Circle starks

math/src/circle/cfft.rs

@@ -0,0 +1,220 @@
+extern crate alloc;
+use crate::field::{element::FieldElement, fields::mersenne31::field::Mersenne31Field};
+use alloc::vec::Vec;
+
+#[cfg(feature = "alloc")]
+/// fft in place algorithm used to evaluate a polynomial of degree 2^n - 1 in 2^n points.
+/// Input must be of size 2^n for some n.
+pub fn cfft(
+    input: &mut [FieldElement<Mersenne31Field>],
+    twiddles: Vec<Vec<FieldElement<Mersenne31Field>>>,
+) {
+    // If the input size is 2^n, then log_2_size is n.
+    let log_2_size = input.len().trailing_zeros();
+
+    // The cfft has n layers.
+    (0..log_2_size).for_each(|i| {
+        // In each layer i we split the current input in chunks of size 2^{i+1}.
+        let chunk_size = 1 << (i + 1);
+        let half_chunk_size = 1 << i;
+        input.chunks_mut(chunk_size).for_each(|chunk| {
+            // We split each chunk in half, calling the first half hi_part and the second hal low_part.
+            let (hi_part, low_part) = chunk.split_at_mut(half_chunk_size);
+
+            // We apply the corresponding butterfly for every element j of the high and low part.
+            hi_part
+                .iter_mut()
+                .zip(low_part)
+                .enumerate()
+                .for_each(|(j, (hi, low))| {
+                    let temp = *low * twiddles[i as usize][j];
+                    *low = *hi - temp;
+                    *hi += temp
+                });
+        });
+    });
+}
+
+#[cfg(feature = "alloc")]
+/// The inverse fft algorithm used to interpolate 2^n points.
+/// Input must be of size 2^n for some n.
+pub fn icfft(
+    input: &mut [FieldElement<Mersenne31Field>],
+    twiddles: Vec<Vec<FieldElement<Mersenne31Field>>>,
+) {
+    // If the input size is 2^n, then log_2_size is n.
+    let log_2_size = input.len().trailing_zeros();
+
+    // The icfft has n layers.
+    (0..log_2_size).for_each(|i| {
+        // In each layer i we split the current input in chunks of size 2^{n - i}.
+        let chunk_size = 1 << (log_2_size - i);
+        let half_chunk_size = chunk_size >> 1;
+        input.chunks_mut(chunk_size).for_each(|chunk| {
+            // We split each chunk in half, calling the first half hi_part and the second hal low_part.
+            let (hi_part, low_part) = chunk.split_at_mut(half_chunk_size);
+
+            // We apply the corresponding butterfly for every element j of the high and low part.
+            hi_part
+                .iter_mut()
+                .zip(low_part)
+                .enumerate()
+                .for_each(|(j, (hi, low))| {
+                    let temp = *hi + *low;
+                    *low = (*hi - *low) * twiddles[i as usize][j];
+                    *hi = temp;
+                });
+        });
+    });
+}
+
+/// This function permutes a slice of field elements to order the result of the cfft in the natural way.
+/// We call the natural order to [P(x0, y0), P(x1, y1), P(x2, y2), ...],
+/// where (x0, y0) is the first point of the corresponding coset.
+/// The cfft doesn't return the evaluations in the natural order.
+/// For example, if we apply the cfft to 8 coefficients of a polynomial of degree 7 we'll get the evaluations in this order:
+/// [P(x0, y0), P(x2, y2), P(x4, y4), P(x6, y6), P(x7, y7), P(x5, y5), P(x3, y3), P(x1, y1)],
+/// where the even indices are found first in ascending order and then the odd indices in descending order.
+/// This function permutes the slice [0, 2, 4, 6, 7, 5, 3, 1] into [0, 1, 2, 3, 4, 5, 6, 7].
+/// TODO: This can be optimized by performing in-place value swapping (WIP).  
+pub fn order_cfft_result_naive(
+    input: &mut [FieldElement<Mersenne31Field>],
+) -> Vec<FieldElement<Mersenne31Field>> {
+    let mut result = Vec::new();
+    let length = input.len();
