From 1858a869a71039bb5b5301c96997ec7b6f22b92e Mon Sep 17 00:00:00 2001
From: wborgeaud <williamborgeaud@gmail.com>
Date: Thu, 1 Jul 2021 15:57:55 +0200
Subject: [PATCH] Optimize products of 1 element

---
 src/util/partial_products.rs | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/src/util/partial_products.rs b/src/util/partial_products.rs
index 2d9d33e2..f22ba01e 100644
--- a/src/util/partial_products.rs
+++ b/src/util/partial_products.rs
@@ -9,11 +9,17 @@ pub fn partial_products<T: Product + Copy>(v: &[T], max_degree: usize) -> Vec<T>
     while remainder.len() >= max_degree {
         let new_partials = remainder
             .chunks(max_degree)
-            // TODO: If `chunk.len()=1`, there's some redundant data.
+            .filter(|chunk| chunk.len() != 1)
             .map(|chunk| chunk.iter().copied().product())
             .collect::<Vec<_>>();
         res.extend_from_slice(&new_partials);
+        let addendum = if remainder.len() % max_degree == 1 {
+            vec![*remainder.last().unwrap()]
+        } else {
+            vec![]
+        };
         remainder = new_partials;
+        remainder.extend(addendum);
     }
 
     res
@@ -24,7 +30,8 @@ pub fn num_partial_products(n: usize, max_degree: usize) -> (usize, usize) {
     let mut remainder = n;
     while remainder >= max_degree {
         let new_partials_len = ceil_div_usize(remainder, max_degree);
-        res += new_partials_len;
+        let addendum = if remainder % max_degree == 1 { 1 } else { 0 };
+        res += new_partials_len - addendum;
         remainder = new_partials_len;
     }
 
@@ -42,10 +49,17 @@ pub fn check_partial_products<T: Product + Copy + Sub<Output = T>>(
     while remainder.len() >= max_degree {
         let products = remainder
             .chunks(max_degree)
+            .filter(|chunk| chunk.len() != 1)
             .map(|chunk| chunk.iter().copied().product())
             .collect::<Vec<T>>();
         res.extend(products.iter().zip(&partials).map(|(&a, &b)| a - b));
+        let addendum = if remainder.len() % max_degree == 1 {
+            vec![*remainder.last().unwrap()]
+        } else {
+            vec![]
+        };
         remainder = partials.drain(..products.len()).collect();
+        remainder.extend(addendum)
     }
 
     res
@@ -61,7 +75,7 @@ mod tests {
     fn test_partial_products() {
         let v = vec![1, 2, 3, 4, 5, 6];
         let p = partial_products(&v, 2);
-        assert_eq!(p, vec![2, 12, 30, 24, 30, 720]);
+        assert_eq!(p, vec![2, 12, 30, 24, 720]);
         assert_eq!(p.len(), num_partial_products(v.len(), 2).0);
         assert!(check_partial_products(&v, &p, 2)
             .iter()