is_point_on_line_segment in C++

gistfile1.txt

// See if pt is between pt0 and pt1.
// via stackoverflow https://stackoverflow.com/a/328122
auto is_between = [](VertexType pt, VertexType p0, VertexType p1) {
    auto z = (pt.y - p0.y) * (p1.x - p0.x)
        - (pt.x - p0.x) * (p1.y - p0.y);
    if (abs(z) > .0001) return false;
    auto d = (pt.x - p0.x) * (p1.x - p0.x)
        + (pt.y - p0.y) * (p1.y - p0.y);
    if (d < 0) return false;
    auto n2 = (p1.x - p0.x) * (p1.x - p0.x)
        + (p1.y - p0.y) * (p1.y - p0.y);
    if (d > n2) return false;
    return true;
};