Feature: generate FRI parameters compatible with L1 verifier (#7)

Problem: the FRI parameters generation is not compatible with the L1
verifier. The step list needs to start with 0 and the last entry cannot
be 1.

Solution: implement an alternative computation method for the L1
verifier and allow the user to select his verifier.

src/fri.rs

@@ -1,4 +1,8 @@
-use crate::models::{FriParameters, ProverParameters, StarkParameters};
+use crate::models::{FriParameters, ProverParameters, StarkParameters, Verifier};
+
+const DEFAULT_LAST_LAYER_DEGREE_BOUND: u32 = 64;
+const DEFAULT_N_QUERIES: u32 = 18;
+const DEFAULT_PROOF_OF_WORK_BITS: u32 = 24;
 
 /// Implements ceil(log2(x)).
 fn ceil_log2(x: u32) -> u32 {
@@ -9,48 +13,114 @@ fn ceil_log2(x: u32) -> u32 {
     log
 }
 
-/// Computes the FRI steps list based on the number of Cairo steps of the program.
+/// Computes the FRI steps list based on the specified parameters.
 ///
 /// This computation is based on the documentation of the Stone prover:
 /// # log₂(#steps) + 4 = log₂(last_layer_degree_bound) + ∑fri_step_list
 /// # log₂(#steps) = log₂(last_layer_degree_bound) + ∑fri_step_list - 4
 /// # ∑fri_step_list = log₂(#steps) + 4 - log₂(last_layer_degree_bound)
 ///
-/// * `nb_steps`: Number of Cairo steps of the program.
-/// * `last_layer_degree_bound`: Last layer degree bound.
+/// * `nb_steps_log`: Ceiled log₂ of the number of Cairo steps of the program.
+/// * `last_layer_degree_bound_log`: Ceiled log₂ of the last layer degree bound.
+/// * `max_step_value`: Maximum value for each step. All elements will be in the range
+///   [0, `max_step_value`].
 ///
 /// Returns The FRI steps list.
-pub fn compute_fri_steps(nb_steps: u32, last_layer_degree_bound: u32) -> Vec<u32> {
-    let nb_steps_log = ceil_log2(nb_steps);
-    let last_layer_degree_bound_log = ceil_log2(last_layer_degree_bound);
-
-    let sigma_fri_step_list = nb_steps_log + 4 - last_layer_degree_bound_log;
-    let quotient = (sigma_fri_step_list / 4) as usize;
-    let remainder = sigma_fri_step_list % 4;
+fn compute_fri_steps(
+    nb_steps_log: u32,
+    last_layer_degree_bound_log: u32,
+    max_step_value: u32,
+) -> Vec<u32> {
+    let sum_of_fri_steps = nb_steps_log + 4 - last_layer_degree_bound_log;
+    let quotient = (sum_of_fri_steps / max_step_value) as usize;
+    let remainder = sum_of_fri_steps % max_step_value;
 
-    let mut fri_steps = vec![4; quotient];
+    let mut fri_steps = vec![max_step_value; quotient];
     if remainder > 0 {
         fri_steps.push(remainder);
     }
 
