it's not fast but it works

06/index.ts

@@ -1,37 +1,55 @@
 import fs from "node:fs";
 import { printTime, now } from "../util";
 
-type tile = { guard: boolean; blocked: boolean; visited: boolean };
+type tile = {
+  guard: boolean;
+  blocked: boolean;
+  visited: boolean;
+  hit: { north: number; east: number; south: number; west: number };
+};
 
-const test = true;
+const test = false;
 
 const data = fs.readFileSync(
   test ? "./inputs/testinput" : "./inputs/input",
   "utf8"
 );
 
+const generateFloor = (data: string): Record<string, tile> => {
+  return data
+    .split("\n")
+    .slice(0, -1)
+    .reduce((floor, row, i) => {
+      row.split("").forEach((v, col) => {
+        floor[`${i}|${col}`] = {
+          guard: v === "^",
+          blocked: v === "#",
+          visited: false,
+          hit: {
+            north: 0,
+            south: 0,
+            east: 0,
+            west: 0,
+          },
+        };
+      });
+      return floor;
+    }, {} as Record<string, tile>);
+};
+
+const keyToCoord = (string): [number, number] => {
+  return string.split("|").map((v) => parseInt(v, 10));
+};
+
 let timer = now.instant();
 console.log(
   "part one:",
   (() => {
-    let floor = data
-      .split("\n")
-      .slice(0, -1)
-      .reduce((floor, row, i) => {
-        row.split("").forEach((v, col) => {
-          floor[`${i}|${col}`] = {
-            guard: v === "^",
-            blocked: v === "#",
-            visited: false,
-          };
-        });
-        return floor;
-      }, {} as Record<string, tile>);
-
+    let floor = generateFloor(data);
     let direction = "north";
     let [guard] = Object.entries(floor).filter((tile) => tile[1].guard)[0];
     while (true) {
-      let [x, y] = guard.split("|").map((v) => parseInt(v, 10));
+      let [x, y] = keyToCoord(guard);
       floor[guard].visited = true;
       let next = "";
       switch (direction) {
@@ -73,3 +91,128 @@ console.log(
   })(),
   printTime(now.instant().since(timer))
 );
+
+timer = now.instant();
+console.log(
+  "part two:",
+  (() => {
+    const floor = generateFloor(data);
+
+    let direction = "north";
+    let guard = Object.entries(floor).filter((tile) => tile[1].guard)[0][0];
+    const initialGuardPosition = guard;
+    while (true) {
+      let [x, y] = keyToCoord(guard);
+      floor[guard].visited = true;
+      let next = "";
+      switch (direction) {
+        case "north":
+          next = `${x - 1}|${y}`;
+          break;
+        case "south":
+          next = `${x + 1}|${y}`;
+          break;
+        case "east":
+          next = `${x}|${y + 1}`;
+          break;
+        case "west":
+          next = `${x}|${y - 1}`;
+          break;
+      }
+      if (floor[next] === undefined) {
+        // guard left the floor, exit
+        break;
+      }
+      if (!floor[next].blocked) {
+        // move the guard
+        floor[next].guard = true;
+        floor[guard].guard = false;
+        guard = next;
+      } else {
+        // guard is blocked, rotate
+        if (direction === "north") {
+          direction = "east";
+        } else if (direction === "east") {
+          direction = "south";
+        } else if (direction === "south") {
+          direction = "west";
+        } else if (direction === "west") {
+          direction = "north";
+        }
+      }
+    }
+    let newObstructions = Object.entries(floor).reduce((total, tile) => {
+      if (tile[1].visited && tile[0] !== initialGuardPosition) {
+        return [...total, tile[0]];
+      }
+      return total;
+    }, [] as string[]);
+    return newObstructions.reduce((total, curr) => {
+      const floor = generateFloor(data);
+      floor[curr].blocked = true;
+
+      // iterate as above, but this time we tract how many times the guard hits any obstruction from the same direction.
+      // if the guard ever hits one from the same direction twice, we know they're in a loop.
+      let direction = "north";
+      let guard = Object.entries(floor).filter((tile) => tile[1].guard)[0][0];
+      while (true) {
+        let [x, y] = keyToCoord(guard);
+        let next = "";
+        switch (direction) {
+          case "north":
+            next = `${x - 1}|${y}`;
+            break;
+          case "south":
+            next = `${x + 1}|${y}`;
+            break;
+          case "east":
+            next = `${x}|${y + 1}`;
+            break;
+          case "west":
+            next = `${x}|${y - 1}`;
+            break;
+        }
+        if (floor[next] === undefined) {
+          // guard left the floor, exit
+          break;
+        }
+        if (!floor[next].blocked) {
+          // move the guard
+          floor[next].guard = true;
+          floor[guard].guard = false;
+          guard = next;
+        } else {
+          // guard is blocked, log impact and rotate
+          if (direction === "north") {
+            floor[next].hit.north++;
+            if (floor[next].hit.north === 2) {
+              return total + 1;
+            }
+            direction = "east";
+          } else if (direction === "east") {
+            floor[next].hit.east++;
+            if (floor[next].hit.east === 2) {
+              return total + 1;
+            }
+            direction = "south";
+          } else if (direction === "south") {
+            floor[next].hit.south++;
+            if (floor[next].hit.south === 2) {
+              return total + 1;
+            }
+            direction = "west";
+          } else if (direction === "west") {
+            floor[next].hit.west++;
+            if (floor[next].hit.west === 2) {
+              return total + 1;
+            }
+            direction = "north";
+          }
+        }
+      }
+
+      return total;
+    }, 0);
+  })(),
+  printTime(now.instant().since(timer))
+);