Day 15

15/input

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+Sensor at x=3556832, y=3209801: closest beacon is at x=3520475, y=3164417
+Sensor at x=3068970, y=3071952: closest beacon is at x=3520475, y=3164417
+Sensor at x=636397, y=1899889: closest beacon is at x=338784, y=1935796
+Sensor at x=3856769, y=3377079: closest beacon is at x=3520475, y=3164417
+Sensor at x=2876227, y=2633203: closest beacon is at x=2595700, y=2684432
+Sensor at x=1435445, y=1194830: closest beacon is at x=925348, y=2000000
+Sensor at x=3764673, y=3881970: closest beacon is at x=3520475, y=3164417
+Sensor at x=3171272, y=1098717: closest beacon is at x=3778277, y=740547
+Sensor at x=3646837, y=966534: closest beacon is at x=3778277, y=740547
+Sensor at x=1736390, y=3309102: closest beacon is at x=1623417, y=4114070
+Sensor at x=1086601, y=2272573: closest beacon is at x=925348, y=2000000
+Sensor at x=3793954, y=2346914: closest beacon is at x=3520475, y=3164417
+Sensor at x=1896054, y=2706210: closest beacon is at x=2595700, y=2684432
+Sensor at x=2298950, y=3449308: closest beacon is at x=2205069, y=3958831
+Sensor at x=1911518, y=3848874: closest beacon is at x=2205069, y=3958831
+Sensor at x=2566355, y=1516144: closest beacon is at x=2595700, y=2684432
+Sensor at x=246553, y=343125: closest beacon is at x=338784, y=1935796
+Sensor at x=2197183, y=3975039: closest beacon is at x=2205069, y=3958831
+Sensor at x=552775, y=3494740: closest beacon is at x=-138318, y=2857049
+Sensor at x=128870, y=1935711: closest beacon is at x=338784, y=1935796
+Sensor at x=2197078, y=3999879: closest beacon is at x=2205069, y=3958831
+Sensor at x=2502533, y=3911039: closest beacon is at x=2205069, y=3958831
+Sensor at x=2289309, y=3024440: closest beacon is at x=2595700, y=2684432
+Sensor at x=3999523, y=551710: closest beacon is at x=3778277, y=740547
+Sensor at x=2246061, y=3999936: closest beacon is at x=2205069, y=3958831
+Sensor at x=3982782, y=1306639: closest beacon is at x=3778277, y=740547
+Sensor at x=1166660, y=2766482: closest beacon is at x=925348, y=2000000
+Sensor at x=3744391, y=440575: closest beacon is at x=3778277, y=740547
+Sensor at x=1480453, y=3997346: closest beacon is at x=1623417, y=4114070
+Sensor at x=9770, y=1844797: closest beacon is at x=338784, y=1935796
+Sensor at x=202829, y=2427690: closest beacon is at x=338784, y=1935796
+Sensor at x=3051096, y=3631595: closest beacon is at x=3147080, y=4258152
+Sensor at x=2111052, y=297293: closest beacon is at x=1552534, y=-431081
+Sensor at x=864326, y=2053355: closest beacon is at x=925348, y=2000000
+Sensor at x=2422495, y=2130146: closest beacon is at x=2595700, y=2684432
+Sensor at x=3655670, y=100751: closest beacon is at x=3778277, y=740547
+Sensor at x=535656, y=2133259: closest beacon is at x=338784, y=1935796
+Sensor at x=263229, y=2101270: closest beacon is at x=338784, y=1935796

15/solution.mjs

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+import { readFileSync } from 'node:fs';
+
+const input = readFileSync('input', 'utf-8');
+
+const sensors = input
+  .split('\n')
+  .filter(Boolean)
+  .map(line => {
+    const match = line.match(/.*?x=(-?\d+), y=(-?\d+).*?x=(-?\d+), y=(-?\d+)/);
+    if (!match) {
+      throw new Error(`Invalid sensor readout: ${line}`);
+    }
+    const [x, y, bx, by] = match
+      .slice(1)
+      .map(n => parseInt(n));
+    return {x, y, bx, by};
+  });
+
+function rangesOverlap(range1, range2, tollerance=0) {
+  const t = tollerance;
+  range1 = [...range1].sort((a, b) => a - b);
+  range2 = [...range2].sort((a, b) => a - b);
+  return (
+    (range1[0] >= range2[0] - t && range1[0] <= range2[1] + t) ||
+    (range1[1] >= range2[0] - t && range1[1] <= range2[1] + t) ||
+    (range2[0] >= range1[0] - t && range2[0] <= range1[1] + t) ||
+    (range2[1] >= range1[0] - t && range2[1] <= range1[1] + t)
+  );
+}
+
+function mergeCoverage(coverage) {
+  return [...coverage]
+    .map(range => [...range].sort((a, b) => a - b))
+    .sort((a, b) => a[0] - b[0])
+    .reduce((newCoverage, range) => {
+      if (newCoverage.length) {
+        const lastRange = newCoverage[newCoverage.length - 1];
+        if (rangesOverlap(lastRange, range, 1)) {
+          newCoverage[newCoverage.length - 1] = [
+            Math.min(lastRange[0], range[0]),
+            Math.max(lastRange[1], range[1])
+          ];
+          return newCoverage;
+        }
+      }
+      newCoverage.push(range);
+      return newCoverage;
+    }, []);
+}
+
+function getCoverageForRow(y) {
+  const coverage = [];
+
+  for (const s of sensors) {
+    const range = Math.abs(s.x - s.bx) + Math.abs(s.y - s.by);
+    const distanceToSensor = Math.abs(s.y - y);
+    const reducedRange = range - distanceToSensor;
+
+    if (reducedRange < 0) continue;
+
+    coverage.push([s.x - reducedRange, s.x + reducedRange]);
+  }
+
+  return mergeCoverage(coverage);
+}
+
+function countBeaconsForRow(y) {
+  return sensors
+    .map(s => [s.bx, s.by])
+    .filter((coord, i, coords) => coords.findLastIndex(
+      c => c[0] === coord[0] && c[1] === coord[1]
+    ) === i)
+    .reduce((total, coord) => total + Number(coord[1] === y), 0);
+}
+
+function countCoverageForRow(y) {
+  const coverageCount = getCoverageForRow(y)
+    .reduce((total, range) => total + range[1] - range[0] + 1, 0);
+  const beaconCount = countBeaconsForRow(y);
+  return coverageCount - beaconCount;
+}
+
+const row = 2000000;
+const coverageCount = countCoverageForRow(2000000);
+console.log(
+  `There are ${coverageCount} positions that cannot contain a beacon on ` +
+    `row: ${row}`
+);
+
+
+function limitCoverage(coverage, min, max) {
+  return coverage
+    .filter(range => rangesOverlap([min, max], range))
+    .map(range => [Math.max(min, range[0]), Math.min(max, range[1])]);
+}
+
+console.log('Locating distress beacon...');
+let tuningFrequency;
+for (let y = 0; y <= 4000000; y++) {
+  const coverage = limitCoverage(getCoverageForRow(y), 0, 4000000);
+  if (coverage.length > 1) {
+    const x = coverage[0][1] + 1;
+    tuningFrequency = x * 4000000 + y;
+  }
+}
+console.log(`Tuning frequency for distress beacon: ${tuningFrequency}`);