advent-of-code

commit 3eefdb2273f806d24906a42adde838478009e9ac
parent 247b36212dd6ce2855956d336deedbf0dbd53917
Author: finwo <finwo@pm.me>
Date:   Wed, 24 Dec 2025 17:42:11 +0100

Completed 2025/04

Diffstat:
A
2025/04/example.txt
 | 
10
++++++++++
A
2025/04/index.js
 | 
111
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

2 files changed, 121 insertions(+), 0 deletions(-)
diff --git a/2025/04/example.txt b/2025/04/example.txt
@@ -0,0 +1,10 @@
+..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.
diff --git a/2025/04/index.js b/2025/04/index.js
@@ -0,0 +1,111 @@
+#!/usr/bin/env node
+
+const fs = require('node:fs');
+
+const grid = fs
+  .readFileSync('input', 'utf-8')
+  .split('\r\n').join('\n')
+  .split('\r').join('\n')
+  .split('\n')
+  .map(str => str.trim())
+  .filter(str => str)
+  .map(str => str.split(''))
+  .map(row => row.map(c => c === '@'))
+  ;
+
+const buildNeighbours = (grid) => {
+  const neighbours = grid
+    .map(row => row.map(() => 0));
+
+  // Mark adjacent locations as having 'current' as neighbour
+  for(let y = 0; y < grid.length; y++) {
+    const row = grid[y];
+    for(let x = 0; x < row.length ; x++) {
+      // Skip if we're empty
+      if (!grid[y][x]) continue;
+      for(let ny = -1; ny <= 1; ny++) {
+        if ((y+ny) < 0) continue;
+        if ((y+ny) >= grid.length) continue;
+        for(let nx = -1; nx <= 1; nx++) {
+          if ((x+nx) < 0) continue;
+          if ((x+nx) >= row.length) continue;
+          // Skip self
+          if (nx === 0 && ny === 0) continue;
+          neighbours[y+ny][x+nx]++;
+        }
+      }
+    }
+  }
+
+  return neighbours;
+};
+
+const buildReachable = (grid, neighbours) => {
+  // Mark reachable rolls in part 1
+  let count = 0;
+  const _map = grid.map(row => row.map(() => false));
+  for(let y = 0; y < grid.length; y++) {
+    const row = grid[y];
+    for(let x = 0; x < row.length ; x++) {
+      // Skip if we're empty
+      if (!grid[y][x]) continue;
+      if (neighbours[y][x] >= 4) continue;
+      _map[y][x] = true;
+      count++;
+    }
+  }
+  return {
+    _map,
+    count,
+  };
+};
+
+const removeReachable = (grid, reachableMap) => {
+  for(let y = 0; y < grid.length; y++) {
+    const row = grid[y];
+    for(let x = 0; x < row.length ; x++) {
+      if (reachableMap[y][x]) row[x] = false;
+    }
+  }
+};
+
+// Debug display origin grid
+const displayGrid = grid => {
+  process.stdout.write(
+    grid.map(row => row.map(c => c ? '@' : '.').join('')).join('\n') + '\n\n'
+  );
+};
+
+// Debug display
+const displayReachable = (grid, reachableMap) => {
+  process.stdout.write(
+    grid.map(
+      (row,y) => row.map(
+        (roll,x) => reachableMap[y][x] ? 'x' : roll ? '@' : '.'
+      ).join('')
+    ).join('\n') + '\n\n'
+  );
+}
+
+let neighbours = buildNeighbours(grid);
+let reachable = buildReachable(grid, neighbours);
+const part1 = reachable.count;
+
+let totalRemoved = 0;
+
+console.log(`Initial state:`);
+displayGrid(grid);
+
+while(reachable.count) {
+  console.log(`Remove ${reachable.count} rolls of paper:`);
+  totalRemoved += reachable.count;
+  displayReachable(grid, reachable._map);
+
+  removeReachable(grid, reachable._map);
+  neighbours = buildNeighbours(grid);
+  reachable = buildReachable(grid, neighbours);
+}
+
+process.stdout.write('\n');
+process.stdout.write(`------[ Part 1: ${part1} ]------\n`);
+process.stdout.write(`------[ Part 2: ${totalRemoved} ]------\n`);