constructor( radius = 1, segments = 32, thetaStart = 0, thetaLength = Math.PI * 2 ) {

		super();

		this.type = 'CircleGeometry';

		/**
		 * Holds the constructor parameters that have been
		 * used to generate the geometry. Any modification
		 * after instantiation does not change the geometry.
		 *
		 * @type {Object}
		 */
		this.parameters = {
			radius: radius,
			segments: segments,
			thetaStart: thetaStart,
			thetaLength: thetaLength
		};

		segments = Math.max( 3, segments );

		// buffers

		const indices = [];
		const vertices = [];
		const normals = [];
		const uvs = [];

		// helper variables

		const vertex = new Vector3();
		const uv = new Vector2();

		// center point

		vertices.push( 0, 0, 0 );
		normals.push( 0, 0, 1 );
		uvs.push( 0.5, 0.5 );

		for ( let s = 0, i = 3; s <= segments; s ++, i += 3 ) {

			const segment = thetaStart + s / segments * thetaLength;

			// vertex

			vertex.x = radius * Math.cos( segment );
			vertex.y = radius * Math.sin( segment );

			vertices.push( vertex.x, vertex.y, vertex.z );

			// normal

			normals.push( 0, 0, 1 );

			// uvs

			uv.x = ( vertices[ i ] / radius + 1 ) / 2;
			uv.y = ( vertices[ i + 1 ] / radius + 1 ) / 2;

			uvs.push( uv.x, uv.y );

		}

		// indices

		for ( let i = 1; i <= segments; i ++ ) {

			indices.push( i, i + 1, 0 );

		}

		// build geometry

		this.setIndex( indices );
		this.setAttribute( 'position', new Float32BufferAttribute( vertices, 3 ) );
		this.setAttribute( 'normal', new Float32BufferAttribute( normals, 3 ) );
		this.setAttribute( 'uv', new Float32BufferAttribute( uvs, 2 ) );

	}
Constructs a new circle geometry.

@param {number} [radius=1] - Radius of the circle.
@param {number} [segments=32] - Number of segments (triangles), minimum = `3`.
@param {number} [thetaStart=0] - Start angle for first segment in radians.
@param {number} [thetaLength=Math.PI*2] - The central angle, often called theta,
of the circular sector in radians. The default value results in a complete circle.