add vanishing polynomial of a coset

provers/circle_stark/src/constraints.rs

@@ -0,0 +1,42 @@
+use lambdaworks_math::{
+    circle::point::CirclePoint,
+    field::{element::FieldElement, fields::mersenne31::field::Mersenne31Field},
+};
+
+pub fn evaluate_vanishing_poly(
+    log_2_size: u32,
+    point: CirclePoint<Mersenne31Field>,
+) -> FieldElement<Mersenne31Field> {
+    let mut x = point.x;
+    for _ in 1..log_2_size {
+        x = x.square().double() - FieldElement::one();
+    }
+    x
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use lambdaworks_math::circle::cosets::Coset;
+
+    type FE = FieldElement<Mersenne31Field>;
+
+    #[test]
+    fn vanishing_poly_vanishes_in_coset() {
+        let log_2_size = 3;
+        let coset = Coset::new_standard(log_2_size);
+        let points = Coset::get_coset_points(&coset);
+        for point in points {
+            assert_eq!(evaluate_vanishing_poly(log_2_size, point), FE::zero());
+        }
+    }
+    #[test]
+    fn vanishing_poly_doesnt_vanishe_outside_coset() {
+        let log_2_size = 3;
+        let coset = Coset::new_standard(log_2_size + 1);
+        let points = Coset::get_coset_points(&coset);
+        for point in points {
+            assert_ne!(evaluate_vanishing_poly(log_2_size, point), FE::zero());
+        }
+    }
+}

provers/circle_stark/src/lib.rs

@@ -1,2 +1,3 @@
 pub mod config;
+pub mod constraints;
 pub mod prover;