From e3c1f9bb5866e460a48ed62918d23b097203a1d2 Mon Sep 17 00:00:00 2001
From: Olivia Appleton <olivia.kay.appleton@gmail.com>
Date: Thu, 22 Aug 2024 18:12:47 -0500
Subject: [PATCH] Attempt to make p357 not slow

---
 rust/src/include/factors.rs | 71 +++++++++++++++++++++++++++++++++++++
 rust/src/main.rs            |  2 +-
 rust/src/p0357.rs           |  5 +--
 3 files changed, 75 insertions(+), 3 deletions(-)
 create mode 100644 rust/src/include/factors.rs

diff --git a/rust/src/include/factors.rs b/rust/src/include/factors.rs
new file mode 100644
index 00000000..f529c19f
--- /dev/null
+++ b/rust/src/include/factors.rs
@@ -0,0 +1,71 @@
+use std::collections::HashSet;
+use std::cmp::Ord;
+use std::hash::Hash;
+use std::ops::{Add,Div,Mul,Rem};
+
+use num_traits::{one,One,Zero};
+use itertools::Itertools;
+
+use crate::include::primes::prime_factors;
+
+pub struct ProperDivisors<I>
+{
+    seen: HashSet<I>,
+    factors: Vec<I>,
+    current_batch: Vec<I>,
+    curr_index: usize,
+    next_size: usize,
+}
+
+pub fn proper_divisors<I>(num: I) -> ProperDivisors<I>
+{
+    return ProperDivisors::<I>::new(num);
+}
+
+impl<I> ProperDivisors<I>
+where I: Hash + Zero + One + Add + Ord + Copy + Div<Output=I> + Rem<Output=I>
+{
+    pub fn new(num: I) -> Self {
+        return ProperDivisors::<I>{
+            seen: HashSet::new(),
+            factors: prime_factors(num).collect(),
+            current_batch: vec![],
+            curr_index: 0,
+            next_size: 1,
+        };
+    }
+}
+
+impl<I> Iterator for ProperDivisors<I>
+where I: Hash + Zero + One + Add + Ord + Copy + Div<Output=I> + Rem<Output=I> + Mul<Output=I>
+{
+    type Item = I;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        loop {
+            if self.next_size > self.factors.len() {
+                return None;
+            }
+            if self.curr_index == self.current_batch.len() {
+                self.current_batch = self.factors
+                                        .iter()
+                                        .cloned()
+                                        .combinations(self.next_size)
+                                        .map(|v| v.into_iter().fold(one(), |x, y| x * y))
+                                        .collect();
+                self.next_size += 1;
+                self.curr_index = 0;
+            }
+            while self.curr_index < self.current_batch.len() {
+                let result = self.current_batch[self.curr_index];
+                self.curr_index += 1;
+                if !self.seen.contains(&result) {
+                    self.seen.insert(result);
+                    return Some(result);
+                }
+            }
+            self.current_batch.clear();
+            self.curr_index = 0;
+        }
+    }
+}
\ No newline at end of file
diff --git a/rust/src/main.rs b/rust/src/main.rs
index f7db81e2..8ad20052 100644
--- a/rust/src/main.rs
+++ b/rust/src/main.rs
@@ -73,7 +73,7 @@ seq!(N in 01..=20 {
 #[case::problem_76(76)]
 #[case::problem_77(77)]
 #[case::problem_87(87)]
-// #[case::problem_357(357)]
+#[case::problem_357(357)]
 #[case::problem_836(836)]
 fn test_problem(#[case] id: usize) -> Result<(), String> {
     let Some((_, func, _slow)) = get_problem(id) else { panic!() };
diff --git a/rust/src/p0357.rs b/rust/src/p0357.rs
index 63193c7f..650dfef5 100644
--- a/rust/src/p0357.rs
+++ b/rust/src/p0357.rs
@@ -13,7 +13,8 @@ such that for every divisor d of n, d+n/d is prime.
 */
 use std::collections::HashSet;
 
-use crate::include::primes::{is_prime,primes_until,proper_divisors};
+use crate::include::primes::{is_prime,primes_until};
+use crate::include::factors::proper_divisors;
 use crate::include::utils::Answer;
 
 pub fn p0357() -> Answer {
@@ -24,7 +25,7 @@ pub fn p0357() -> Answer {
     for n in (6..100000000).step_by(4) {
         // n can't be odd (unless 1) because then n + n/d is even, and can't be a multiple of 4 as shown below
         let mut broken = false;
-        for d in proper_divisors(n).into_iter().chain(vec![n].into_iter()) {
+        for d in proper_divisors(n).chain(vec![n].into_iter()) {
             if prime_squares.contains(&d) || !is_prime(d + n / d) {
                 // this works because if n=kp^2, then whenever d=p, (p + kp^2/p) = (k+1)p, which isn't prime
                 // but since detecting if n % d^2 == 0 is expensive, I just wait for p^2 to show up