     fri_steps
 }
 
+pub trait FriComputer {
+    fn compute_fri_parameters(&self, nb_steps: u32) -> FriParameters;
+}
+
+pub struct DefaultFriComputer;
+
+impl FriComputer for DefaultFriComputer {
+    fn compute_fri_parameters(&self, nb_steps: u32) -> FriParameters {
+        let last_layer_degree_bound = 64;
+
+        let nb_steps_log = ceil_log2(nb_steps);
+        let last_layer_degree_bound_log = ceil_log2(last_layer_degree_bound);
+        let max_step_value = 4;
+
+        let fri_steps =
+            compute_fri_steps(nb_steps_log, last_layer_degree_bound_log, max_step_value);
+
+        FriParameters {
+            fri_step_list: fri_steps,
+            last_layer_degree_bound,
+            n_queries: DEFAULT_N_QUERIES,
+            proof_of_work_bits: DEFAULT_PROOF_OF_WORK_BITS,
+        }
+    }
+}
+
+pub struct L1VerifierFriComputer;
+
+impl FriComputer for L1VerifierFriComputer {
+    fn compute_fri_parameters(&self, nb_steps: u32) -> FriParameters {
+        // The L1 verifier accepts FRI steps in [0, 1, 2].
+        let max_step_value = 2;
+
+        let nb_steps_log = ceil_log2(nb_steps);
+
+        let (last_layer_degree_bound, last_layer_degree_bound_log) = {
+            let mut lldb = DEFAULT_LAST_LAYER_DEGREE_BOUND;
+            // The last step cannot be 1, prevent this by reducing the last layer degree bound.
+            // Using log₂(#steps) + 4 = log₂(last_layer_degree_bound) + ∑fri_step_list,
+            // we just need log₂(#steps) - log₂(last_layer_degree_bound) to be even.
+            let mut lldb_log = ceil_log2(lldb);
+            if ((nb_steps_log - lldb_log) % 2) != 0 {
+                lldb /= 2;
+                lldb_log -= 1;
+            }
+            (lldb, lldb_log)
+        };
+
+        // The first FRI step must be 0
+        let mut fri_steps = vec![0];
+        fri_steps.extend(compute_fri_steps(
+            nb_steps_log,
+            last_layer_degree_bound_log,
+            max_step_value,
+        ));
+
+        FriParameters {
+            fri_step_list: fri_steps,
+            last_layer_degree_bound,
+            n_queries: DEFAULT_N_QUERIES,
+            proof_of_work_bits: DEFAULT_PROOF_OF_WORK_BITS,
+        }
+    }
+}
+
 /// Generates prover parameters based on program parameters.
 ///
 /// * `nb_steps`: Number of Cairo steps of the program.
 /// * `last_layer_degree_bound`: Last layer degree bound.
-pub fn generate_prover_parameters(nb_steps: u32, last_layer_degree_bound: u32) -> ProverParameters {
-    let fri_steps = compute_fri_steps(nb_steps, last_layer_degree_bound);
+pub fn generate_prover_parameters(nb_steps: u32, verifier: Verifier) -> ProverParameters {
+    let fri_parameters = match verifier {
+        Verifier::L1 => L1VerifierFriComputer.compute_fri_parameters(nb_steps),
+        _ => DefaultFriComputer.compute_fri_parameters(nb_steps),
+    };
     ProverParameters {
         field: "PrimeField0".to_string(),
         stark: StarkParameters {
-            fri: FriParameters {
-                fri_step_list: fri_steps,
-                last_layer_degree_bound,
-                n_queries: 18,
-                proof_of_work_bits: 24,
-            },
+            fri: fri_parameters,
             log_n_cosets: 4,
         },
         use_extension_field: false,
@@ -76,9 +146,33 @@ mod tests {
     #[case(32768, vec ! [4, 4, 4, 1])]
     #[case(524288, vec ! [4, 4, 4, 4, 1])]
     #[case(768, vec ! [4, 4])]
-    fn test_compute_fri_step_list(#[case] nb_steps: u32, #[case] expected: Vec<u32>) {
-        let last_layer_degree_bound = 64;
-        let step_list = compute_fri_steps(nb_steps, last_layer_degree_bound);
-        assert_eq!(step_list, expected);
+    fn test_compute_fri_parameters_default(#[case] nb_steps: u32, #[case] expected: Vec<u32>) {
+        let expected_last_layer_degree_bound = 64;
+        let fri_parameters = DefaultFriComputer.compute_fri_parameters(nb_steps);
+
+        assert_eq!(fri_parameters.fri_step_list, expected);
+        assert_eq!(
+            fri_parameters.last_layer_degree_bound,
+            expected_last_layer_degree_bound
+        );
+    }
+
+    /// # ∑fri_step_list = log₂(#steps) + 4 - log₂(last_layer_degree_bound)
+    #[rstest]
+    #[case(32768, vec ! [0, 2, 2, 2, 2, 2, 2, 2], 32)]
+    #[case(524288, vec ! [0, 2, 2, 2, 2, 2, 2, 2, 2, 2], 32)]
+    #[case(768, vec ! [0, 2, 2, 2, 2], 64)]
+    fn test_compute_fri_parameters_l1_verifier(
+        #[case] nb_steps: u32,
+        #[case] expected_fri_steps: Vec<u32>,
+        #[case] expected_last_layer_degree_bound: u32,
+    ) {
+        let fri_parameters = L1VerifierFriComputer.compute_fri_parameters(nb_steps);
+
+        assert_eq!(fri_parameters.fri_step_list, expected_fri_steps);
+        assert_eq!(
+            fri_parameters.last_layer_degree_bound,
+            expected_last_layer_degree_bound
+        );
     }
 }

src/models.rs

@@ -8,6 +8,38 @@ use std::str::FromStr;
 use serde::{Deserialize, Serialize};
 use serde_json::Value;
 
+#[derive(Debug, Clone)]
+pub enum Verifier {
+    Stone,
+    L1,
+}
+
+impl FromStr for Verifier {
+    type Err = String;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let verifier = match s {
+            "stone" => Self::Stone,
+            "l1" => Self::L1,
+            other => {
+                return Err(format!("unknown verifier: {other}"));
+            }
+        };
+
+        Ok(verifier)
+    }
+}
+
+impl Display for Verifier {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        let s = match self {
+            Self::Stone => "stone",
+            Self::L1 => "l1",
+        };
+        write!(f, "{}", s)
+    }
+}
+
 #[derive(Serialize, Deserialize, Debug, PartialEq)]
 pub struct CachedLdeConfig {
     pub store_full_lde: bool,