More ergonomic usage of batch_mul with table (arkworks-rs#757)

* More ergonomic usage of `batch_mul` with table

* Tweak

ec/src/scalar_mul/mod.rs

@@ -115,18 +115,47 @@ pub trait ScalarMul:
     /// ```
     fn batch_mul(self, v: &[Self::ScalarField]) -> Vec<Self::MulBase> {
         let table = BatchMulPreprocessing::new(self, v.len());
-        self.batch_mul_with_preprocessing(v, &table)
+        Self::batch_mul_with_preprocessing(&table, v)
     }
 
+    /// Compute the vector v[0].G, v[1].G, ..., v[n-1].G, given:
+    /// - an element `g`
+    /// - a list `v` of n scalars
+    ///
+    /// This method allows the user to provide a precomputed table of multiples of `g`.
+    /// A more ergonomic way to call this would be to use [`BatchMulPreprocessing::batch_mul`].
+    ///
+    /// # Example
+    /// ```
+    /// use ark_std::{One, UniformRand};
+    /// use ark_ec::pairing::Pairing;
+    /// use ark_test_curves::bls12_381::G1Projective as G;
+    /// use ark_test_curves::bls12_381::Fr;
+    /// use ark_ec::scalar_mul::*;
+    ///
+    /// // Compute G, s.G, s^2.G, ..., s^9.G
+    /// let mut rng = ark_std::test_rng();
+    /// let max_degree = 10;
+    /// let s = Fr::rand(&mut rng);
+    /// let g = G::rand(&mut rng);
+    /// let mut powers_of_s = vec![Fr::one()];
+    /// let mut cur = s;
+    /// for _ in 0..max_degree {
+    ///     powers_of_s.push(cur);
+    ///     cur *= &s;
+    /// }
+    /// let table = BatchMulPreprocessing::new(g, powers_of_s.len());
+    /// let powers_of_g = G::batch_mul_with_preprocessing(&table, &powers_of_s);
+    /// let powers_of_g_2 = table.batch_mul(&powers_of_s);
+    /// let naive_powers_of_g: Vec<G> = powers_of_s.iter().map(|e| g * e).collect();
+    /// assert_eq!(powers_of_g, naive_powers_of_g);
+    /// assert_eq!(powers_of_g_2, naive_powers_of_g);
+    /// ```
     fn batch_mul_with_preprocessing(
-        self,
+        table: &BatchMulPreprocessing<Self>,
         v: &[Self::ScalarField],
-        preprocessing: &BatchMulPreprocessing<Self>,
     ) -> Vec<Self::MulBase> {
-        let result = cfg_iter!(v)
-            .map(|e| preprocessing.windowed_mul(e))
-            .collect::<Vec<_>>();
-        Self::batch_convert_to_mul_base(&result)
+        table.batch_mul(v)
     }
 }
 
@@ -137,25 +166,22 @@ pub trait ScalarMul:
 pub struct BatchMulPreprocessing<T: ScalarMul> {
     pub window: usize,
     pub max_scalar_size: usize,
+    pub max_num_scalars: usize,
     pub table: Vec<Vec<T::MulBase>>,
 }
 
 impl<T: ScalarMul> BatchMulPreprocessing<T> {
     pub fn new(base: T, num_scalars: usize) -> Self {
-        let window = Self::compute_window_size(num_scalars);
         let scalar_size = T::ScalarField::MODULUS_BIT_SIZE as usize;
-        Self::with_window_and_scalar_size(base, window, scalar_size)
-    }
-
-    fn compute_window_size(num_scalars: usize) -> usize {
-        if num_scalars < 32 {
-            3
-        } else {
-            ln_without_floats(num_scalars)
-        }
+        Self::with_num_scalars_and_scalar_size(base, num_scalars, scalar_size)
     }
 
-    pub fn with_window_and_scalar_size(base: T, window: usize, max_scalar_size: usize) -> Self {
+    pub fn with_num_scalars_and_scalar_size(
+        base: T,
+        max_num_scalars: usize,
+        max_scalar_size: usize,
+    ) -> Self {
+        let window = Self::compute_window_size(max_num_scalars);
         let in_window = 1 << window;
         let outerc = (max_scalar_size + window - 1) / window;
         let last_in_window = 1 << (max_scalar_size - (outerc - 1) * window);
@@ -193,10 +219,30 @@ impl<T: ScalarMul> BatchMulPreprocessing<T> {
         Self {
             window,
             max_scalar_size,
+            max_num_scalars,
             table,
         }
     }
 
+    pub fn compute_window_size(num_scalars: usize) -> usize {
+        if num_scalars < 32 {
+            3
+        } else {
+            ln_without_floats(num_scalars)
+        }
+    }
+
+    pub fn batch_mul(&self, v: &[T::ScalarField]) -> Vec<T::MulBase> {
+        assert!(
+            v.len() <= self.max_num_scalars,
+            "number of scalars exceeds the maximum number of scalars supported by this table"
+        );
+        let result = cfg_iter!(v)
+            .map(|e| self.windowed_mul(e))
+            .collect::<Vec<_>>();
+        T::batch_convert_to_mul_base(&result)
+    }
+
     fn windowed_mul(&self, scalar: &T::ScalarField) -> T {
         let outerc = (self.max_scalar_size + self.window - 1) / self.window;
         let modulus_size = T::ScalarField::MODULUS_BIT_SIZE as usize;