Poseidon with logup (#712)

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crates/prover/src/constraint_framework/logup.rs

@@ -1,8 +1,9 @@
 use std::ops::{Add, Mul, Sub};
 
 use itertools::Itertools;
-use num_traits::Zero;
+use num_traits::{One, Zero};
 
+use super::EvalAtRow;
 use crate::core::backend::simd::column::SecureColumn;
 use crate::core::backend::simd::m31::LOG_N_LANES;
 use crate::core::backend::simd::prefix_sum::inclusive_prefix_sum;
@@ -19,8 +20,102 @@ use crate::core::poly::BitReversedOrder;
 use crate::core::utils::shifted_secure_combination;
 use crate::core::ColumnVec;
 
+/// Evaluates constraints for batched logups.
+/// These constraint enforce the sum of multiplicity_i / (z + sum_j alpha^j * x_j) = claimed_sum.
+/// BATCH_SIZE is the number of fractions to batch together. The degree of the resulting constraints
+/// will be BATCH_SIZE + 1.
+pub struct LogupAtRow<const BATCH_SIZE: usize, E: EvalAtRow> {
+    /// The index of the interaction used for the cumulative sum columns.
+    pub interaction: usize,
+    /// Queue of fractions waiting to be batched together.
+    pub queue: [(E::EF, E::EF); BATCH_SIZE],
+    /// Number of fractions in the queue.
+    pub queue_size: usize,
+    /// The claimed sum of all the fractions.
+    pub claimed_sum: SecureField,
+    /// The evaluation of the last cumulative sum column.
+    pub prev_col_cumsum: E::EF,
+    /// The value of the `is_first` constant column at current row.
+    /// See [`super::constant_columns::gen_is_first()`].
+    pub is_first: E::F,
+}
+impl<const BATCH_SIZE: usize, E: EvalAtRow> LogupAtRow<BATCH_SIZE, E> {
+    pub fn new(interaction: usize, claimed_sum: SecureField, is_first: E::F) -> Self {
+        Self {
+            interaction,
+            queue: [(E::EF::zero(), E::EF::zero()); BATCH_SIZE],
+            queue_size: 0,
+            claimed_sum,
+            prev_col_cumsum: E::EF::zero(),
+            is_first,
+        }
+    }
+    pub fn push_lookup(
+        &mut self,
+        eval: &mut E,
+        numerator: E::EF,
+        values: &[E::F],
+        lookup_elements: LookupElements,
+    ) {
+        let shifted_value = shifted_secure_combination(
+            values,
+            E::EF::from(lookup_elements.alpha),
+            E::EF::from(lookup_elements.z),
+        );
+        self.push_frac(eval, numerator, shifted_value);
+    }
+
+    pub fn push_frac(&mut self, eval: &mut E, numerator: E::EF, denominator: E::EF) {
+        if self.queue_size < BATCH_SIZE {
+            self.queue[self.queue_size] = (numerator, denominator);
+            self.queue_size += 1;
+            return;
+        }
+
+        // Compute sum_i pi/qi over batch, as a fraction, num/denom.
+        let (num, denom) = self
+            .queue
+            .iter()
+            .copied()
+            .fold((E::EF::zero(), E::EF::one()), |(p0, q0), (pi, qi)| {
+                (p0 * qi + pi * q0, qi * q0)
+            });
+
+        self.queue[0] = (numerator, denominator);
+        self.queue_size = 1;
+
+        // Add a constraint that num / denom = diff.
+        let cur_cumsum = E::combine_ef(std::array::from_fn(|_| {
+            eval.next_interaction_mask(self.interaction, [0])[0]
+        }));
+        let diff = cur_cumsum - self.prev_col_cumsum;
+        self.prev_col_cumsum = cur_cumsum;
+        eval.add_constraint(diff * denom - num);
+    }
+
+    pub fn finalize(self, eval: &mut E) {
+        let (num, denom) = self.queue[0..self.queue_size]
+            .iter()
+            .copied()
+            .fold((E::EF::zero(), E::EF::one()), |(p0, q0), (pi, qi)| {
+                (p0 * qi + pi * q0, qi * q0)
+            });
+
+        let cumsum_mask =
+            std::array::from_fn(|_| eval.next_interaction_mask(self.interaction, [0, -1]));
+        let cur_cumsum = E::combine_ef(cumsum_mask.map(|[cur_row, _prev_row]| cur_row));
+        let prev_row_cumsum = E::combine_ef(cumsum_mask.map(|[_cur_row, prev_row]| prev_row));
+
+        // Fix `prev_row_cumsum` by subtracting `claimed_sum` if this is the first row.
+        let fixed_prev_row_cumsum = prev_row_cumsum - self.is_first * self.claimed_sum;
+        let diff = cur_cumsum - fixed_prev_row_cumsum - self.prev_col_cumsum;
+
+        eval.add_constraint(diff * denom - num);
+    }
+}
+
 /// Interaction elements for the logup protocol.
-#[derive(Copy, Clone, Debug)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub struct LookupElements {
     pub z: SecureField,
     pub alpha: SecureField,

crates/prover/src/constraint_framework/mod.rs

@@ -8,7 +8,7 @@ mod simd_domain;
 
 use std::array;
 use std::fmt::Debug;
-use std::ops::{Add, AddAssign, Mul, Sub};
+use std::ops::{Add, AddAssign, Mul, Neg, Sub};
 
 pub use assert::{assert_constraints, AssertEvaluator};
 pub use info::InfoEvaluator;
@@ -45,13 +45,15 @@ pub trait EvalAtRow {
         + Copy
         + Debug
         + Zero
+        + Neg<Output = Self::EF>
         + Add<SecureField, Output = Self::EF>
         + Sub<SecureField, Output = Self::EF>
         + Mul<SecureField, Output = Self::EF>
         + Add<Self::F, Output = Self::EF>
         + Mul<Self::F, Output = Self::EF>
         + Sub<Self::EF, Output = Self::EF>
-        + Mul<Self::EF, Output = Self::EF>;
+        + Mul<Self::EF, Output = Self::EF>
+        + From<SecureField>;
 
     /// Returns the next mask value for the first interaction at offset 0.
     fn next_trace_mask(&mut self) -> Self::F {