Three points are collinear when the slope between (x1,y1)→(x2,y2) equals the slope between (x1,y1)→(x3,y3). Compare cross products (y2−y1)·(x3−x1) === (x2−x1)·(y3−y1) instead of using division to avoid floating-point errors.

Fix the first two points as a reference, then loop from index 2 onward, checking each point against that reference slope.

Handle the vertical line case: if all points share the same x-coordinate, the denominator dx is 0 — the cross-product check already handles this correctly without special-casing.