add day 18

day18/Cargo.lock

@@ -0,0 +1,25 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "day18"
+version = "0.1.0"
+dependencies = [
+ "itertools",
+]
+
+[[package]]
+name = "either"
+version = "1.16.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "91622ff5e7162018101f2fea40d6ebf4a78bbe5a49736a2020649edf9693679e"
+
+[[package]]
+name = "itertools"
+version = "0.15.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8b4baf93f58d4425749ca49a51c50ebab072c5df6994d08fed93541c331481dc"
+dependencies = [
+ "either",
+]

day18/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "day18"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+itertools = "*"

day18/src/main.rs

@@ -0,0 +1,42 @@
+use itertools::Itertools;
+
+#[cfg(debug_assertions)]
+const START: &str = ".^^.^.^^^^";
+
+#[cfg(not(debug_assertions))]
+const START: &str = "^.^^^.^..^....^^....^^^^.^^.^...^^.^.^^.^^.^^..^.^...^.^..^.^^.^..^.....^^^.^.^^^..^^...^^^...^...^.";
+
+fn main() {
+    let mut row = START.to_string();
+
+    let mut counter = 0;
+
+    for _ in 0..400_000 {
+        counter += row.chars().filter(|c| *c == '.').count();
+        let next_row = generate_next_row(row);
+        row = next_row;
+    }
+    println!("{counter}");
+}
+
+fn generate_next_row(row: String) -> String {
+    ['.']
+        .into_iter()
+        .chain(row.chars())
+        .chain(['.'])
+        .map(|c| c == '^')
+        .tuple_windows()
+        .map(|(a, b, c)| is_trap(&[a, b, c]))
+        .map(|b| if b { '^' } else { '.' })
+        .collect()
+}
+
+#[allow(clippy::nonminimal_bool)]
+fn is_trap(are_traps: &[bool; 3]) -> bool {
+    let &[left, center, right] = are_traps;
+
+    (left && center && !right)
+        || (!left && center && right)
+        || (left && !center && !right)
+        || (!left && !center && right)
+}