d3-geo-projection/src/project/contains.js

Extended geographic projections for d3-geo.

export default function(ring, point) {
  var x = point[0],
      y = point[1],
      contains = false;
  for (var i = 0, n = ring.length, j = n - 1; i < n; j = i++) {
    var pi = ring[i], xi = pi[0], yi = pi[1],
        pj = ring[j], xj = pj[0], yj = pj[1];
    if (((yi > y) ^ (yj > y)) && (x < (xj - xi) * (y - yi) / (yj - yi) + xi)) contains = !contains;
  }
  return contains;
}