Part 2

2021/1.ts

@@ -1,11 +1,53 @@
 import { readLines } from "./deps.ts";
 
 // Day 1
-let increases = 0;
+/*
+ * This solution is relatively simple. Keep track of the last depth we measured
+ * in `lastDepth` (skipping the first line of input by defaulting it to
+ * `Number.MAX_VALUE`) and compare it to each line of input as we read it. Once
+ * we've compared, set the `lastDepth` to the current depth for the next
+ * iteration.
+ *
+ * This solution should be O(n) for compute and O(n) for memory since we
+ * iterate the input once and only read a line at a time, we actually have O(1)
+ * memory, but since the input is on disk, that's kind of unfair to say.
+ */
+let part1Increases = 0;
 let lastDepth = Number.MAX_VALUE;
 for await (const depthString of readLines(await Deno.open("1.input"))) {
   const depth = parseInt(depthString);
-  if (depth > lastDepth) increases++;
+  if (depth > lastDepth) part1Increases++;
   lastDepth = depth;
 }
-console.log("Number of depth increases:", increases);
+console.log("Number of depth increases (Part 1):", part1Increases);
+
+// Day 2
+/*
+ * Since the windows we need to compare overlap, we really only need to compare
+ * the values unique to each window, which will be the first of the first window
+ * and the last of the last window. Since the windows have size 3, we really
+ * only need to compare a value with the "three-lines-ago" value. This solution
+ * otherwise is identical to the previous one.
+ */
+let part2Increases = 0;
+const WINDOW_SIZE = 3;
+const input = readLines(await Deno.open("1.input"));
+
+// TODO: use a proper FIFO queue data structure for this?
+// read the first `WINDOW_SIZE` values from our input in preparation
+let window: number[] = await Promise.all(Array.from(
+  Array(WINDOW_SIZE),
+  async (_x, _i) => parseInt((await input.next()).value),
+));
+
+for await (const depthString of input) {
+  const depth = parseInt(depthString);
+  // compare the current depth to the depth we read `WINDOW_SIZE` ago
+  if (depth > window[0]) part2Increases++;
+  // now we push the current depth onto the window and cut the one we just compared off
+  // TODO: if this was a proper queue, we could just push it on the queue and
+  // expect the first-in value to fall out
+  window = window.slice(1);
+  window.push(depth);
+}
+console.log("Number of summed depth increases (Part 2):", part2Increases);