From 1480876c9a3ea55c90e3dbe0f37db6f79545cc81 Mon Sep 17 00:00:00 2001
From: Daniel Lubarov <daniel@lubarov.com>
Date: Wed, 17 Feb 2021 22:19:18 -0800
Subject: [PATCH] Add some FRI params & clean up FFT a bit

---
 src/field/crandall_field.rs |  2 +-
 src/field/fft.rs            | 12 ++++++------
 src/field/field.rs          |  6 +++++-
 src/fri.rs                  | 21 +++++++++++++++++++++
 src/main.rs                 |  1 +
 5 files changed, 34 insertions(+), 8 deletions(-)
 create mode 100644 src/fri.rs

diff --git a/src/field/crandall_field.rs b/src/field/crandall_field.rs
index 9180e1e1..fd1a178e 100644
--- a/src/field/crandall_field.rs
+++ b/src/field/crandall_field.rs
@@ -50,7 +50,7 @@ impl Field for CrandallField {
         *self * *self * *self
     }
 
-    fn inverse(&self) -> Self {
+    fn try_inverse(&self) -> Option<Self> {
         unimplemented!()
     }
 
diff --git a/src/field/fft.rs b/src/field/fft.rs
index 243b2767..a75f1de5 100644
--- a/src/field/fft.rs
+++ b/src/field/fft.rs
@@ -34,10 +34,10 @@ impl<F: Field> FftPrecomputation<F> {
     }
 }
 
-// pub fn fft<F: Field>(coefficients: &[F]) -> Vec<F> {
-//     let precomputation = fft_precompute(coefficients.len());
-//     fft_with_precomputation(coefficients, &precomputation)
-// }
+pub fn fft<F: Field>(coefficients: Vec<F>) -> Vec<F> {
+    let precomputation = fft_precompute(coefficients.len());
+    fft_with_precomputation_power_of_2(coefficients, &precomputation)
+}
 
 pub fn fft_precompute<F: Field>(degree: usize) -> FftPrecomputation<F> {
     let degree_pow = log2_ceil(degree);
@@ -54,11 +54,11 @@ pub fn fft_precompute<F: Field>(degree: usize) -> FftPrecomputation<F> {
 }
 
 pub fn ifft_with_precomputation_power_of_2<F: Field>(
-    points: &[F],
+    points: Vec<F>,
     precomputation: &FftPrecomputation<F>,
 ) -> Vec<F> {
     let n = points.len();
-    let n_inv = Field(n as u64).multiplicative_inverse().unwrap();
+    let n_inv = F::from_canonical_usize(n).try_inverse().unwrap();
     let mut result = fft_with_precomputation_power_of_2(points, precomputation);
 
     // We reverse all values except the first, and divide each by n.
diff --git a/src/field/field.rs b/src/field/field.rs
index ecb70adc..34aa25af 100644
--- a/src/field/field.rs
+++ b/src/field/field.rs
@@ -25,7 +25,11 @@ pub trait Field: 'static
     fn cube(&self) -> Self;
 
     /// Compute the multiplicative inverse of this field element.
-    fn inverse(&self) -> Self;
+    fn try_inverse(&self) -> Option<Self>;
+
+    fn inverse(&self) -> Self {
+        self.try_inverse().expect("Tried to invert zero")
+    }
 
     fn primitive_root_of_unity(n_power: usize) -> Self;
 
diff --git a/src/fri.rs b/src/fri.rs
new file mode 100644
index 00000000..d7ebfee5
--- /dev/null
+++ b/src/fri.rs
@@ -0,0 +1,21 @@
+/// Somewhat arbitrary. Smaller values will increase delta, but with diminishing returns,
+/// while increasing L, potentially requiring more challenge points.
+const EPSILON: f64 = 0.01;
+
+fn fri_delta(rate_log: usize, conjecture: bool) -> f64 {
+    let rate = (1 << rate_log) as f64;
+    if conjecture {
+        todo!()
+    } else {
+        return 1.0 - rate.sqrt() - EPSILON;
+    }
+}
+
+fn fri_l(rate_log: usize, conjecture: bool) -> f64 {
+    let rate = (1 << rate_log) as f64;
+    if conjecture {
+        todo!()
+    } else {
+        return 1.0 / (2.0 * EPSILON * rate.sqrt());
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index a790c66d..0fe2cf14 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -13,6 +13,7 @@ use crate::util::log2_ceil;
 mod circuit_data;
 mod constraint_polynomial;
 mod field;
+mod fri;
 mod gates;
 mod generator;
 mod gmimc;