day 24 part 2

src/day24.rs

@@ -19,9 +19,10 @@ fn part1(input: &str) -> RiddleResult {
             let (lhs, rhs) = l.split_once(" -> ").unwrap();
             (rhs, lhs)
         })
-        .collect_vec();
-    let program = HashMap::from_iter(program.into_iter());
-    let start_values = HashMap::from_iter(a.lines().map(|l| {
+        .collect_map();
+    let start_values = a
+        .lines()
+        .map(|l| {
             let (x, y) = l.split_once(": ").unwrap();
             let b = match y {
                 "0" => false,
@@ -29,7 +30,8 @@ fn part1(input: &str) -> RiddleResult {
                 _ => panic!("bad values {y}"),
             };
             (x, b)
-    }));
+        })
+        .collect_map();
     let binary_result = program
         .keys()
         .filter(|k| k.starts_with("z"))
@@ -61,8 +63,88 @@ fn compute(program: &HashMap<&str, &str>, start_values: &HashMap<&str, bool>, st
     }
 }
 
-fn part2(_input: &str) -> RiddleResult {
-    0
+const SWAPS: [(&'static str, &'static str); 4] = [
+    ("rkf", "z09"),
+    ("jgb", "z20"),
+    ("vcg", "z24"),
+    ("rvc", "rrs"),
+];
+
+fn swap(s: &str) -> &str {
+    for (a, b) in &SWAPS {
+        if *a == s {
+            return b;
+        }
+        if *b == s {
+            return a;
+        }
+    }
+    s
+}
+
+fn part2(input: &str) -> String {
+    let (_, b) = input.split_once("\n\n").unwrap();
+    let program = b
+        .lines()
+        .map(|l| l.split_once(" -> ").unwrap())
+        .map(|(def, name)| (def, swap(name)))
+        .collect_map();
+
+    if program["y00 XOR x00"] != "z00" {
+        panic!("first bit is bad already");
+    }
+    let Some(mut c_prev) = find(&program, "y00", "AND", "x00") else {
+        panic!("first carrier missing");
+    };
+    for i in 1..=44 {
+        // println!();
+        // println!("## {i:02}");
+        let x_i = format!("x{:02}", i);
+        let y_i = format!("y{:02}", i);
+        let op = "XOR";
+        let Some(xor_i) = find(&program, x_i.as_str(), op, y_i.as_str()) else {
+            println!("xor_{:02} missing", i);
+            break;
+        };
+        // println!("xor_i = {xor_i}");
+        let Some(z_i) = find(&program, &xor_i, "XOR", &c_prev) else {
+            println!("z_{:02} missing", i);
+            break;
+        };
+        if format!("z{i:02}") != z_i {
+            println!("z_i is bad: {z_i}");
+            break;
+        }
+        let Some(and_inputs_i) = find(&program, &x_i, "AND", &y_i) else {
+            println!("and_inputs_{:02} missing", i);
+            break;
+        };
+        // println!("and_inputs_i = {and_inputs_i}");
+        let Some(and_all_i) = find(&program, &c_prev, "AND", &xor_i) else {
+            println!("and of last carrier with xor_{:02} missing", i);
+            break;
+        };
+        // println!("and_all_i = {and_all_i}");
+        let Some(c_i) = find(&program, &and_all_i, "OR", &and_inputs_i) else {
+            println!("c_{:02} missing", i);
+            break;
+        };
+        // println!("c_i = {c_i}");
+        c_prev = c_i;
+    }
+    SWAPS
+        .iter()
+        .map(|(a, b)| [a, b])
+        .flatten()
+        .sorted()
+        .join(",")
+}
+
+fn find(program: &HashMap<&str, &str>, a: &str, op: &str, b: &str) -> Option<String> {
+    program
+        .get(format!("{a} {op} {b}").as_str())
+        .or_else(|| program.get(format!("{b} {op} {a}").as_str()))
+        .map(|s| s.to_string())
 }
 
 /// Trait that provides a `collect_map` method for iterators over key-value tuples.