Day 7

day7.peggy

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+lines = (@num ":" _  @num|1.., _|"\n")*
+
+num = n:$[0-9]+ { return parseInt(n, 10) }
+_ = [ \t]+

day7.ts

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+import { type MainArgs, parseFile } from './lib/utils.ts';
+
+type Parsed = [number, number[]][];
+
+// Starting from the front of the reversed list (the endq), see if we can rule
+// out any of the operations
+function calc(left: number, operands: number[], concat = false): number {
+  if ((left <= 0) || (Math.floor(left) !== left) || !operands.length) {
+    return left;
+  }
+  const [first, ...rest] = operands;
+  if (concat) { // Part 2
+    // This is most likely to rule out an operand.
+    // 20 || 123 = 20123.  The opposite op is 20123 -- 123, which fails if
+    // LHS doesn't end in RHS, then truncates.
+    const sfirst = String(first);
+    const sleft = String(left);
+    if (sleft.endsWith(sfirst)) {
+      if (calc(Number(sleft.slice(0, -sfirst.length)), rest, true) === 0) {
+        return 0;
+      }
+    }
+  }
+  if (calc(left - first, rest, concat) === 0) {
+    return 0;
+  };
+  if (calc(left / first, rest, concat) === 0) {
+    return 0;
+  }
+  return left;
+}
+
+function part1(inp: Parsed): number {
+  let sum = 0;
+  for (const [tot, operands] of inp) {
+    if (calc(tot, operands.reverse()) === 0) {
+      sum += tot;
+    }
+  }
+  return sum;
+}
+
+function part2(inp: Parsed): number {
+  let sum = 0;
+  for (const [tot, operands] of inp) {
+    // Already reversed
+    if (calc(tot, operands, true) === 0) {
+      sum += tot;
+    }
+  }
+  return sum;
+}
+
+export default async function main(args: MainArgs): Promise<[number, number]> {
+  const inp = await parseFile<Parsed>(args);
+  return [part1(inp), part2(inp)];
+}

test/day7.js

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+export default [1038838357795, 254136560217241];