Overflow checks enabled. (#156)

* Sanity workflow in debug mode.
  * Overflow occurs while calculating the Collatz sequence in a solution.
  * By some weird coincidence, this did not affect the result.
  * Running in debug mode will terminate the program upon overflow.
* Switched back to release mode, but enabled overflow checks.
* Improved logic to avoid overflow on 32-bit systems.
* Formatted.
* `Dockerfile`: more precise path.

Cargo.toml

@@ -12,4 +12,5 @@ harness = false
 
 [profile.release]
 incremental = true
+overflow-checks = true
 strip = "debuginfo"

Dockerfile

@@ -1,4 +1,4 @@
 FROM --platform=linux/i386 alpine:3.19
 RUN apk add cargo
-WORKDIR /project-euler
+WORKDIR /github.com/tfpf/project-euler
 COPY . .

src/solutions/longest_collatz_sequence.rs

@@ -1,27 +1,38 @@
 /// Find the length of the Collatz sequence starting from the given number
 /// and ending at 1.
 ///
-/// * `collatz_lengths` - Array to cache the lengths.
+/// * `collatz_lengths` - Array to cache the lengths. Indexable with `num`.
 /// * `num` - Number to find the length of the Collatz sequence for.
 ///
 /// -> Length of Collatz sequence.
 fn get_collatz_length(collatz_lengths: &mut Vec<i32>, num: usize) -> i32 {
-    if num == 1 {
-        return 1;
-    }
-    if num < collatz_lengths.len() && collatz_lengths[num] != 0 {
+    if collatz_lengths[num] != 0 {
         return collatz_lengths[num];
     }
-    let num_next = if num & 1 == 0 { num >> 1 } else { 3 * num + 1 };
-    let len = 1 + get_collatz_length(collatz_lengths, num_next);
-    if num < collatz_lengths.len() {
-        collatz_lengths[num] = len;
+    // In the course of Collatz iterations, a 32-bit number will overflow,
+    // which could lead to problems on 32-bit systems. Temporarily use a 64-bit
+    // number.
+    let mut num_next = num as u64;
+    for count in 1.. {
+        // Keep transforming this number until it can be used as an index into
+        // the cache.
+        num_next = if num_next % 2 == 0 {
+            num_next / 2
+        } else {
+            3 * num_next + 1
+        };
+        if num_next >= collatz_lengths.len() as u64 {
+            continue;
+        }
+        collatz_lengths[num] = count + get_collatz_length(collatz_lengths, num_next as usize);
+        return collatz_lengths[num];
     }
-    len
+    unreachable!();
 }
 
 pub fn solve() -> i64 {
     let mut collatz_lengths = vec![0; 1000000];
+    collatz_lengths[1] = 1;
     let result = (2..collatz_lengths.len())
         .map(|num| (get_collatz_length(&mut collatz_lengths, num), num))
         .max()