import {
	BufferAttribute,
	BufferGeometry,
	Float32BufferAttribute,
	InstancedBufferAttribute,
	InterleavedBuffer,
	InterleavedBufferAttribute,
	TriangleFanDrawMode,
	TriangleStripDrawMode,
	TrianglesDrawMode,
	Vector3,
} from 'three';

function computeMikkTSpaceTangents( geometry, MikkTSpace, negateSign = true ) {

	if ( ! MikkTSpace || ! MikkTSpace.isReady ) {

		throw new Error( 'BufferGeometryUtils: Initialized MikkTSpace library required.' );

	}

	if ( ! geometry.hasAttribute( 'position' ) || ! geometry.hasAttribute( 'normal' ) || ! geometry.hasAttribute( 'uv' ) ) {

		throw new Error( 'BufferGeometryUtils: Tangents require "position", "normal", and "uv" attributes.' );

	}

	function getAttributeArray( attribute ) {

		if ( attribute.normalized || attribute.isInterleavedBufferAttribute ) {

			const dstArray = new Float32Array( attribute.count * attribute.itemSize );

			for ( let i = 0, j = 0; i < attribute.count; i ++ ) {

				dstArray[ j ++ ] = attribute.getX( i );
				dstArray[ j ++ ] = attribute.getY( i );

				if ( attribute.itemSize > 2 ) {

					dstArray[ j ++ ] = attribute.getZ( i );

				}

			}

			return dstArray;

		}

		if ( attribute.array instanceof Float32Array ) {

			return attribute.array;

		}

		return new Float32Array( attribute.array );

	}

	// MikkTSpace algorithm requires non-indexed input.

	const _geometry = geometry.index ? geometry.toNonIndexed() : geometry;

	// Compute vertex tangents.

	const tangents = MikkTSpace.generateTangents(

		getAttributeArray( _geometry.attributes.position ),
		getAttributeArray( _geometry.attributes.normal ),
		getAttributeArray( _geometry.attributes.uv )

	);

	// Texture coordinate convention of glTF differs from the apparent
	// default of the MikkTSpace library; .w component must be flipped.

	if ( negateSign ) {

		for ( let i = 3; i < tangents.length; i += 4 ) {

			tangents[ i ] *= - 1;

		}

	}

	//

	_geometry.setAttribute( 'tangent', new BufferAttribute( tangents, 4 ) );

	if ( geometry !== _geometry ) {

		geometry.copy( _geometry );

	}

	return geometry;

}

/**
 * @param  {Array<BufferGeometry>} geometries
 * @param  {Boolean} useGroups
 * @return {BufferGeometry}
 */
function mergeGeometries( geometries, useGroups = false ) {

	const isIndexed = geometries[ 0 ].index !== null;

	const attributesUsed = new Set( Object.keys( geometries[ 0 ].attributes ) );
	const morphAttributesUsed = new Set( Object.keys( geometries[ 0 ].morphAttributes ) );

	const attributes = {};
	const morphAttributes = {};

	const morphTargetsRelative = geometries[ 0 ].morphTargetsRelative;

	const mergedGeometry = new BufferGeometry();

	let offset = 0;

	for ( let i = 0; i < geometries.length; ++ i ) {

		const geometry = geometries[ i ];
		let attributesCount = 0;

		// ensure that all geometries are indexed, or none

		if ( isIndexed !== ( geometry.index !== null ) ) {

			console.error( 'THREE.BufferGeometryUtils: .mergeGeometries() failed with geometry at index ' + i + '. All geometries must have compatible attributes; make sure index attribute exists among all geometries, or in none of them.' );
			return null;

		}

		// gather attributes, exit early if they're different

		for ( const name in geometry.attributes ) {

			if ( ! attributesUsed.has( name ) ) {

				console.error( 'THREE.BufferGeometryUtils: .mergeGeometries() failed with geometry at index ' + i + '. All geometries must have compatible attributes; make sure "' + name + '" attribute exists among all geometries, or in none of them.' );
				return null;

			}

			if ( attributes[ name ] === undefined ) attributes[ name ] = [];

			attributes[ name ].push( geometry.attributes[ name ] );

			attributesCount ++;

