part-one.js

const { input, folds } = require('./input');
const { InfiniteGrid } = require('./infinite-grid');

let grid = new InfiniteGrid({
	defaultFactory: () => 0,
	string_map: {
		0: ' ',
		1: '#',
	},
});

for (let [x, y] of input) {
	grid.set(x, y, 1);
}

let { axis, line } = folds[0];

let points_to_fold_across = [];
for (let [id, cell] of grid) {
	// This shouldn't happen, but just in case
	if (cell === 0) {
		continue;
	}

	let [x, y] = InfiniteGrid.toCoords(id);
	const compare = axis === 'x' ? x : y;

	if (compare < line) {
		// This point is above / to the left of the line, it stays
		continue;
	}

	// This point is below the line (if folding along y) or to the right (if folding along x)
	points_to_fold_across.push([x, y]);
}

for (let [x, y] of points_to_fold_across) {
	if (axis === 'x') {
		let new_x = line - Math.abs(x - line);
		grid.set(new_x, y, 1);
	} else {
		let new_y = line - Math.abs(y - line);
		grid.set(x, new_y, 1);
	}

	// Remove point from grid map directly. Don't worry about resizing just yet, we can do that at the very end.
	grid.grid.delete(InfiniteGrid.toId(x, y));
}

grid.resize();

console.log(grid.sum());