where the game assigns points to a player that has completed a level.
+    let mut builder = SignedPodBuilder::new(&params);
+    builder.insert("player", "Alice");
+    builder.insert("level", 1);
+    builder.insert("points", 3512);
+    let pod_points_lvl_1 = builder.sign(&mut game_signer)?;
+    pod_points_lvl_1.verify()?;
+    println!("# pod_points_lvl_1:\n{}", pod_points_lvl_1);
+
+    let mut builder = SignedPodBuilder::new(&params);
+    builder.insert("player", "Alice");
+    builder.insert("level", 2);
+    builder.insert("points", 5771);
+    let pod_points_lvl_2 = builder.sign(&mut game_signer)?;
+    pod_points_lvl_2.verify()?;
+    println!("# pod_points_lvl_2:\n{}", pod_points_lvl_2);
+
+    // Build a MainPod to prove >9000 points from sum of level 1 and 2
+
+    // Declare the custom predicate
+    let input = format!(
+        r#"
+        points(player, level, points, private: points_pod) = AND(
+            Equal(?points_pod["{key_type}"], {pod_type})
+            Equal(?points_pod["{key_signer}"], {game_pk:#})
+            Equal(?points_pod["player"], ?player)
+            Equal(?points_pod["level"], ?level)
+            Equal(?points_pod["points"], ?points)
+        )
+
+        over_9000(player, private: points_lvl_1, points_lvl_2, points_total) = AND(
+            points(?player, 1, ?points_lvl_1)
+            points(?player, 2, ?points_lvl_2)
+            SumOf(?points_total, ?points_lvl_1, ?points_lvl_2)
+            Gt(?points_total, 9000)
+        )
+    "#,
+        key_type = KEY_TYPE,
+        key_signer = KEY_SIGNER,
+        pod_type = PodType::Signed as usize,
+        game_pk = Value::from(game_pk).raw(),
+    );
+    println!("# custom predicate batch:{}", input);
+    let batch = parse(&input, &params, &[])?.custom_batch;
+    let points_pred = batch.predicate_ref_by_name("points").unwrap();
+    let over_9000_pred = batch.predicate_ref_by_name("over_9000").unwrap();
+
+    // Build a pod to prove the statement `points("Alice", 1, 3512)`
+    let mut builder = MainPodBuilder::new(&params, vd_set);
+    builder.add_signed_pod(&pod_points_lvl_1);
+    let st_type = builder.priv_op(op!(eq, (&pod_points_lvl_1, KEY_TYPE), PodType::Signed))?;
+    let st_signer = builder.priv_op(op!(eq, (&pod_points_lvl_1, KEY_SIGNER), game_pk))?;
+    let st_player = builder.priv_op(op!(eq, (&pod_points_lvl_1, "player"), "Alice"))?;
+    let st_level = builder.priv_op(op!(eq, (&pod_points_lvl_1, "level"), 1))?;
+    let st_points = builder.priv_op(op!(eq, (&pod_points_lvl_1, "points"), 3512))?;
+    let st_points_lvl_1 = builder.pub_op(op!(
+        custom,
+        points_pred.clone(),
+        st_type,
+        st_signer,
+        st_player,
+        st_level,
+        st_points
+    ))?;
+    let pod_alice_lvl_1_points = builder.prove(prover, &params).unwrap();
+    println!("# pod_alice_lvl_1_points\n:{}", pod_alice_lvl_1_points);
+    pod_alice_lvl_1_points.pod.verify().unwrap();
+
+    // Build a pod to prove the statement `points("Alice", 2, 5771)`
+    let mut builder = MainPodBuilder::new(&params, vd_set);
+    builder.add_signed_pod(&pod_points_lvl_2);
+    let st_type = builder.priv_op(op!(eq, (&pod_points_lvl_2, KEY_TYPE), PodType::Signed))?;
+    let st_signer = builder.priv_op(op!(eq, (&pod_points_lvl_2, KEY_SIGNER), game_pk))?;
+    let st_player = builder.priv_op(op!(eq, (&pod_points_lvl_2, "player"), "Alice"))?;
+    let st_level = builder.priv_op(op!(eq, (&pod_points_lvl_2, "level"), 2))?;
+    let st_points = builder.priv_op(op!(eq, (&pod_points_lvl_2, "points"), 5771))?;
+    let st_points_lvl_2 = builder.pub_op(op!(
+        custom,
+        points_pred,
+        st_type,
+        st_signer,
+        st_player,
+        st_level,
+        st_points
+    ))?;
+    let pod_alice_lvl_2_points = builder.prove(prover, &params).unwrap();
+    println!("# pod_alice_lvl_2_points\n:{}", pod_alice_lvl_2_points);
+    pod_alice_lvl_2_points.pod.verify().unwrap();
+
+    // Build a pod to prove the statement `over_9000("Alice")`
+    let mut builder = MainPodBuilder::new(&params, vd_set);
+    builder.add_recursive_pod(pod_alice_lvl_1_points);
+    builder.add_recursive_pod(pod_alice_lvl_2_points);
+    let st_points_total = builder.priv_op(op!(sum_of, 3512 + 5771, 3512, 5771))?;
+    let st_gt_9000 = builder.priv_op(op!(gt, 3512 + 5771, 9000))?;
+    let _st_over_9000 = builder.pub_op(op!(
+        custom,
+        over_9000_pred,
+        st_points_lvl_1,
+        st_points_lvl_2,
+        st_points_total,
+        st_gt_9000
+    ));
+    let pod_alice_over_9000 = builder.prove(prover, &params).unwrap();
+    println!("# pod_alice_over_9000\n:{}", pod_alice_over_9000);
+    pod_alice_over_9000.pod.verify().unwrap();
+
+    Ok(())
+}