source: node_modules/d3-geo/src/circle.js@ e4c61dd

import {cartesian, cartesianNormalizeInPlace, spherical} from "./cartesian.js";
import constant from "./constant.js";
import {acos, cos, degrees, epsilon, radians, sin, tau} from "./math.js";
import {rotateRadians} from "./rotation.js";

// Generates a circle centered at [0°, 0°], with a given radius and precision.
export function circleStream(stream, radius, delta, direction, t0, t1) {
  if (!delta) return;
  var cosRadius = cos(radius),
      sinRadius = sin(radius),
      step = direction * delta;
  if (t0 == null) {
    t0 = radius + direction * tau;
    t1 = radius - step / 2;
  } else {
    t0 = circleRadius(cosRadius, t0);
    t1 = circleRadius(cosRadius, t1);
    if (direction > 0 ? t0 < t1 : t0 > t1) t0 += direction * tau;
  }
  for (var point, t = t0; direction > 0 ? t > t1 : t < t1; t -= step) {
    point = spherical([cosRadius, -sinRadius * cos(t), -sinRadius * sin(t)]);
    stream.point(point[0], point[1]);
  }
}

// Returns the signed angle of a cartesian point relative to [cosRadius, 0, 0].
function circleRadius(cosRadius, point) {
  point = cartesian(point), point[0] -= cosRadius;
  cartesianNormalizeInPlace(point);
  var radius = acos(-point[1]);
  return ((-point[2] < 0 ? -radius : radius) + tau - epsilon) % tau;
}

export default function() {
  var center = constant([0, 0]),
      radius = constant(90),
      precision = constant(2),
      ring,
      rotate,
      stream = {point: point};

  function point(x, y) {
    ring.push(x = rotate(x, y));
    x[0] *= degrees, x[1] *= degrees;
  }

  function circle() {
    var c = center.apply(this, arguments),
        r = radius.apply(this, arguments) * radians,
        p = precision.apply(this, arguments) * radians;
    ring = [];
    rotate = rotateRadians(-c[0] * radians, -c[1] * radians, 0).invert;
    circleStream(stream, r, p, 1);
    c = {type: "Polygon", coordinates: [ring]};
    ring = rotate = null;
    return c;
  }

  circle.center = function(_) {
    return arguments.length ? (center = typeof _ === "function" ? _ : constant([+_[0], +_[1]]), circle) : center;
  };

  circle.radius = function(_) {
    return arguments.length ? (radius = typeof _ === "function" ? _ : constant(+_), circle) : radius;
  };

  circle.precision = function(_) {
    return arguments.length ? (precision = typeof _ === "function" ? _ : constant(+_), circle) : precision;
  };

  return circle;
}