From 70141f8762956666ff44f11bf4d84c82089003bc Mon Sep 17 00:00:00 2001
From: Tim Kurvers <tim@moonsphere.net>
Date: Wed, 18 Dec 2024 12:12:37 +0100
Subject: [PATCH] feat(js): add solutions for 2024 day 17

---
 js/src/2024/17/index.js       | 124 ++++++++++++++++++++++++++++++++++
 puzzles/2024/17/examples.yaml |  17 +++++
 2 files changed, 141 insertions(+)
 create mode 100644 js/src/2024/17/index.js
 create mode 100644 puzzles/2024/17/examples.yaml

diff --git a/js/src/2024/17/index.js b/js/src/2024/17/index.js
new file mode 100644
index 0000000..99a87df
--- /dev/null
+++ b/js/src/2024/17/index.js
@@ -0,0 +1,124 @@
+import { mod, solution } from '../../utils/index.js';
+
+const NUMBER_MATCHER = /\d+/g;
+
+const parse = (input) => {
+  const parts = input.trim().split('\n\n');
+  const registers = Array.from(parts[0].matchAll(NUMBER_MATCHER)).map(BigInt);
+  const program = parts[1].slice(8).split(',').map(BigInt);
+  return { program, registers };
+};
+
+const run = (program, registers) => {
+  const [a, b, c] = [0, 1, 2];
+
+  const resolve = (operand) => {
+    if (operand >= 0n && operand <= 3n) return operand;
+    if (operand === 7n) throw new Error('reserved');
+    return registers[operand - 4n];
+  };
+
+  const output = [];
+
+  let ip = 0n;
+
+  const operations = {
+    // adv
+    0: (operand) => {
+      registers[a] = (registers[a] / 2n ** resolve(operand)) | 0n;
+    },
+
+    // bxl
+    1: (operand) => {
+      registers[b] ^= operand;
+    },
+
+    // bst
+    2: (operand) => {
+      registers[b] = mod(resolve(operand), 8n);
+    },
+
+    // jnz
+    3: (operand) => {
+      if (registers[a] !== 0n) {
+        ip = operand;
+      }
+    },
+
+    // bxc
+    4: (_operand) => {
+      registers[b] ^= registers[c];
+    },
+
+    // out
+    5: (operand) => {
+      output.push(mod(resolve(operand), 8n));
+    },
+
+    // bdv
+    6: (operand) => {
+      registers[b] = (registers[a] / 2n ** resolve(operand)) | 0n;
+    },
+
+    // cdv
+    7: (operand) => {
+      registers[c] = (registers[a] / 2n ** resolve(operand)) | 0n;
+    },
+  };
+
+  while (ip < program.length) {
+    const opcode = program[ip++];
+    const operand = program[ip++];
+
+    const operation = operations[opcode];
+    if (!operation) {
+      throw new Error(`no operation for opcode: ${opcode}`);
+    }
+    operation(operand);
+  }
+
+  return output;
+};
+
+export const partOne = solution((input) => {
+  const { program, registers } = parse(input);
+  return run(program, registers).join(',');
+});
+
+export const partTwo = solution((input) => {
+  const { program, registers } = parse(input);
+
+  // Samples of `a` with the resulting outputs:
+  //
+  //   a = 7n    =>                 [ 0n ]
+  //   a = 15n   =>             [ 1n, 0n ]
+  //   a = 16n   =>             [ 2n, 0n ]
+  //   a = 87n   =>         [ 2n, 1n, 0n ]
+  //   a = 88n   =>         [ 3n, 1n, 0n ]
+  //   a = 120n  =>         [ 7n, 1n, 0n ]  (a = 15n << 3)
+  //   a = 960n  =>     [ 0n, 7n, 1n, 0n ]  (a = 15n << 6)
+  //   a = 7680n => [ 0n, 0n, 7n, 1n, 0n ]  (a = 15n << 9)
+  //
+  // As seen in the last three examples, a position's output is directly linked to its triple (three-bit)
+  // position in the `a` value, but in reverse: most significant bytes affect least significant output slots.
+
+  let candidates = [0n];
+
+  // For each output slot in the program, attempt to find candidate `a` values, until eventually we have
+  // candidates that match the full program output
+  for (const [index, _] of program.entries()) {
+    const next = [];
+    for (const shifta of candidates) {
+      for (let a = 0n; a < 8n; ++a) {
+        const candidate = (shifta << 3n) + a;
+        const output = run(program, [candidate, ...registers.slice(1)]);
+        if (output.at(-index - 1) === program.at(-index - 1)) {
+          next.push(candidate);
+        }
+      }
+    }
+    candidates = next;
+  }
+
+  return Math.min(...candidates.map(Number));
+});
diff --git a/puzzles/2024/17/examples.yaml b/puzzles/2024/17/examples.yaml
new file mode 100644
index 0000000..4c63a32
--- /dev/null
+++ b/puzzles/2024/17/examples.yaml
@@ -0,0 +1,17 @@
+part-one:
+  - input: |
+      Register A: 729
+      Register B: 0
+      Register C: 0
+
+      Program: 0,1,5,4,3,0
+    answer: 4,6,3,5,6,3,5,2,1,0
+
+part-two:
+  - input: |
+      Register A: 2024
+      Register B: 0
+      Register C: 0
+
+      Program: 0,3,5,4,3,0
+    answer: 117440