ugg :(

.gitignore

@@ -0,0 +1,3 @@
+package.json
+package-lock.json
+node_modules

day4/part2.lua

@@ -28,8 +28,6 @@ local lines = lines_from(file)
 
 ---@diagnostic disable-next-line: redefined-local
 local function count_xmas_at_index(lines, line_idx, char_idx)
-	local line = lines[line_idx]
-
 	local up_idx = line_idx - 1
 	local down_idx = line_idx + 1
 	local right_idx = char_idx + 1
@@ -39,7 +37,7 @@ local function count_xmas_at_index(lines, line_idx, char_idx)
 	char_count["M"] = 0
 	char_count["S"] = 0
 	if up_idx >= 1 then
-		if right_idx <= #line then
+		if right_idx <= #lines[up_idx] then
 			local key = lines[up_idx]:sub(right_idx, right_idx)
 			if char_count[key] ~= nil then
 				char_count[key] = char_count[key] + 1
@@ -55,7 +53,7 @@ local function count_xmas_at_index(lines, line_idx, char_idx)
 	end
 
 	if down_idx <= #lines then
-		if right_idx <= #line then
+		if right_idx <= #lines[down_idx] then
 			local key = lines[down_idx]:sub(right_idx, right_idx)
 			if char_count[key] ~= nil then
 				char_count[key] = char_count[key] + 1

day4/part2.ts

@@ -0,0 +1,49 @@
+const inputFile = await Bun.file("input").text();
+
+const lines = inputFile.split("\n").filter((x) => x);
+
+function between(x: number, min: number, max: number): boolean {
+  return x >= min && x < max;
+}
+
+function countMs(str: string) {
+  return (str.match(/M/g) || []).length;
+}
+
+function countSs(str: string) {
+  return (str.match(/S/g) || []).length;
+}
+
+let xmasCount = 0;
+
+for (let lineIdx = 0; lineIdx < lines.length; lineIdx++) {
+  for (let charIdx of lines[lineIdx]
+    .matchAll(/A/g)
+    .toArray()
+    .map((m) => m.index)) {
+    let charPositions = [
+      [lineIdx - 1, charIdx - 1], // up left
+      [lineIdx - 1, charIdx + 1], // up right
+      [lineIdx + 1, charIdx - 1], // down left
+      [lineIdx + 1, charIdx + 1], // down right
+    ].filter(
+      ([li, ci]) =>
+        between(li, 0, lines.length) && between(ci, 0, lines[lineIdx].length),
+    );
+
+    if (charPositions.length === 4) {
+      let corners = "";
+      for (let [li, ci] of charPositions) {
+        corners += lines[li][ci];
+      }
+
+      if (countMs(corners) == 2 && countSs(corners) == 2) {
+        xmasCount += 1;
+      }
+    }
+  }
+}
+
+console.log(xmasCount);
+
+export {};