+    for i in 0..length / 2 {
+        result.push(input[i]); // We push the left index.
+        result.push(input[length - i - 1]); // We push the right index.
+    }
+    result
+}
+
+/// This function permutes a slice of field elements to order the input of the icfft in a specific way.
+/// For example, if we want to interpolate 8 points we should input them in the icfft in this order:
+/// [(x0, y0), (x2, y2), (x4, y4), (x6, y6), (x7, y7), (x5, y5), (x3, y3), (x1, y1)],
+/// where the even indices are found first in ascending order and then the odd indices in descending order.
+/// This function permutes the slice [0, 1, 2, 3, 4, 5, 6, 7] into [0, 2, 4, 6, 7, 5, 3, 1].
+/// TODO: This can be optimized by performing in-place value swapping (WIP).  
+pub fn order_icfft_input_naive(
+    input: &mut [FieldElement<Mersenne31Field>],
+) -> Vec<FieldElement<Mersenne31Field>> {
+    let mut result = Vec::new();
+
+    // We push the even indices.
+    (0..input.len()).step_by(2).for_each(|i| {
+        result.push(input[i]);
+    });
+
+    // We push the odd indices.
+    (1..input.len()).step_by(2).rev().for_each(|i| {
+        result.push(input[i]);
+    });
+    result
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    type FE = FieldElement<Mersenne31Field>;
+
+    #[test]
+    fn ordering_cfft_result_works_for_4_points() {
+        let expected_slice = [FE::from(0), FE::from(1), FE::from(2), FE::from(3)];
+
+        let mut slice = [FE::from(0), FE::from(2), FE::from(3), FE::from(1)];
+
+        let res = order_cfft_result_naive(&mut slice);
+
+        assert_eq!(res, expected_slice)
+    }
+
+    #[test]
+    fn ordering_cfft_result_works_for_16_points() {
+        let expected_slice = [
+            FE::from(0),
+            FE::from(1),
+            FE::from(2),
+            FE::from(3),
+            FE::from(4),
+            FE::from(5),
+            FE::from(6),
+            FE::from(7),
+            FE::from(8),
+            FE::from(9),
+            FE::from(10),
+            FE::from(11),
+            FE::from(12),
+            FE::from(13),
+            FE::from(14),
+            FE::from(15),
+        ];
+
+        let mut slice = [
+            FE::from(0),
+            FE::from(2),
+            FE::from(4),
+            FE::from(6),
+            FE::from(8),
+            FE::from(10),
+            FE::from(12),
+            FE::from(14),
+            FE::from(15),
+            FE::from(13),
+            FE::from(11),
+            FE::from(9),
+            FE::from(7),
+            FE::from(5),
+            FE::from(3),
+            FE::from(1),
+        ];
+
+        let res = order_cfft_result_naive(&mut slice);
+
+        assert_eq!(res, expected_slice)
+    }
+
+    #[test]
+    fn from_natural_to_icfft_input_order_works() {
+        let mut slice = [
+            FE::from(0),
+            FE::from(1),
+            FE::from(2),
+            FE::from(3),
+            FE::from(4),
+            FE::from(5),
+            FE::from(6),
+            FE::from(7),
+            FE::from(8),
+            FE::from(9),
+            FE::from(10),
+            FE::from(11),
+            FE::from(12),
+            FE::from(13),
+            FE::from(14),
+            FE::from(15),
+        ];
+
+        let expected_slice = [
+            FE::from(0),
+            FE::from(2),
+            FE::from(4),
+            FE::from(6),
+            FE::from(8),
+            FE::from(10),
+            FE::from(12),
+            FE::from(14),
+            FE::from(15),
+            FE::from(13),
+            FE::from(11),
+            FE::from(9),
+            FE::from(7),
+            FE::from(5),
+            FE::from(3),
+            FE::from(1),
+        ];
+
+        let res = order_icfft_input_naive(&mut slice);
+
+        assert_eq!(res, expected_slice)
+    }
+}