		}

		// ensure geometries have the same number of attributes

		if ( attributesCount !== attributesUsed.size ) {

			console.error( 'THREE.BufferGeometryUtils: .mergeGeometries() failed with geometry at index ' + i + '. Make sure all geometries have the same number of attributes.' );
			return null;

		}

		// gather morph attributes, exit early if they're different

		if ( morphTargetsRelative !== geometry.morphTargetsRelative ) {

			console.error( 'THREE.BufferGeometryUtils: .mergeGeometries() failed with geometry at index ' + i + '. .morphTargetsRelative must be consistent throughout all geometries.' );
			return null;

		}

		for ( const name in geometry.morphAttributes ) {

			if ( ! morphAttributesUsed.has( name ) ) {

				console.error( 'THREE.BufferGeometryUtils: .mergeGeometries() failed with geometry at index ' + i + '.  .morphAttributes must be consistent throughout all geometries.' );
				return null;

			}

			if ( morphAttributes[ name ] === undefined ) morphAttributes[ name ] = [];

			morphAttributes[ name ].push( geometry.morphAttributes[ name ] );

		}

		if ( useGroups ) {

			let count;

			if ( isIndexed ) {

				count = geometry.index.count;

			} else if ( geometry.attributes.position !== undefined ) {

				count = geometry.attributes.position.count;

			} else {

				console.error( 'THREE.BufferGeometryUtils: .mergeGeometries() failed with geometry at index ' + i + '. The geometry must have either an index or a position attribute' );
				return null;

			}

			mergedGeometry.addGroup( offset, count, i );

			offset += count;

		}

	}

	// merge indices

	if ( isIndexed ) {

		let indexOffset = 0;
		const mergedIndex = [];

		for ( let i = 0; i < geometries.length; ++ i ) {

			const index = geometries[ i ].index;

			for ( let j = 0; j < index.count; ++ j ) {

				mergedIndex.push( index.getX( j ) + indexOffset );

			}

			indexOffset += geometries[ i ].attributes.position.count;

		}

		mergedGeometry.setIndex( mergedIndex );

	}

	// merge attributes

	for ( const name in attributes ) {

		const mergedAttribute = mergeAttributes( attributes[ name ] );

		if ( ! mergedAttribute ) {

			console.error( 'THREE.BufferGeometryUtils: .mergeGeometries() failed while trying to merge the ' + name + ' attribute.' );
			return null;

		}

		mergedGeometry.setAttribute( name, mergedAttribute );

	}

	// merge morph attributes

	for ( const name in morphAttributes ) {

		const numMorphTargets = morphAttributes[ name ][ 0 ].length;

		if ( numMorphTargets === 0 ) break;

		mergedGeometry.morphAttributes = mergedGeometry.morphAttributes || {};
		mergedGeometry.morphAttributes[ name ] = [];

		for ( let i = 0; i < numMorphTargets; ++ i ) {

			const morphAttributesToMerge = [];

			for ( let j = 0; j < morphAttributes[ name ].length; ++ j ) {

				morphAttributesToMerge.push( morphAttributes[ name ][ j ][ i ] );

			}

			const mergedMorphAttribute = mergeAttributes( morphAttributesToMerge );

			if ( ! mergedMorphAttribute ) {

				console.error( 'THREE.BufferGeometryUtils: .mergeGeometries() failed while trying to merge the ' + name + ' morphAttribute.' );
				return null;

			}

			mergedGeometry.morphAttributes[ name ].push( mergedMorphAttribute );

		}

	}

	return mergedGeometry;

}

/**
 * @param {Array<BufferAttribute>} attributes
 * @return {BufferAttribute}
 */
function mergeAttributes( attributes ) {

	let TypedArray;
	let itemSize;
	let normalized;
	let gpuType = - 1;
	let arrayLength = 0;

	for ( let i = 0; i < attributes.length; ++ i ) {

		const attribute = attributes[ i ];

		if ( TypedArray === undefined ) TypedArray = attribute.array.constructor;
		if ( TypedArray !== attribute.array.constructor ) {

			console.error( 'THREE.BufferGeometryUtils: .mergeAttributes() failed. BufferAttribute.array must be of consistent array types across matching attributes.' );
			return null;

