1 files changed, 66 insertions, 0 deletions
diff --git a/2023/rs/code/eight/src/main.rs b/2023/rs/code/eight/src/main.rs
new file mode 100644
index 0000000..9e58f07
--- /dev/null
+++ b/2023/rs/code/eight/src/main.rs
@@ -0,0 +1,66 @@
+use std::collections::BTreeMap;
+
+static INPUT: &str = include_str!("input.txt");
+
+fn gcd(mut a: u128, mut b: u128) -> u128 {
+    while b > 0 {
+        (a, b) = (b, a % b)
+    }
+    a
+}
+
+fn lcm(a: u128, b: u128) -> u128 {
+  a * b / gcd(a, b)
+}
+
+fn main() {
+  let (valueified_instr, valueified_triples) = {
+    let mut iter = INPUT.lines();
+    let instr = iter.next().unwrap().bytes().map(|c| c == b'R');
+    iter.next().unwrap();
+    let triples = iter.map(|s| (&s[0..3], (&s[7..10], &s[12..15]))).collect::<BTreeMap<_, _>>();
+    (instr, triples)
+  };
+
+  let mut last = "AAA";
+  for (n, instr) in valueified_instr.clone().cycle().enumerate() {
+    if last == "ZZZ" {
+      println!("Steps to reach `ZZZ`: {n}");
+      break;
+    }
+    let pair = valueified_triples[last];
+    last = if instr { pair.1 } else { pair.0 };
+  }
+
+/*
+  let mut lasts = valueified_triples.keys().filter(|s| s.as_bytes()[2] == b'A').cloned().map(|a| (a, 0u128)).collect::<Vec<_>>();
+  for last in &mut lasts {
+    for (n, instr) in valueified_instr.clone().cycle().enumerate() {
+      let pair = valueified_triples[last.0];
+      last.0 = if instr { pair.1 } else { pair.0 };
+      if last.0.as_bytes()[2] == b'Z' {
+        last.1 = n as u128;
+        break;
+      }
+    }
+  }
+  dbg!(&lasts);
+  // yes, i could optimize the maths here, but i do not feel like it.
+  let total = lasts.into_iter().fold(1, |a, (_, n)| lcm(n, a));
+  println!("Steps for all to reach `Z`: {total}");
+
+  OK there's totally a way to do this cleverly like above but I can't think of it. This method overshoots, apparently.
+  It assumes the period here is the time it takes to reach Z. What I really need to do is like, find the LCM of the real periods but offset such that the final LCM is n below and blah blah ugh im gonna write the brute forcer and shit and hope it's done.
+  */
+  let mut lasts = valueified_triples.keys().filter(|s| s.as_bytes()[2] == b'A').cloned().collect::<Vec<_>>();
+  for (n, instr) in valueified_instr.clone().cycle().enumerate() {
+    for last in &mut lasts {
+      let pair = valueified_triples[*last];
+      *last = if instr { pair.1 } else { pair.0 };
+    }
+    if lasts.iter().all(|a| a.as_bytes()[2] == b'Z') {
+      println!("Steps for all to reach `Z`: {n}");
+      break;
+    }
+  }
+}