less involved solution

shahata · shahata · commit e128e934e443 · 2024-12-25T00:49:28.000+02:00
diff --git a/src/2024/day24.js b/src/2024/day24.js
@@ -11,14 +11,15 @@ function run(gates, wiresMap) {
     gates.forEach(({ a, op, b, wire }) => {
       a = wiresMap.get(a);
       b = wiresMap.get(b);
-      if (a !== undefined && b !== undefined) {
+      let c = wiresMap.get(wire);
+      if (a !== undefined && b !== undefined && c === undefined) {
+        done = false;
         if (op === "AND") wiresMap.set(wire, a & b);
         if (op === "OR") wiresMap.set(wire, a | b);
         if (op === "XOR") wiresMap.set(wire, a ^ b);
-      } else done = false;
+      }
     });
   }
-  return parseInt(read(wiresMap, "z"), 2);
 }
 
 function parse(input) {
@@ -38,37 +39,111 @@ function parse(input) {
 
 export function part1(input) {
   let { wiresMap, gates } = parse(input);
-  return run(gates, wiresMap);
+  run(gates, wiresMap);
+  return parseInt(read(wiresMap, "z"), 2);
+}
+
+function extractUnit(gates, i) {
+  let num = i.toString().padStart(2, "0");
+  let next = (i + 1).toString().padStart(2, "0");
+  let unit = gates.filter(x => x.a === `x${num}` && x.b === `y${num}`);
+  let not = gates
+    .filter(x => x.a === `x${next}` && x.b === `y${next}`)
+    .map(x => x.wire);
+  let wires = unit.map(x => x.wire);
+  let done = false;
+  while (!done) {
+    done = true;
+    let more = gates.filter(x => wires.includes(x.a) || wires.includes(x.b));
+    more = more.filter(x => !unit.includes(x));
+    more = more.filter(x => !not.includes(x.a) && !not.includes(x.b));
+    if (more.length) {
+      done = false;
+      unit.push(...more);
+      wires.push(...more.map(x => x.wire));
+    }
+  }
+  return unit;
+}
+
+function unitTest(gates) {
+  let inp = new Set(gates.flatMap(x => [x.a, x.b]));
+  let out = new Set(gates.map(x => x.wire));
+  let inputs = [...inp.difference(out)].sort();
+  let outputs = [...out.difference(inp)].sort();
+  if (outputs.every(x => x.startsWith("z"))) outputs.reverse();
+  for (let i = 0; i < 2 ** inputs.length; i++) {
+    let wiresMap = new Map();
+    let bin = i.toString(2);
+    inputs.forEach((x, j) => wiresMap.set(x, +bin[j]));
+    run(gates, wiresMap);
+    let result = parseInt(outputs.map(x => wiresMap.get(x)).join(""), 2);
+    let expect = bin.split("").reduce((a, b) => +a + +b, 0);
+    if (result !== expect) return false;
+  }
+  return true;
+}
+
+function getSwamps(gates) {
+  let options = [];
+  let out = gates.map(x => x.wire);
+  for (let i = 0; i < out.length; i++) {
+    for (let j = i + 1; j < out.length; j++) {
+      let option = gates.map(x => {
+        let next = { ...x };
+        if (x.wire === out[i]) next.wire = out[j];
+        if (x.wire === out[j]) next.wire = out[i];
+        return next;
+      });
+      options.push({ swap: [out[i], out[j]], gates: option });
+    }
+  }
+  return options;
+}
+
+export function part2(input) {
+  let { wiresMap, gates } = parse(input);
+  let { length } = read(wiresMap, "x");
+  let swapped = [];
+  for (let i = 0; i < length; i++) {
+    let unit = extractUnit(gates, i);
+    let pass = unitTest(unit);
+    if (!pass) {
+      let { swap } = getSwamps(unit).find(x => unitTest(x.gates));
+      swapped.push(...swap);
+    }
+  }
+  return swapped.sort().join(",");
 }
 
-// full adder
+// Shorter but much more involved solution adapted from reddit
 // https://www.build-electronic-circuits.com/wp-content/uploads/2022/10/fullAdder-1-1024x473.png
 // X XOR Y => M
 // X AND Y => N
 // CIN XOR M => Z
 // CIN AND M => R
 // R OR  N => COUT
-export function part2(input) {
-  let { gates, wiresMap } = parse(input);
-  function find(a, b, op) {
-    [a, b] = [a, b].sort();
-    return gates.find(x => x.a === a && x.b === b && x.op === op)?.wire;
-  }
-  let swapped = [];
-  let { length } = read(wiresMap, "x");
-  let cin = find("x00", "y00", "AND");
-  for (let i = 1; i < length; i++) {
-    let num = i.toString().padStart(2, "0");
-    let m = find(`x${num}`, `y${num}`, "XOR");
-    let n = find(`x${num}`, `y${num}`, "AND");
-    if (find(cin, n, "XOR")) swapped.push(...([m, n] = [n, m]));
-    let z = find(cin, m, "XOR");
-    let r = find(cin, m, "AND");
-    if (find(r, z, "OR")) swapped.push(...([n, z] = [z, n]));
-    if (find(n, z, "OR")) swapped.push(...([r, z] = [z, r]));
-    let cout = find(r, n, "OR");
-    if (cout === `z${num}`) swapped.push(...([z, cout] = [cout, z]));
-    cin = cout;
-  }
-  return swapped.sort().join(",");
-}
+// export function part2(input) {
+//   let { wiresMap, gates } = parse(input);
+//   function find(a, b, op) {
+//     [a, b] = [a, b].sort();
+//     return gates.find(x => x.a === a && x.b === b && x.op === op)?.wire;
+//   }
+//   let swapped = [];
+//   let { length } = read(wiresMap, "x");
+//   let cin = find("x00", "y00", "AND");
+//   for (let i = 1; i < length; i++) {
+//     let num = i.toString().padStart(2, "0");
+//     let m = find(`x${num}`, `y${num}`, "XOR");
+//     let n = find(`x${num}`, `y${num}`, "AND");
+//     if (find(cin, n, "XOR")) swapped.push(...([m, n] = [n, m]));
+//     let z = find(cin, m, "XOR");
+//     let r = find(cin, m, "AND");
+//     if (find(r, z, "OR")) swapped.push(...([n, z] = [z, n]));
+//     if (find(n, z, "OR")) swapped.push(...([r, z] = [z, r]));
+//     let cout = find(r, n, "OR");
+//     if (cout === `z${num}`) swapped.push(...([z, cout] = [cout, z]));
+//     cin = cout;
+//   }
+//   return swapped.sort().join(",");
+// }
