Advent of Code: 2024 Day 6 / Jo Wood

Jo Wood

Professor of Visual Analytics interested in design, geovisualization, visual storytelling and computational thinking.

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Ξ🎄Advent of Code 2024

Edited

Dec 14, 2024

Paused

function parse(input) {

const grid = input.split("\n").map((d) => d.split(""));

const [nRows, nCols] = [grid.length, grid[0].length];

const p = grid.flat().indexOf("^");

const [grdRow, grdCol] = [Math.floor(p / nCols), p % nCols];

grid[grdRow][grdCol] = ".";

return { start: [grdRow, grdCol], dir: 0, grid: grid, nRows, nCols };

}

dirs = [

[-1, 0],

[0, 1],

[1, 0],

[0, -1]

]

function guardPositions(lab) {

const positions = [];

let [[row, col], dir] = [lab.start, lab.dir];

while (true) {

positions.push([row, col, dir]);

const [nextR, nextC] = [row + dirs[dir][0], col + dirs[dir][1]];

if (nextR < 0 || nextR >= lab.nRows || nextC < 0 || nextC >= lab.nCols) {

return positions;

}

if (lab.grid[nextR][nextC] === "#") {

dir = (dir + 1) % 4; // Turn right if hitting an obstruction

}

[row, col] = [row + dirs[dir][0], col + dirs[dir][1]];

}

function part1(input) {

return new Set(guardPositions(parse(input)).map((d) => (d[0] << 8) | d[1]))

.size;

}

function hasLoop(lab) {

const positions = new Set();

let [row, col] = lab.start;

let dir = lab.dir;

let turn = false;

while (true) {

if (turn) {

const pos = (row << 16) | (col << 8) | dir;

if (positions.has(pos)) {

return true;

}

positions.add(pos);

}

const [nextR, nextC] = [row + dirs[dir][0], col + dirs[dir][1]];

if (nextR < 0 || nextR >= lab.nRows || nextC < 0 || nextC > lab.nCols) {

return false;

}

if (lab.grid[nextR][nextC] === "#") {

// If we hit an obstruction, turn, but don't move

dir = (dir + 1) % 4;

turn = true;

} else {

turn = false;

row = nextR;

col = nextC;

}

function numObstructions(lab) {

const route = [

...new Set(guardPositions(lab).map((d) => (d[0] << 8) | d[1]))

].map((d) => [(d >> 8) & 0xff, d & 0xff]);

const loopPos = new Set();

route.forEach((pos) => {

lab.grid[pos[0]][pos[1]] = "#";

if (hasLoop(lab)) {

loopPos.add((pos[0] << 8) | pos[1]);

}

lab.grid[pos[0]][pos[1]] = ".";

});

return loopPos.size;

}

function part2(input) {

return numObstructions(parse(input));

}

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