Day 9 part 1

2024/Day_09/README.md

@@ -65,3 +65,27 @@ The final step of this file-compacting process is to update the **filesystem che
 Continuing the first example, the first few blocks' position multiplied by its file ID number are `0 * 0 = 0`, `1 * 0 = 0`, `2 * 9 = 18`, `3 * 9 = 27`, `4 * 8 = 32`, and so on. In this example, the checksum is the sum of these, **`1928`**.
 
 Compact the amphipod's hard drive using the process he requested. **What is the resulting filesystem checksum?** 
+
+---
+
+## --- Part Two ---
+
+Upon completion, two things immediately become clear. First, the disk definitely has a lot more contiguous free space, just like the amphipod hoped. Second, the computer is running much more slowly! Maybe introducing all of that [file system fragmentation](https://en.wikipedia.org/wiki/File_system_fragmentation) was a bad idea?
+
+The eager amphipod already has a new plan: rather than move individual blocks, he'd like to try compacting the files on his disk by moving **whole files** instead.
+
+This time, attempt to move whole files to the leftmost span of free space blocks that could fit the file. Attempt to move each file exactly once in order of **decreasing file ID number** starting with the file with the highest file ID number. If there is no span of free space to the left of a file that is large enough to fit the file, the file does not move.
+
+The first example from above now proceeds differently:
+
+```
+00...111...2...333.44.5555.6666.777.888899
+0099.111...2...333.44.5555.6666.777.8888..
+0099.1117772...333.44.5555.6666.....8888..
+0099.111777244.333....5555.6666.....8888..
+00992111777.44.333....5555.6666.....8888..
+```
+
+The process of updating the filesystem checksum is the same; now, this example's checksum would be **`2858`**.
+
+Start over, now compacting the amphipod's hard drive using this new method instead. **What is the resulting filesystem checksum?**

2024/Day_09/part_1.js

@@ -8,16 +8,35 @@ const __dirname = path.dirname(fileURLToPath(import.meta.url));
 // = Copyright (c) NullDev = //
 // ========================= //
 
-const INPUT = String(fs.readFileSync(path.join(__dirname, "input.txt"))).trim().split("\n");
+/* eslint-disable one-var */
+
+const INPUT = String(fs.readFileSync(path.join(__dirname, "input.txt"))).trim().split("");
 
 const pStart = performance.now();
 
-//
-// YOUR CODE HERE
-//
-const result = "...";
+const res = INPUT
+    .flatMap((bc, i) => Array.from({ length: Number(bc) }, () => (i % 2 ? "." : i / 2))) // @ts-ignore
+    .reduce((_, __, idx, arr) => {
+        if (idx === 0){
+            const fileBlockCount = arr.filter((blk) => blk !== ".").length; // @ts-ignore
+            let left = arr.indexOf("."), right = arr.findLastIndex(blk => blk !== ".");
+            while (left < fileBlockCount){
+                if (arr[left] !== "."){
+                    left++;
+                    continue;
+                }
+                if (arr[right] === "."){
+                    right--;
+                    continue;
+                } // @ts-ignore
+                [arr[left], arr[right]] = [arr[right], arr[left]];
+            }
+        }
+        return arr;
+    }, []) // @ts-ignore
+    .reduce((total, nxId, i) => total + (nxId === "." ? 0 : nxId * i), 0);
 
 const pEnd = performance.now();
 
-console.log("<DESCRIPTION>: " + result);
+console.log("FS CHECKSUM: " + res);
 console.log(pEnd - pStart);