function parse(input) {
return input.split("\n").map((d) => d.split("").map(Number));
}
function nPaths(r, c, dem, trod) {
const z = dem[r][c];
const [nr, nc] = [dem.length, dem[0].length];
if (trod?.has(r * nc + c)) {
return 0; // We've been here before
}
trod?.add(r * nc + c);
if (dem[r][c] === 9) {
return 1; // Summit!
}
// Move to neighbours
return (
(r > 0 && dem[r - 1][c] - z === 1 && nPaths(r - 1, c, dem, trod)) +
(c > 0 && dem[r][c - 1] - z === 1 && nPaths(r, c - 1, dem, trod)) +
(r < nr - 1 && dem[r + 1][c] - z === 1 && nPaths(r + 1, c, dem, trod)) +
(c < nc - 1 && dem[r][c + 1] - z === 1 && nPaths(r, c + 1, dem, trod))
);
}
function part1(input) {
const dem = parse(input);
return d3.sum(
dem.flatMap((row, r) =>
row.map((elev, c) => (elev === 0 ? nPaths(r, c, dem, new Set()) : 0))
)
);
}
function part2(input) {
const dem = parse(input);
return d3.sum(
dem.flatMap((row, r) =>
row.map((elev, c) => (elev === 0 ? nPaths(r, c, dem) : 0))
)
);
}
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