Day 23

23/input

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23/solution.mjs

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+import { readFileSync } from 'node:fs';
+
+const input = readFileSync('input', 'utf-8');
+
+export class InfiniteGrid {
+  constructor() {
+    this.ox = null;
+    this.oy = null;
+    this.rows = [];
+  }
+
+  get height() {
+    return this.rows.length;
+  }
+
+  get width() {
+    return this.rows[0]?.length ?? 0;
+  }
+
+  get(x, y) {
+    return this.rows[y - (this.oy ?? y)]?.[x - (this.ox ?? x)];
+  }
+
+  set(x, y, v) {
+    this.expandToFit(x, y);
+    
+    const ax = x - this.ox;
+    const ay = y - this.oy;
+
+    this.rows[ay][ax] = v;
+  }
+
+  expandToFit(x, y) {
+    if (this.ox === null) this.ox = x;
+    if (this.oy === null) this.oy = y;
+
+    const left = Math.max(0, this.ox - x);
+    const up = Math.max(0, this.oy - y);
+    const right = Math.max(0, (x - this.ox + 1) - this.width);
+    const down = Math.max(0, (y - this.oy + 1) - this.height);
+    
+    if (up || right || down || left) {
+      this.rows = [
+        ...new Array(up).fill(new Array(this.width + left + right).fill()),
+        ...this.rows.map(row =>
+          [...new Array(left), ...row, ...new Array(right)]
+        ),
+        ...new Array(down).fill(new Array(this.width + left + right).fill())
+      ];
+    }
+
+    this.ox -= left;
+    this.oy -= up;
+  }
+
+  getBounds() {
+    return {
+      minX: this.ox,
+      maxX: this.ox + (this.rows[0]?.length ?? 1) - 1,
+      minY: this.oy,
+      maxY: this.oy + (this.rows.length || 1) - 1
+    };
+  }
+
+  printDefined() {
+    console.log(this.rows
+      .map(row => row
+        .map(cell => cell ? '#' : '.')
+        .join('')
+      )
+      .join('\n')
+    );
+  }
+
+  countEmpty() {
+    return this.rows.reduce(
+      (total, row) => total + row.reduce(
+        (rowTotal, cell) => rowTotal + Number(!cell),
+        0
+      ),
+      0
+    );
+  }
+}
+
+class Elf {
+  constructor(grid) {
+    this.grid = grid;
+    this.pos = undefined;
+  }
+
+  moveTo(x, y) {
+    if (this.pos) {
+      this.grid.set(...this.pos, undefined);
+    }
+    this.pos = [x, y];
+    this.grid.set(...this.pos, this);
+  }
+
+  proposePosition(directions) {
+    if (!this.pos) throw new Error('Elf is not on the grid');
+
+    const [x, y] = this.pos;
+    const N = this.grid.get(x, y - 1);
+    const NE = this.grid.get(x + 1, y - 1);
+    const E = this.grid.get(x + 1, y);
+    const SE = this.grid.get(x + 1, y + 1);
+    const S = this.grid.get(x, y + 1);
+    const SW = this.grid.get(x - 1, y + 1);
+    const W = this.grid.get(x - 1, y);
+    const NW = this.grid.get(x - 1, y - 1);
+
+    if (!(N || NE || E || SE || S || SW || W || NW)) {
+      // Don't move
+      return;
+    }
+
+    for (const direction of directions) {
+      if (direction === 'N' && !(NW || N || NE)) {
+        return [x, y - 1];
+      } else if (direction === 'E' && !(NE || E || SE)) {
+        return [x + 1, y];
+      } else if (direction === 'S' && !(SW || S || SE)) {
+        return [x, y + 1];
+      } else if (direction === 'W' && !(NW || W || SW)) {
+        return [x - 1, y];
+      }
+    }
+  }
+}
+
+const elfPositions = input
+  .split('\n')
+  .map(line => line.trim())
+  .filter(line => !!line)
+  .flatMap((line, y) => line
+    .split('')
+    .map((c, x) => {
+      if (c === '#') {
+        return [x, y];
+      }
+    })
+    .filter(coords => !!coords)
+  );
+
+const grid = new InfiniteGrid();
+const elves = [];
+for (const pos of elfPositions) {
+  const elf = new Elf(grid);
+  elves.push(elf);
+  elf.moveTo(...pos);
+}
+const directions = ['N', 'S', 'W', 'E'];
+let round = 0;
+
+function processRound() {
+  const propositions = elves.flatMap(elf => {
+    const pos = elf.proposePosition(directions);
+    if (!pos) return [];
+    return [{elf, x: pos[0], y: pos[1]}];
+  });
+
+  propositions.sort((p1, p2) => (p1.x - p2.x) || (p1.y - p2.y));
+  const validPropositions = propositions
+    .reduce((groups, proposition) => {
+      if (groups.length) {
+        const lastGroup = groups[groups.length - 1];
+        const {x: lastX, y: lastY} = lastGroup[0];
+        if (proposition.x === lastX && proposition.y === lastY) {
+          lastGroup.push(proposition);
+          return groups;
+        }
+      }
+
+      groups.push([proposition]);
+      return groups;
+    }, [])
+    .filter(group => group.length === 1)
+    .flat();
+
+  const moved = validPropositions.length;
+
+  for (const proposition of validPropositions) {
+    proposition.elf.moveTo(proposition.x, proposition.y);
+  }
+  
+  // Reorder directions
+  directions.push(directions.shift());
+
+  round++;
+  return moved;
+}
+
+do {
+  processRound();
+} while (round <= 10)
+console.log('The number of empty ground tiles is:', grid.countEmpty());
+
+while (processRound() !== 0) {};
+console.log('The number of rounds to completion is:', round);