AdventOfCode2024/06/index.ts

import fs from "node:fs";
import { printTime, now } from "../util";

type tile = {
  guard: boolean;
  blocked: boolean;
  visited: boolean;
  hit: { north: number; east: number; south: number; west: number };
};

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 = generateFloor(data);
    let direction = "north";
    let [guard] = Object.entries(floor).filter((tile) => tile[1].guard)[0];
    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) {
        break;
      }
      if (!floor[next].blocked) {
        floor[next].guard = true;
        floor[guard].guard = false;
        guard = next;
      } else {
        if (direction === "north") {
          direction = "east";
        } else if (direction === "east") {
          direction = "south";
        } else if (direction === "south") {
          direction = "west";
        } else if (direction === "west") {
          direction = "north";
        }
      }
    }
    return Object.entries(floor).reduce((total, tile) => {
      return total + (tile[1].visited ? 1 : 0);
    }, 0);
  })(),
  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))
);