feat: 2024 Day 17

Author: mfulton26

2024/day/17/part/1/solve.ts

@@ -1,12 +1,12 @@
-function run(a: number, b: number, c: number, program: number[]) {
+function run({ a, b, c }: Record<"a" | "b" | "c", bigint>, program: number[]) {
   const output: number[] = [];
-  function combo(operand: number) {
-    if (operand < 4) return operand;
+  function combo(operand: number): bigint {
     switch (operand) {
       case 0:
       case 1:
       case 2:
-        return operand;
+      case 3:
+        return BigInt(operand);
       case 4:
         return a;
       case 5:
@@ -18,7 +18,7 @@ function run(a: number, b: number, c: number, program: number[]) {
     }
   }
   function div(operand: number) {
-    return Math.trunc(a / 2 ** combo(operand));
+    return a / 2n ** combo(operand);
   }
   for (let ip = 0; ip in program;) {
     const opcode = program[ip++];
@@ -28,10 +28,10 @@ function run(a: number, b: number, c: number, program: number[]) {
         a = div(operand);
         break;
       case 1: // bxl
-        b = b ^ operand;
+        b = b ^ BigInt(operand);
         break;
       case 2: // bst
-        b = combo(operand) % 8;
+        b = combo(operand) % 8n;
         break;
       case 3: // jnz
         if (a) ip = operand;
@@ -40,7 +40,7 @@ function run(a: number, b: number, c: number, program: number[]) {
         b ^= c;
         break;
       case 5: // out
-        output.push(combo(operand) % 8);
+        output.push(Number(combo(operand) % 8n));
         break;
       case 6: // bdv
         b = div(operand);
@@ -55,7 +55,7 @@ function run(a: number, b: number, c: number, program: number[]) {
 
 export default function solve(input: string) {
   const [top, bottom] = input.split("\n\n");
-  const [a, b, c] = top.matchAll(/\d+/g)!.map(([v]) => +v);
+  const [a, b, c] = top.matchAll(/\d+/g)!.map(([v]) => BigInt(v));
   const program = bottom.split(": ")[1].split(",").map(Number);
-  return run(a, b, c, program).join(",");
+  return run({ a, b, c }, program).join(",");
 }

2024/day/17/part/2/solve.test.ts

@@ -0,0 +1,14 @@
+import solve from "./solve.ts";
+
+import { assertEquals } from "@std/assert";
+
+Deno.test("example", () => {
+  const input = `\
+Register A: 2024
+Register B: 0
+Register C: 0
+
+Program: 0,3,5,4,3,0`;
+
+  assertEquals(solve(input), "117440");
+});

2024/day/17/part/2/solve.ts

@@ -0,0 +1,82 @@
+type Registers = Record<"a" | "b" | "c", bigint>;
+
+function run({ a, b, c }: Registers, program: number[]) {
+  const output: number[] = [];
+  function combo(operand: number): bigint {
+    switch (operand) {
+      case 0:
+      case 1:
+      case 2:
+      case 3:
+        return BigInt(operand);
+      case 4:
+        return a;
+      case 5:
+        return b;
+      case 6:
+        return c;
+      default:
+        throw new Error(`unsupported combo literal: ${operand}`);
+    }
+  }
+  function div(operand: number) {
+    return a / 2n ** combo(operand);
+  }
+  for (let ip = 0; ip in program;) {
+    const opcode = program[ip++];
+    const operand = program[ip++];
+    switch (opcode) {
+      case 0: // adv
+        a = div(operand);
+        break;
+      case 1: // bxl
+        b = b ^ BigInt(operand);
+        break;
+      case 2: // bst
+        b = combo(operand) % 8n;
+        break;
+      case 3: // jnz
+        if (a) ip = operand;
+        break;
+      case 4: // bxc
+        b ^= c;
+        break;
+      case 5: // out
+        output.push(Number(combo(operand) % 8n));
+        break;
+      case 6: // bdv
+        b = div(operand);
+        break;
+      case 7: // cdv
+        c = div(operand);
+        break;
+    }
+  }
+  return output;
+}
+
+/** adapted from https://github.com/gbasov/aoc24/blob/0c8149623f7f7ccf6da1d072d439bb60136aea10/17/part2.ts */
+function findA(
+  { b, c }: Omit<Registers, "a">,
+  program: number[],
+  input = 0n,
+  offset = program.length - 1,
+): bigint | undefined {
+  if (offset === -1) return input;
+
+  for (let i = 0n; i < 8n; i++) {
+    const a = (input << 3n) + i;
+    const output = run({ a, b, c }, program);
+    if (output[0] !== program.at(offset)) continue;
+    const min = findA({ b, c }, program, a, offset - 1);
+    if (min === undefined) continue;
+    return min;
+  }
+}
+
+export default function solve(input: string) {
+  const [top, bottom] = input.split("\n\n");
+  const [b, c] = top.matchAll(/\d+/g)!.map(([v]) => BigInt(v));
+  const program = bottom.split(": ")[1].split(",").map(Number);
+  return findA({ b, c }, program)?.toString();
+}