From 5cb8d0cfe7321dbea5befc34261ccab5785e9859 Mon Sep 17 00:00:00 2001
From: "Dr. Matthias Ratajczak" <m.ratajczak@hzdr.de>
Date: Wed, 28 Jun 2023 14:41:10 +0200
Subject: [PATCH] basic version of 95, very slow

---
 src/euler95/Cargo.toml  |   7 ++-
 src/euler95/src/main.rs | 117 +++++++++++++++++++++++++++-------------
 2 files changed, 86 insertions(+), 38 deletions(-)

diff --git a/src/euler95/Cargo.toml b/src/euler95/Cargo.toml
index 38eb5dd..269262f 100644
--- a/src/euler95/Cargo.toml
+++ b/src/euler95/Cargo.toml
@@ -5,4 +5,9 @@ edition = "2021"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
-[dependencies]
+[dev-dependencies]
+rstest = "0.17"
+
+[profile.release]
+debug = true
+lto = "fat"
diff --git a/src/euler95/src/main.rs b/src/euler95/src/main.rs
index 0bf37d4..f059047 100644
--- a/src/euler95/src/main.rs
+++ b/src/euler95/src/main.rs
@@ -1,51 +1,94 @@
-const MAX: usize = 1_000;
+use std::collections::HashSet;
+
+const MAX: usize = 1_000_000;
 
 fn main() {
-    let mut numbers: Vec<_> = (0_usize..MAX + 1).map(|_| true).collect();
-
     let mut chains = vec![];
 
-    let mut i = 0_usize;
-    loop {
-        if i > MAX {
-            break;
-        }
-        let mut num = if numbers[i] {
-            numbers[i] = false;
-            i
-        } else {
-            i += 1;
+    let mut part_of_any_chain = HashSet::new();
+
+    for current in 1..MAX {
+        if part_of_any_chain.contains(&current) {
             continue;
-        };
-
-        let mut maybe_chain = vec![];
-
-        loop {
-            let next: usize = get_proper_divisors(num).sum();
-            if next > MAX {
-                break;
-            }
-
-            if !numbers[next] {
-                if maybe_chain.contains(&next) {
-                    chains.push(maybe_chain);
-                }
-                break;
-            }
-
-            numbers[next] = false;
-            maybe_chain.push(num);
-
-            num = next;
         }
-
-        i += 1;
+        try_build_chain_from_number(MAX, current, &mut chains, &mut part_of_any_chain);
     }
 
-    dbg!(chains);
+    chains.sort_by_key(|chain| chain.len());
+
+    let longest = chains.last().unwrap();
+
+    let smallest = longest.iter().min().unwrap().to_owned();
+
+    println!("{smallest}");
+}
+
+fn try_build_chain_from_number(
+    max: usize,
+    mut current: usize,
+    chains: &mut Vec<Vec<usize>>,
+    part_of_any_chain: &mut HashSet<usize>,
+) {
+    let mut maybe_chain = vec![];
+
+    loop {
+        maybe_chain.push(current);
+
+        let next: usize = get_proper_divisors(current).sum();
+
+        if next > max || next == 0 {
+            break;
+        }
+
+        if maybe_chain.contains(&next) {
+            if maybe_chain.len() > 1 && *maybe_chain.first().unwrap() == next {
+                for number in &maybe_chain {
+                    part_of_any_chain.insert(*number);
+                }
+                println!("{}", maybe_chain.first().unwrap());
+                chains.push(maybe_chain);
+            }
+            break;
+        }
+
+        current = next;
+    }
 }
 
 fn get_proper_divisors(x: usize) -> impl Iterator<Item = usize> {
     let limit = x / 2 + 1;
     (1..limit).filter(move |i| x % *i == 0)
 }
+
+#[cfg(test)]
+mod tests {
+
+    use rstest::rstest;
+
+    #[rstest]
+    #[case(6, vec![1, 2, 3])]
+    #[case(28, vec![1, 2, 4, 7, 14])]
+    #[case(12_496, vec![1, 2, 4, 8, 11, 16, 22, 44, 71, 88, 142, 176, 284, 568, 781, 1136, 1562, 3124, 6248])]
+    fn proper_divisor(#[case] input: usize, #[case] expected: Vec<usize>) {
+        use crate::get_proper_divisors;
+
+        assert_eq!(get_proper_divisors(input).collect::<Vec<_>>(), expected);
+    }
+
+    #[rstest]
+    #[case(220, vec![vec![220, 284]])]
+    #[case(12_496, vec![vec![12_496, 14_288, 15_472, 14_536, 14_264]])]
+    fn chain(#[case] input: usize, #[case] expected: Vec<Vec<usize>>) {
+        use std::collections::HashSet;
+
+        use crate::try_build_chain_from_number;
+
+        let max = expected.iter().flatten().max().unwrap().to_owned();
+        let mut part_of_any_chain = HashSet::new();
+
+        let mut chains = vec![];
+        try_build_chain_from_number(max, input, &mut chains, &mut part_of_any_chain);
+
+        assert_eq!(chains, expected);
+    }
+}