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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;
}
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