source: node_modules/d3-polygon/src/hull.js@ e4c61dd

import cross from "./cross.js";

function lexicographicOrder(a, b) {
  return a[0] - b[0] || a[1] - b[1];
}

// Computes the upper convex hull per the monotone chain algorithm.
// Assumes points.length >= 3, is sorted by x, unique in y.
// Returns an array of indices into points in left-to-right order.
function computeUpperHullIndexes(points) {
  const n = points.length,
      indexes = [0, 1];
  let size = 2, i;

  for (i = 2; i < n; ++i) {
    while (size > 1 && cross(points[indexes[size - 2]], points[indexes[size - 1]], points[i]) <= 0) --size;
    indexes[size++] = i;
  }

  return indexes.slice(0, size); // remove popped points
}

export default function(points) {
  if ((n = points.length) < 3) return null;

  var i,
      n,
      sortedPoints = new Array(n),
      flippedPoints = new Array(n);

  for (i = 0; i < n; ++i) sortedPoints[i] = [+points[i][0], +points[i][1], i];
  sortedPoints.sort(lexicographicOrder);
  for (i = 0; i < n; ++i) flippedPoints[i] = [sortedPoints[i][0], -sortedPoints[i][1]];

  var upperIndexes = computeUpperHullIndexes(sortedPoints),
      lowerIndexes = computeUpperHullIndexes(flippedPoints);

  // Construct the hull polygon, removing possible duplicate endpoints.
  var skipLeft = lowerIndexes[0] === upperIndexes[0],
      skipRight = lowerIndexes[lowerIndexes.length - 1] === upperIndexes[upperIndexes.length - 1],
      hull = [];

  // Add upper hull in right-to-l order.
  // Then add lower hull in left-to-right order.
  for (i = upperIndexes.length - 1; i >= 0; --i) hull.push(points[sortedPoints[upperIndexes[i]][2]]);
  for (i = +skipLeft; i < lowerIndexes.length - skipRight; ++i) hull.push(points[sortedPoints[lowerIndexes[i]][2]]);

  return hull;
}