		}

		if ( itemSize === undefined ) itemSize = attribute.itemSize;
		if ( itemSize !== attribute.itemSize ) {

			console.error( 'THREE.BufferGeometryUtils: .mergeAttributes() failed. BufferAttribute.itemSize must be consistent across matching attributes.' );
			return null;

		}

		if ( normalized === undefined ) normalized = attribute.normalized;
		if ( normalized !== attribute.normalized ) {

			console.error( 'THREE.BufferGeometryUtils: .mergeAttributes() failed. BufferAttribute.normalized must be consistent across matching attributes.' );
			return null;

		}

		if ( gpuType === - 1 ) gpuType = attribute.gpuType;
		if ( gpuType !== attribute.gpuType ) {

			console.error( 'THREE.BufferGeometryUtils: .mergeAttributes() failed. BufferAttribute.gpuType must be consistent across matching attributes.' );
			return null;

		}

		arrayLength += attribute.count * itemSize;

	}

	const array = new TypedArray( arrayLength );
	const result = new BufferAttribute( array, itemSize, normalized );
	let offset = 0;

	for ( let i = 0; i < attributes.length; ++ i ) {

		const attribute = attributes[ i ];
		if ( attribute.isInterleavedBufferAttribute ) {

			const tupleOffset = offset / itemSize;
			for ( let j = 0, l = attribute.count; j < l; j ++ ) {

				for ( let c = 0; c < itemSize; c ++ ) {

					const value = attribute.getComponent( j, c );
					result.setComponent( j + tupleOffset, c, value );

				}

			}

		} else {

			array.set( attribute.array, offset );

		}

		offset += attribute.count * itemSize;

	}

	if ( gpuType !== undefined ) {

		result.gpuType = gpuType;

	}

	return result;

}

/**
 * @param {BufferAttribute}
 * @return {BufferAttribute}
 */
export function deepCloneAttribute( attribute ) {

	if ( attribute.isInstancedInterleavedBufferAttribute || attribute.isInterleavedBufferAttribute ) {

		return deinterleaveAttribute( attribute );

	}

	if ( attribute.isInstancedBufferAttribute ) {

		return new InstancedBufferAttribute().copy( attribute );

	}

	return new BufferAttribute().copy( attribute );

}

/**
 * @param {Array<BufferAttribute>} attributes
 * @return {Array<InterleavedBufferAttribute>}
 */
function interleaveAttributes( attributes ) {

	// Interleaves the provided attributes into an InterleavedBuffer and returns
	// a set of InterleavedBufferAttributes for each attribute
	let TypedArray;
	let arrayLength = 0;
	let stride = 0;

	// calculate the length and type of the interleavedBuffer
	for ( let i = 0, l = attributes.length; i < l; ++ i ) {

		const attribute = attributes[ i ];

		if ( TypedArray === undefined ) TypedArray = attribute.array.constructor;
		if ( TypedArray !== attribute.array.constructor ) {

			console.error( 'AttributeBuffers of different types cannot be interleaved' );
			return null;

		}

		arrayLength += attribute.array.length;
		stride += attribute.itemSize;

	}

	// Create the set of buffer attributes
	const interleavedBuffer = new InterleavedBuffer( new TypedArray( arrayLength ), stride );
	let offset = 0;
	const res = [];
	const getters = [ 'getX', 'getY', 'getZ', 'getW' ];
	const setters = [ 'setX', 'setY', 'setZ', 'setW' ];

	for ( let j = 0, l = attributes.length; j < l; j ++ ) {

		const attribute = attributes[ j ];
		const itemSize = attribute.itemSize;
		const count = attribute.count;
		const iba = new InterleavedBufferAttribute( interleavedBuffer, itemSize, offset, attribute.normalized );
		res.push( iba );

		offset += itemSize;

		// Move the data for each attribute into the new interleavedBuffer
		// at the appropriate offset
		for ( let c = 0; c < count; c ++ ) {

			for ( let k = 0; k < itemSize; k ++ ) {

				iba[ setters[ k ] ]( c, attribute[ getters[ k ] ]( c ) );

			}

		}

	}

	return res;

