Function isValidDiagonal

docs/tests/v2/es6/js/sanddance.js:69784–69789

(a, b)

// check if a diagonal between two polygon nodes is valid (lies in polygon interior)
function isValidDiagonal(a, b) {
    return a.next.i !== b.i && a.prev.i !== b.i && !intersectsPolygon(a, b) && // dones't intersect other edges
           (locallyInside(a, b) && locallyInside(b, a) && middleInside(a, b) && // locally visible
            (area(a.prev, a, b.prev) || area(a, b.prev, b)) || // does not create opposite-facing sectors
            equals(a, b) && area(a.prev, a, a.next) > 0 && area(b.prev, b, b.next) > 0); // special zero-length case
}

// signed area of a triangle
function area(p, q, r) {