aoc_day06_part1.ts

const input = await Deno.readTextFile("input.txt");

type Position = { x: number; y: number };
type Direction = "up" | "down" | "left" | "right";

type Guard = {
  position: Position;
  direction: Direction;
};

const DirectionDeltas: Record<Direction, Position> = {
  up: { x: 0, y: -1 },
  down: { x: 0, y: 1 },
  left: { x: -1, y: 0 },
  right: { x: 1, y: 0 },
};

const DirectionMarkers: Record<Direction, string> = {
  up: "▲",
  down: "▼",
  left: "◄",
  right: "►",
};

function initializeGuard(grid: string[][]): Guard {
  for (let y = 0; y < grid.length; y++) {
    for (let x = 0; x < grid[y].length; x++) {
      if (grid[y][x] === "^") {
        grid[y][x] = "S";
        return { position: { x, y }, direction: "up" };
      }
    }
  }
  throw new Error("Guard starting position not found.");
}

function getNextTile(grid: string[][], guard: Guard): string {
  const { x, y } = guard.position;
  const delta = DirectionDeltas[guard.direction];
  return grid[y + delta.y]?.[x + delta.x] || "";
}

function changeDirection(guard: Guard): void {
  const directionOrder: Direction[] = ["up", "right", "down", "left"];
  const currentIndex = directionOrder.indexOf(guard.direction);
  guard.direction = directionOrder[(currentIndex + 1) % directionOrder.length];
}

function moveGuard(grid: string[][], guard: Guard): void {
  const { position, direction } = guard;
  grid[position.y][position.x] = DirectionMarkers[direction];

  const delta = DirectionDeltas[direction];
  position.x += delta.x;
  position.y += delta.y;
}

function markExit(grid: string[][], guard: Guard): void {
  const { x, y } = guard.position;
  grid[y][x] = "E";
  console.log("Guard is out of the grid", guard.position);
}

function countVisitedTiles(grid: string[][], markers: Set<string>): number {
  return grid.flat().filter((tile) => markers.has(tile)).length;
}

function renderGrid(grid: string[][]): string[] {
  return grid.map((line) => line.join(""));
}

const grid = input.split("\n").map((line) => line.split(""));
const guard: Guard = initializeGuard(grid);

const visitedMarkers = new Set(["▲", "▼", "◄", "►", "S", "E"]);

while (true) {
  const nextTile = getNextTile(grid, guard);

  if (!nextTile) {
    markExit(grid, guard);
    break;
  }

  if (nextTile === "#") {
    changeDirection(guard);
  } else {
    moveGuard(grid, guard);
  }
}

const theGrid = renderGrid(grid);
const visited = countVisitedTiles(grid, visitedMarkers);

console.log(theGrid);
console.log({ visited });