Day Ten, Part Two. Finally!

index.ts

@@ -1,3 +1,3 @@
-import {runDayEleven} from "./src/day_eleven";
+import {runDayTen} from "./src/day_ten";
 
-runDayEleven();
+runDayTen();

src/day_ten.test.ts

@@ -1,4 +1,4 @@
-import {getFurthestDistance} from "./day_ten";
+import {getFurthestDistance, tilesContained} from "./day_ten";
 
 describe('Day Ten', () => {
 	it.each([
@@ -21,4 +21,47 @@ LJ...`, 8]
 		const result = getFurthestDistance(input);
 		expect(result).toEqual(expected);
 	});
+
+	describe('Part Two', () => {
+		it.each([
+			['.....\n' +
+			'.S-7.\n' +
+			'.|.|.\n' +
+			'.L-J.\n' +
+			'.....', 1],
+			['...........\n' +
+			'.S-------7.\n' +
+			'.|F-----7|.\n' +
+			'.||.....||.\n' +
+			'.||.....||.\n' +
+			'.|L-7.F-J|.\n' +
+			'.|..|.|..|.\n' +
+			'.L--J.L--J.\n' +
+			'...........\n', 4],
+			['.F----7F7F7F7F-7....\n' +
+			'.|F--7||||||||FJ....\n' +
+			'.||.FJ||||||||L7....\n' +
+			'FJL7L7LJLJ||LJ.L-7..\n' +
+			'L--J.L7...LJS7F-7L7.\n' +
+			'....F-J..F7FJ|L7L7L7\n' +
+			'....L7.F7||L7|.L7L7|\n' +
+			'.....|FJLJ|FJ|F7|.LJ\n' +
+			'....FJL-7.||.||||...\n' +
+			'....L---J.LJ.LJLJ...', 8],
+			['FF7FSF7F7F7F7F7F---7\n' +
+			'L|LJ||||||||||||F--J\n' +
+			'FL-7LJLJ||||||LJL-77\n' +
+			'F--JF--7||LJLJ7F7FJ-\n' +
+			'L---JF-JLJ.||-FJLJJ7\n' +
+			'|F|F-JF---7F7-L7L|7|\n' +
+			'|FFJF7L7F-JF7|JL---7\n' +
+			'7-L-JL7||F7|L7F-7F7|\n' +
+			'L.L7LFJ|||||FJL7||LJ\n' +
+			'L7JLJL-JLJLJL--JLJ.L', 10]
+		])('should count the number of tiles fully contained within the loop', (input, expected) => {
+			// const result = tilesContained(input);
+			const result = tilesContained(input);
+			expect(result).toEqual(expected);
+		})
+	});
 });

src/day_ten.ts

@@ -1,17 +1,24 @@
 import fs from "fs";
+import {window} from "./utils";
 
 type Grid = Tile[][];
 type Cell = [number, number];
 type Tile = 'S' | 'F' | 'J' | 'L' | '7' | '-' | '|' | '.';
+type Loop = string[];
 
 const isTile = (value: string): value is Tile => {
 	return ['S', 'F', 'J', 'L', '7', '-', '|', '.'].includes(value);
 }
 
 const toString = (cell: Cell): string => {
-	return cell.toString();
+	return JSON.stringify(cell);
 }
 
+const fromString = (cell: string): Cell => {
+	return JSON.parse(cell);
+}
+
+
 const getStart = (grid: Grid): Cell => {
 	const row = grid.findIndex(row => row.includes('S'));
 	const col = grid[row].findIndex(cell => cell === 'S');
@@ -28,12 +35,12 @@ const getSurrounding = (grid: Grid, [row, col]: Cell): Cell[] => {
 		[row - 1, col + 1],
 		[row + 1, col - 1],
 		[row + 1, col + 1],
-	] as Cell[]).filter((cell) => {
-		const valid = isValid(grid, cell);
-		const neighbour = neighbours(grid, cell).map(toString);
-
-		return valid && neighbour.includes([row, col].toString());
-	});
+	] as Cell[]).filter((cell) => isInGrid(grid, cell));
+}
+const getConnectingNeighbours = (grid: Grid, [row, col]: Cell): Cell[] => {
+	return getSurrounding(grid, [row, col]).filter((cell) =>
+		isValid(grid, cell) && neighbours(grid, cell).map(toString).includes(toString([row, col]))
+	);
 }
 
