Offline CDQ Divide & Conquer

// 3D partial order: for each point (t,x,y), count points with t'<t, x'<=x, y'<=y
// CDQ: time dimension handled by divide (left half has smaller t)
// x dimension: sort + two-pointer
// y dimension: Fenwick tree

function cdq3D(points) { // points = [{t, x, y, idx}] sorted by t
    const ans = new Array(points.length).fill(0);
    const n = points.length;
    const bit = new Array(n + 1).fill(0);
    const upd = i => { for (; i <= n; i += i & (-i)) bit[i]++; };
    const qry = i => { let s = 0; for (; i > 0; i -= i & (-i)) s += bit[i]; return s; };
    const undo = [];

    function solve(l, r) {
        if (l === r) return;
        const mid = (l + r) >> 1;
        solve(l, mid); // process left half
        // Compute contribution of left half on right half
        const left = points.slice(l, mid + 1).sort((a, b) => a.x - b.x);
        const right = points.slice(mid + 1, r + 1).sort((a, b) => a.x - b.x);
        let li = 0;
        for (const rp of right) {
            while (li < left.length && left[li].x <= rp.x) {
                upd(left[li].y); undo.push(left[li].y); li++;
            }
            ans[rp.idx] += qry(rp.y);
        }
        // Undo BIT updates
        while (undo.length) { const y = undo.pop(); for (let i = y; i <= n; i += i & (-i)) bit[i]--; }
        solve(mid + 1, r); // process right half
    }

    solve(0, n - 1);
    return ans;
}