add non-circuit ceiling_log2

proof-input/src/gen_input.rs

@@ -5,7 +5,7 @@ use plonky2_field::types::Field;
 use plonky2_poseidon2::poseidon2_hash::poseidon2::Poseidon2;
 use codex_plonky2_circuits::circuits::params::{CircuitParams, HF};
 use crate::params::TestParams;
-use crate::utils::{bits_le_padded_to_usize, calculate_cell_index_bits, usize_to_bits_le};
+use crate::utils::{bits_le_padded_to_usize, calculate_cell_index_bits, ceiling_log2, usize_to_bits_le};
 use codex_plonky2_circuits::merkle_tree::merkle_safe::{MerkleProof, MerkleTree};
 use codex_plonky2_circuits::circuits::sample_cells::{Cell, MerklePath, SampleCircuit, SampleCircuitInput};
 use plonky2::iop::witness::PartialWitness;
@@ -60,9 +60,7 @@ pub fn verify_circuit_input<
     // check dataset level proof
     let slot_proof = circ_input.slot_proof.clone();
     let dataset_path_bits = usize_to_bits_le(slot_index as usize, params.dataset_max_depth());
-    let last_index = params.n_slots - 1;
-    let dataset_last_bits = usize_to_bits_le(last_index, params.dataset_max_depth());
-    let dataset_mask_bits = usize_to_bits_le(last_index, params.dataset_max_depth()+1);
+    let (dataset_last_bits, dataset_mask_bits) = ceiling_log2(params.n_slots, params.dataset_max_depth());
     let reconstructed_slot_root = MerkleProof::<F,D>::reconstruct_root2(
         slot_root,
         dataset_path_bits,
@@ -75,7 +73,6 @@ pub fn verify_circuit_input<
     assert_eq!(reconstructed_slot_root, circ_input.dataset_root.clone());
 
     // check each sampled cell
-
     // get the index for cell from H(slot_root|counter|entropy)
     let mask_bits = usize_to_bits_le(params.n_cells -1, params.max_depth);
     for i in 0..params.n_samples {

proof-input/src/utils.rs

@@ -82,6 +82,31 @@ pub fn bits_le_padded_to_usize(bits: &[bool]) -> usize {
     })
 }
 
+/// computes the `last_index` (the binary decomposition of `inp-1`) and the `mask_bits`
+pub fn ceiling_log2(
+    inp: usize,
+    n: usize,
+) -> (Vec<bool>, Vec<bool>) {
+    // Handle the case when inp is 0
+    let last_index = if inp == 0 { panic!("input to ceiling_log2 is 0") } else { inp - 1 };
+    let last_bits = usize_to_bits_le(last_index, n);
+
+    // Initialize aux, all false
+    let mut aux = vec![false; n+1];
+    aux[n] = true; // aux[n] = 1
+
+    // Initialize mask vector
+    let mut mask = vec![false; n+1];
+
+    // Compute aux and mask bits
+    for i in (0..n).rev() {
+        aux[i] = aux[i + 1] && !last_bits[i];
+        mask[i] = !aux[i];
+    }
+
+    (last_bits, mask)
+}
+
 /// prove given the circuit data and partial witness
 pub fn prove<
     F: RichField + Extendable<D> + Poseidon2,