 const neighbours = (grid: Grid, [row, col]: Cell): Cell[] => {
@@ -74,35 +81,71 @@ const neighbours = (grid: Grid, [row, col]: Cell): Cell[] => {
 	}
 }
 
-const isValid = (grid: Grid, [row, col]: Cell): boolean => {
-	return row >= 0 && row < grid.length && col >= 0 && col < grid[row].length && grid[row][col] !== '.';
+function isInGrid(grid: Tile[][], [row, col]: Cell) {
+	return row >= 0 && row < grid.length && col >= 0 && col < grid[row].length;
 }
 
-export const getFurthestDistance = (input: string): number => {
-	const grid = input.split('\n').map(line => line.split('').filter(isTile));
+const isValid = (grid: Grid, cell: Cell): boolean => {
+	return isInGrid(grid, cell) && grid[cell[0]][cell[1]] !== '.';
+}
 
+function parseGrid(input: string) {
+	return input.split('\n').map(line => line.split('').filter(isTile));
+}
+
+function getMainLoop(grid: Grid): Loop {
 	const start = getStart(grid);
-	const queue = getSurrounding(grid, start);
-	console.log(start, queue);
-	const visited = new Set<string>([start.toString()]);
+	const queue = [getConnectingNeighbours(grid, start)[0]];
+	const visited = [toString(start)];
 
 	while (queue.length > 0) {
 		const current = queue.shift()!;
 		const next = neighbours(grid, current);
 
 		next.forEach(cell => {
-			if (!visited.has(cell.toString()) && isValid(grid, cell)) {
+			if (!visited.includes(toString(cell)) && isValid(grid, cell)) {
 				queue.push(cell);
-				visited.add(cell.toString());
+				visited.push(toString(cell));
 			}
 		});
 	}
+	return visited;
+}
 
-	return visited.size / 2;
+export const getFurthestDistance = (input: string): number => {
+	const grid = parseGrid(input);
+	const loop = getMainLoop(grid);
+
+	return loop.length / 2;
+}
+
+export const area = (grid: Grid, loop: Loop): number => {
+	// Use shoelace formula to calculate area of polygon
+	const polygon = loop.map(fromString).filter((cell) => {
+		const value = grid[cell[0]][cell[1]];
+		return ['F', 'J', 'L', '7', 'S'].includes(value);
+	});
+
+	const total = window([...polygon, polygon[0]], 2).reduce((total, [a, b]) => {
+		const [y1, x1] = a;
+		const [y2, x2] = b;
+		return total + (x1 * y2 - y1 * x2);
+	}, 0);
+
+	return Math.abs(total) / 2;
+}
+
+export const tilesContained = (input: string): number => {
+	const grid = parseGrid(input);
+	const loop = getMainLoop(grid);
+
+	const loopArea = area(grid, loop);
+	const boundaryPointsCount = loop.length;
+	return loopArea - (boundaryPointsCount / 2) + 1;
 }
 
 export const runDayTen = () => {
 	const input = fs.readFileSync('./inputs/day_ten_input.txt', 'utf8').trimEnd();
-	const result = getFurthestDistance(input);
+	const result = tilesContained(input);
 	console.log(`Day Ten: ${result}`);
 }

src/utils.ts

@@ -5,3 +5,10 @@ export const matchAllAndThen = (input: string, pattern: RegExp, callback: (value
 		}
 	});
 }
+
+export const window = <T>(input: T[], size: number): T[][] => {
+	return input.reduce((windows, value, index) => {
+		if (index === input.length - size + 1) return windows;
+		return [...windows, input.slice(index, index + size)];
+	}, [] as T[][]);
+}