}

// returns a new, non-interleaved version of the provided attribute
export function deinterleaveAttribute( attribute ) {

	const cons = attribute.data.array.constructor;
	const count = attribute.count;
	const itemSize = attribute.itemSize;
	const normalized = attribute.normalized;

	const array = new cons( count * itemSize );
	let newAttribute;
	if ( attribute.isInstancedInterleavedBufferAttribute ) {

		newAttribute = new InstancedBufferAttribute( array, itemSize, normalized, attribute.meshPerAttribute );

	} else {

		newAttribute = new BufferAttribute( array, itemSize, normalized );

	}

	for ( let i = 0; i < count; i ++ ) {

		newAttribute.setX( i, attribute.getX( i ) );

		if ( itemSize >= 2 ) {

			newAttribute.setY( i, attribute.getY( i ) );

		}

		if ( itemSize >= 3 ) {

			newAttribute.setZ( i, attribute.getZ( i ) );

		}

		if ( itemSize >= 4 ) {

			newAttribute.setW( i, attribute.getW( i ) );

		}

	}

	return newAttribute;

}

// deinterleaves all attributes on the geometry
export function deinterleaveGeometry( geometry ) {

	const attributes = geometry.attributes;
	const morphTargets = geometry.morphTargets;
	const attrMap = new Map();

	for ( const key in attributes ) {

		const attr = attributes[ key ];
		if ( attr.isInterleavedBufferAttribute ) {

			if ( ! attrMap.has( attr ) ) {

				attrMap.set( attr, deinterleaveAttribute( attr ) );

			}

			attributes[ key ] = attrMap.get( attr );

		}

	}

	for ( const key in morphTargets ) {

		const attr = morphTargets[ key ];
		if ( attr.isInterleavedBufferAttribute ) {

			if ( ! attrMap.has( attr ) ) {

				attrMap.set( attr, deinterleaveAttribute( attr ) );

			}

			morphTargets[ key ] = attrMap.get( attr );

		}

	}

}

/**
 * @param {BufferGeometry} geometry
 * @return {number}
 */
function estimateBytesUsed( geometry ) {

	// Return the estimated memory used by this geometry in bytes
	// Calculate using itemSize, count, and BYTES_PER_ELEMENT to account
	// for InterleavedBufferAttributes.
	let mem = 0;
	for ( const name in geometry.attributes ) {

		const attr = geometry.getAttribute( name );
		mem += attr.count * attr.itemSize * attr.array.BYTES_PER_ELEMENT;

	}

	const indices = geometry.getIndex();
	mem += indices ? indices.count * indices.itemSize * indices.array.BYTES_PER_ELEMENT : 0;
	return mem;

}

/**
 * @param {BufferGeometry} geometry
 * @param {number} tolerance
 * @return {BufferGeometry}
 */
function mergeVertices( geometry, tolerance = 1e-4 ) {

	tolerance = Math.max( tolerance, Number.EPSILON );

	// Generate an index buffer if the geometry doesn't have one, or optimize it
	// if it's already available.
	const hashToIndex = {};
	const indices = geometry.getIndex();
	const positions = geometry.getAttribute( 'position' );
	const vertexCount = indices ? indices.count : positions.count;

	// next value for triangle indices
	let nextIndex = 0;

	// attributes and new attribute arrays
	const attributeNames = Object.keys( geometry.attributes );
	const tmpAttributes = {};
	const tmpMorphAttributes = {};
	const newIndices = [];
	const getters = [ 'getX', 'getY', 'getZ', 'getW' ];
	const setters = [ 'setX', 'setY', 'setZ', 'setW' ];

	// Initialize the arrays, allocating space conservatively. Extra
	// space will be trimmed in the last step.
	for ( let i = 0, l = attributeNames.length; i < l; i ++ ) {

		const name = attributeNames[ i ];
		const attr = geometry.attributes[ name ];

		tmpAttributes[ name ] = new BufferAttribute(
			new attr.array.constructor( attr.count * attr.itemSize ),
			attr.itemSize,
			attr.normalized
		);

		const morphAttr = geometry.morphAttributes[ name ];
		if ( morphAttr ) {

			tmpMorphAttributes[ name ] = new BufferAttribute(
				new morphAttr.array.constructor( morphAttr.count * morphAttr.itemSize ),
				morphAttr.itemSize,
				morphAttr.normalized
			);

