solve 2020d13

data/2020/13.txt

@@ -0,0 +1,2 @@
+1005595
+41,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,557,x,29,x,x,x,x,x,x,x,x,x,x,13,x,x,x,17,x,x,x,x,x,23,x,x,x,x,x,x,x,419,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,19

solutions/2020/13.js

@@ -0,0 +1,56 @@
+const { Glib, BNMath: Math, fn } = require('../../lib');
+
+const parse = (input) => {
+  const [arrivalString, idStrings] = input.split('\n');
+  return {
+    arrival: BigInt(arrivalString),
+    ids: Glib.fromSplit(idStrings, ',').map((s) =>
+      s === 'x' ? 1n : BigInt(s),
+    ),
+  };
+};
+
+const departureAfter = (time, id) => {
+  let next = (time / id) * id;
+  return next < time ? next + id : next;
+};
+
+module.exports = {
+  '1': (input) => {
+    input = parse(input);
+    return input.ids
+      .flatMap((i) => {
+        if (i === 1n) {
+          return [];
+        }
+        return [[i, departureAfter(input.arrival, i) - input.arrival]];
+      })
+      .minScore()
+      .glib.product();
+
+    return id * (time - input.arrival);
+  },
+  '2': (input) => {
+    input = parse(input).ids.array;
+
+    let solved = 1n;
+    let step = input[0];
+    let start = departureAfter(0n, input[0]);
+    while (true) {
+      for (let i = solved; i < input.length; i++) {
+        const currentBus = input[i];
+        const currentTime = start + i;
+        if (departureAfter(currentTime, currentBus) === currentTime) {
+          solved += 1n;
+          step *= currentBus;
+        } else {
+          break;
+        }
+      }
+      if (solved == input.length) {
+        return start;
+      }
+      start += step;
+    }
+  },
+};

testAll.js

@@ -14,6 +14,7 @@ const results = {
     10: [2277, 37024595836928],
     11: [2359, 2131],
     12: [508, 30761],
+    13: [2095, 598411311431841],
   },
   2019: {
     1: [3386686, 5077155],