math/src/circle/cosets.rs

@@ -0,0 +1,95 @@
+extern crate alloc;
+use crate::circle::point::CirclePoint;
+use crate::field::fields::mersenne31::field::Mersenne31Field;
+use alloc::vec::Vec;
+
+/// Given g_n, a generator of the subgroup <g_n> of the circle of size n,
+/// and given a shift, that is a another point of the cirvle,
+/// we define the coset shift + <g_n> which is the set of all the points in
+/// <g_n> plus the shift.
+/// For example, if <g_4> = {p1, p2, p3, p4}, then g_8 + <g_4> = {g_8 + p1, g_8 + p2, g_8 + p3, g_8 + p4}.
+
+#[derive(Debug, Clone)]
+pub struct Coset {
+    // Coset: shift + <g_n> where n = 2^{log_2_size}.
+    // Example: g_16 + <g_8>, n = 8, log_2_size = 3, shift = g_16.
+    pub log_2_size: u32, //TODO: Change log_2_size to u8 because log_2_size < 31.
+    pub shift: CirclePoint<Mersenne31Field>,
+}
+
+impl Coset {
+    pub fn new(log_2_size: u32, shift: CirclePoint<Mersenne31Field>) -> Self {
+        Coset { log_2_size, shift }
+    }
+
+    /// Returns the coset g_2n + <g_n>
+    pub fn new_standard(log_2_size: u32) -> Self {
+        // shift is a generator of the subgroup of order 2n = 2^{log_2_size + 1}.
+        let shift = CirclePoint::get_generator_of_subgroup(log_2_size + 1);
+        Coset { log_2_size, shift }
+    }
+
+    /// Returns g_n, the generator of the subgroup of order n = 2^log_2_size.
+    pub fn get_generator(&self) -> CirclePoint<Mersenne31Field> {
+        CirclePoint::GENERATOR.repeated_double(31 - self.log_2_size)
+    }
+
+    /// Given a standard coset g_2n + <g_n>, returns the subcoset with half size g_2n + <g_{n/2}>
+    pub fn half_coset(coset: Self) -> Self {
+        Coset {
+            log_2_size: coset.log_2_size - 1,
+            shift: coset.shift,
+        }
+    }
+
+    /// Given a coset shift + G returns the coset -shift + G.
+    /// Note that (g_2n + <g_{n/2}>) U (-g_2n + <g_{n/2}>) = g_2n + <g_n>.
+    pub fn conjugate(coset: Self) -> Self {
+        Coset {
+            log_2_size: coset.log_2_size,
+            shift: coset.shift.conjugate(),
+        }
+    }
+
+    /// Returns the vector of shift + g for every g in <g_n>.
+    /// where g = i * g_n for i = 0, ..., n-1.
+    #[cfg(feature = "alloc")]
+    pub fn get_coset_points(coset: &Self) -> Vec<CirclePoint<Mersenne31Field>> {
+        // g_n the generator of the subgroup of order n.
+        let generator_n = CirclePoint::get_generator_of_subgroup(coset.log_2_size);
+        let size: u8 = 1 << coset.log_2_size;
+        core::iter::successors(Some(coset.shift.clone()), move |prev| {
+            Some(prev + &generator_n)
+        })
+        .take(size.into())
+        .collect()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn coset_points_vector_has_right_size() {
+        let coset = Coset::new_standard(3);
+        let points = Coset::get_coset_points(&coset);
+        assert_eq!(1 << coset.log_2_size, points.len())
+    }
+
+    #[test]
+    fn antipode_of_coset_point_is_in_coset() {
+        let coset = Coset::new_standard(3);
+        let points = Coset::get_coset_points(&coset);
+        let point = points[2].clone();
+        let anitpode_point = points[6].clone();
+        assert_eq!(anitpode_point, point.antipode())
+    }
+
+    #[test]
+    fn coset_generator_has_right_order() {
+        let coset = Coset::new(2, CirclePoint::GENERATOR * 3);
+        let generator_n = coset.get_generator();
+        assert_eq!(generator_n.repeated_double(2), CirclePoint::zero());
+    }
+}

math/src/circle/mod.rs

@@ -0,0 +1,5 @@
+pub mod cfft;
+pub mod cosets;
+pub mod point;
+pub mod polynomial;
+pub mod twiddles;