		}

	}

	// convert the error tolerance to an amount of decimal places to truncate to
	const halfTolerance = tolerance * 0.5;
	const exponent = Math.log10( 1 / tolerance );
	const hashMultiplier = Math.pow( 10, exponent );
	const hashAdditive = halfTolerance * hashMultiplier;
	for ( let i = 0; i < vertexCount; i ++ ) {

		const index = indices ? indices.getX( i ) : i;

		// Generate a hash for the vertex attributes at the current index 'i'
		let hash = '';
		for ( let j = 0, l = attributeNames.length; j < l; j ++ ) {

			const name = attributeNames[ j ];
			const attribute = geometry.getAttribute( name );
			const itemSize = attribute.itemSize;

			for ( let k = 0; k < itemSize; k ++ ) {

				// double tilde truncates the decimal value
				hash += `${ ~ ~ ( attribute[ getters[ k ] ]( index ) * hashMultiplier + hashAdditive ) },`;

			}

		}

		// Add another reference to the vertex if it's already
		// used by another index
		if ( hash in hashToIndex ) {

			newIndices.push( hashToIndex[ hash ] );

		} else {

			// copy data to the new index in the temporary attributes
			for ( let j = 0, l = attributeNames.length; j < l; j ++ ) {

				const name = attributeNames[ j ];
				const attribute = geometry.getAttribute( name );
				const morphAttr = geometry.morphAttributes[ name ];
				const itemSize = attribute.itemSize;
				const newarray = tmpAttributes[ name ];
				const newMorphArrays = tmpMorphAttributes[ name ];

				for ( let k = 0; k < itemSize; k ++ ) {

					const getterFunc = getters[ k ];
					const setterFunc = setters[ k ];
					newarray[ setterFunc ]( nextIndex, attribute[ getterFunc ]( index ) );

					if ( morphAttr ) {

						for ( let m = 0, ml = morphAttr.length; m < ml; m ++ ) {

							newMorphArrays[ m ][ setterFunc ]( nextIndex, morphAttr[ m ][ getterFunc ]( index ) );

						}

					}

				}

			}

			hashToIndex[ hash ] = nextIndex;
			newIndices.push( nextIndex );
			nextIndex ++;

		}

	}

	// generate result BufferGeometry
	const result = geometry.clone();
	for ( const name in geometry.attributes ) {

		const tmpAttribute = tmpAttributes[ name ];

		result.setAttribute( name, new BufferAttribute(
			tmpAttribute.array.slice( 0, nextIndex * tmpAttribute.itemSize ),
			tmpAttribute.itemSize,
			tmpAttribute.normalized,
		) );

		if ( ! ( name in tmpMorphAttributes ) ) continue;

		for ( let j = 0; j < tmpMorphAttributes[ name ].length; j ++ ) {

			const tmpMorphAttribute = tmpMorphAttributes[ name ][ j ];

			result.morphAttributes[ name ][ j ] = new BufferAttribute(
				tmpMorphAttribute.array.slice( 0, nextIndex * tmpMorphAttribute.itemSize ),
				tmpMorphAttribute.itemSize,
				tmpMorphAttribute.normalized,
			);

		}

	}

	// indices

	result.setIndex( newIndices );

	return result;

}

/**
 * @param {BufferGeometry} geometry
 * @param {number} drawMode
 * @return {BufferGeometry}
 */
function toTrianglesDrawMode( geometry, drawMode ) {

	if ( drawMode === TrianglesDrawMode ) {

		console.warn( 'THREE.BufferGeometryUtils.toTrianglesDrawMode(): Geometry already defined as triangles.' );
		return geometry;

	}

	if ( drawMode === TriangleFanDrawMode || drawMode === TriangleStripDrawMode ) {

		let index = geometry.getIndex();

		// generate index if not present

		if ( index === null ) {

			const indices = [];

			const position = geometry.getAttribute( 'position' );

			if ( position !== undefined ) {

				for ( let i = 0; i < position.count; i ++ ) {

					indices.push( i );

