/**
 * @author mrdoob / http://mrdoob.com/
 */

THREE.BufferGeometryUtils = {

	computeTangents: function ( geometry ) {

		var index = geometry.index;
		var attributes = geometry.attributes;

		// based on http://www.terathon.com/code/tangent.html
		// (per vertex tangents)

		if ( index === null ||
			 attributes.position === undefined ||
			 attributes.normal === undefined ||
			 attributes.uv === undefined ) {

			console.warn( 'THREE.BufferGeometry: Missing required attributes (index, position, normal or uv) in BufferGeometry.computeTangents()' );
			return;

		}

		var indices = index.array;
		var positions = attributes.position.array;
		var normals = attributes.normal.array;
		var uvs = attributes.uv.array;

		var nVertices = positions.length / 3;

		if ( attributes.tangent === undefined ) {

			geometry.addAttribute( 'tangent', new THREE.BufferAttribute( new Float32Array( 4 * nVertices ), 4 ) );

		}

		var tangents = attributes.tangent.array;

		var tan1 = [], tan2 = [];

		for ( var k = 0; k < nVertices; k ++ ) {

			tan1[ k ] = new THREE.Vector3();
			tan2[ k ] = new THREE.Vector3();

		}

		var vA = new THREE.Vector3(),
			vB = new THREE.Vector3(),
			vC = new THREE.Vector3(),

			uvA = new THREE.Vector2(),
			uvB = new THREE.Vector2(),
			uvC = new THREE.Vector2(),

			sdir = new THREE.Vector3(),
			tdir = new THREE.Vector3();

		function handleTriangle( a, b, c ) {

			vA.fromArray( positions, a * 3 );
			vB.fromArray( positions, b * 3 );
			vC.fromArray( positions, c * 3 );

			uvA.fromArray( uvs, a * 2 );
			uvB.fromArray( uvs, b * 2 );
			uvC.fromArray( uvs, c * 2 );

			var x1 = vB.x - vA.x;
			var x2 = vC.x - vA.x;

			var y1 = vB.y - vA.y;
			var y2 = vC.y - vA.y;

			var z1 = vB.z - vA.z;
			var z2 = vC.z - vA.z;

			var s1 = uvB.x - uvA.x;
			var s2 = uvC.x - uvA.x;

			var t1 = uvB.y - uvA.y;
			var t2 = uvC.y - uvA.y;

			var r = 1.0 / ( s1 * t2 - s2 * t1 );

			sdir.set(
				( t2 * x1 - t1 * x2 ) * r,
				( t2 * y1 - t1 * y2 ) * r,
				( t2 * z1 - t1 * z2 ) * r
			);

			tdir.set(
				( s1 * x2 - s2 * x1 ) * r,
				( s1 * y2 - s2 * y1 ) * r,
				( s1 * z2 - s2 * z1 ) * r
			);

			tan1[ a ].add( sdir );
			tan1[ b ].add( sdir );
			tan1[ c ].add( sdir );

			tan2[ a ].add( tdir );
			tan2[ b ].add( tdir );
			tan2[ c ].add( tdir );

		}

		var groups = geometry.groups;

		if ( groups.length === 0 ) {

			groups = [ {
				start: 0,
				count: indices.length
			} ];

		}

		for ( var j = 0, jl = groups.length; j < jl; ++ j ) {

			var group = groups[ j ];

			var start = group.start;
			var count = group.count;

			for ( var i = start, il = start + count; i < il; i += 3 ) {

				handleTriangle(
					indices[ i + 0 ],
					indices[ i + 1 ],
					indices[ i + 2 ]
				);

			}

		}

		var tmp = new THREE.Vector3(), tmp2 = new THREE.Vector3();
		var n = new THREE.Vector3(), n2 = new THREE.Vector3();
		var w, t, test;

		function handleVertex( v ) {

			n.fromArray( normals, v * 3 );
			n2.copy( n );

			t = tan1[ v ];

			// Gram-Schmidt orthogonalize

			tmp.copy( t );
			tmp.sub( n.multiplyScalar( n.dot( t ) ) ).normalize();

			// Calculate handedness

			tmp2.crossVectors( n2, t );
			test = tmp2.dot( tan2[ v ] );
			w = ( test < 0.0 ) ? - 1.0 : 1.0;

			tangents[ v * 4 ] = tmp.x;
			tangents[ v * 4 + 1 ] = tmp.y;
			tangents[ v * 4 + 2 ] = tmp.z;
			tangents[ v * 4 + 3 ] = w;

		}

		for ( var j = 0, jl = groups.length; j < jl; ++ j ) {

			var group = groups[ j ];

			var start = group.start;
			var count = group.count;

			for ( var i = start, il = start + count; i < il; i += 3 ) {

				handleVertex( indices[ i + 0 ] );
				handleVertex( indices[ i + 1 ] );
				handleVertex( indices[ i + 2 ] );

			}

		}

	}

};