				}

				geometry.setIndex( indices );
				index = geometry.getIndex();

			} else {

				console.error( 'THREE.BufferGeometryUtils.toTrianglesDrawMode(): Undefined position attribute. Processing not possible.' );
				return geometry;

			}

		}

		//

		const numberOfTriangles = index.count - 2;
		const newIndices = [];

		if ( drawMode === TriangleFanDrawMode ) {

			// gl.TRIANGLE_FAN

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

				newIndices.push( index.getX( 0 ) );
				newIndices.push( index.getX( i ) );
				newIndices.push( index.getX( i + 1 ) );

			}

		} else {

			// gl.TRIANGLE_STRIP

			for ( let i = 0; i < numberOfTriangles; i ++ ) {

				if ( i % 2 === 0 ) {

					newIndices.push( index.getX( i ) );
					newIndices.push( index.getX( i + 1 ) );
					newIndices.push( index.getX( i + 2 ) );

				} else {

					newIndices.push( index.getX( i + 2 ) );
					newIndices.push( index.getX( i + 1 ) );
					newIndices.push( index.getX( i ) );

				}

			}

		}

		if ( ( newIndices.length / 3 ) !== numberOfTriangles ) {

			console.error( 'THREE.BufferGeometryUtils.toTrianglesDrawMode(): Unable to generate correct amount of triangles.' );

		}

		// build final geometry

		const newGeometry = geometry.clone();
		newGeometry.setIndex( newIndices );
		newGeometry.clearGroups();

		return newGeometry;

	} else {

		console.error( 'THREE.BufferGeometryUtils.toTrianglesDrawMode(): Unknown draw mode:', drawMode );
		return geometry;

	}

}

/**
 * Calculates the morphed attributes of a morphed/skinned BufferGeometry.
 * Helpful for Raytracing or Decals.
 * @param {Mesh | Line | Points} object An instance of Mesh, Line or Points.
 * @return {Object} An Object with original position/normal attributes and morphed ones.
 */
function computeMorphedAttributes( object ) {

	const _vA = new Vector3();
	const _vB = new Vector3();
	const _vC = new Vector3();

	const _tempA = new Vector3();
	const _tempB = new Vector3();
	const _tempC = new Vector3();

	const _morphA = new Vector3();
	const _morphB = new Vector3();
	const _morphC = new Vector3();

	function _calculateMorphedAttributeData(
		object,
		attribute,
		morphAttribute,
		morphTargetsRelative,
		a,
		b,
		c,
		modifiedAttributeArray
	) {

		_vA.fromBufferAttribute( attribute, a );
		_vB.fromBufferAttribute( attribute, b );
		_vC.fromBufferAttribute( attribute, c );

		const morphInfluences = object.morphTargetInfluences;

		if ( morphAttribute && morphInfluences ) {

			_morphA.set( 0, 0, 0 );
			_morphB.set( 0, 0, 0 );
			_morphC.set( 0, 0, 0 );

			for ( let i = 0, il = morphAttribute.length; i < il; i ++ ) {

				const influence = morphInfluences[ i ];
				const morph = morphAttribute[ i ];

				if ( influence === 0 ) continue;

				_tempA.fromBufferAttribute( morph, a );
				_tempB.fromBufferAttribute( morph, b );
				_tempC.fromBufferAttribute( morph, c );

				if ( morphTargetsRelative ) {

					_morphA.addScaledVector( _tempA, influence );
					_morphB.addScaledVector( _tempB, influence );
					_morphC.addScaledVector( _tempC, influence );

				} else {

					_morphA.addScaledVector( _tempA.sub( _vA ), influence );
					_morphB.addScaledVector( _tempB.sub( _vB ), influence );
					_morphC.addScaledVector( _tempC.sub( _vC ), influence );

				}

			}

			_vA.add( _morphA );
			_vB.add( _morphB );
			_vC.add( _morphC );

		}

		if ( object.isSkinnedMesh ) {

			object.applyBoneTransform( a, _vA );
			object.applyBoneTransform( b, _vB );
			object.applyBoneTransform( c, _vC );

		}

		modifiedAttributeArray[ a * 3 + 0 ] = _vA.x;
		modifiedAttributeArray[ a * 3 + 1 ] = _vA.y;
		modifiedAttributeArray[ a * 3 + 2 ] = _vA.z;
		modifiedAttributeArray[ b * 3 + 0 ] = _vB.x;
		modifiedAttributeArray[ b * 3 + 1 ] = _vB.y;
		modifiedAttributeArray[ b * 3 + 2 ] = _vB.z;
		modifiedAttributeArray[ c * 3 + 0 ] = _vC.x;
		modifiedAttributeArray[ c * 3 + 1 ] = _vC.y;
		modifiedAttributeArray[ c * 3 + 2 ] = _vC.z;

	}

	const geometry = object.geometry;
	const material = object.material;

	let a, b, c;
	const index = geometry.index;
	const positionAttribute = geometry.attributes.position;
	const morphPosition = geometry.morphAttributes.position;
	const morphTargetsRelative = geometry.morphTargetsRelative;
	const normalAttribute = geometry.attributes.normal;
	const morphNormal = geometry.morphAttributes.position;

	const groups = geometry.groups;
	const drawRange = geometry.drawRange;
	let i, j, il, jl;
	let group;
	let start, end;

	const modifiedPosition = new Float32Array( positionAttribute.count * positionAttribute.itemSize );
	const modifiedNormal = new Float32Array( normalAttribute.count * normalAttribute.itemSize );

	if ( index !== null ) {

		// indexed buffer geometry

		if ( Array.isArray( material ) ) {

			for ( i = 0, il = groups.length; i < il; i ++ ) {

				group = groups[ i ];

				start = Math.max( group.start, drawRange.start );
				end = Math.min( ( group.start + group.count ), ( drawRange.start + drawRange.count ) );

				for ( j = start, jl = end; j < jl; j += 3 ) {

					a = index.getX( j );
					b = index.getX( j + 1 );
					c = index.getX( j + 2 );

					_calculateMorphedAttributeData(
						object,
						positionAttribute,
						morphPosition,
						morphTargetsRelative,
						a, b, c,
						modifiedPosition
					);

					_calculateMorphedAttributeData(
						object,
						normalAttribute,
						morphNormal,
						morphTargetsRelative,
						a, b, c,
						modifiedNormal
					);

				}

			}

		} else {

			start = Math.max( 0, drawRange.start );
			end = Math.min( index.count, ( drawRange.start + drawRange.count ) );

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

				a = index.getX( i );
				b = index.getX( i + 1 );
				c = index.getX( i + 2 );

				_calculateMorphedAttributeData(
					object,
					positionAttribute,
					morphPosition,
					morphTargetsRelative,
					a, b, c,
					modifiedPosition
				);

				_calculateMorphedAttributeData(
					object,
					normalAttribute,
					morphNormal,
					morphTargetsRelative,
					a, b, c,
					modifiedNormal
				);

			}

		}

	} else {

		// non-indexed buffer geometry

		if ( Array.isArray( material ) ) {

			for ( i = 0, il = groups.length; i < il; i ++ ) {

				group = groups[ i ];

				start = Math.max( group.start, drawRange.start );
				end = Math.min( ( group.start + group.count ), ( drawRange.start + drawRange.count ) );

				for ( j = start, jl = end; j < jl; j += 3 ) {

					a = j;
					b = j + 1;
					c = j + 2;

					_calculateMorphedAttributeData(
						object,
						positionAttribute,
						morphPosition,
						morphTargetsRelative,
						a, b, c,
						modifiedPosition
					);

					_calculateMorphedAttributeData(
						object,
						normalAttribute,
						morphNormal,
						morphTargetsRelative,
						a, b, c,
						modifiedNormal
					);

				}

			}

		} else {

			start = Math.max( 0, drawRange.start );
			end = Math.min( positionAttribute.count, ( drawRange.start + drawRange.count ) );

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

				a = i;
				b = i + 1;
				c = i + 2;

				_calculateMorphedAttributeData(
					object,
					positionAttribute,
					morphPosition,
					morphTargetsRelative,
					a, b, c,
					modifiedPosition
				);

				_calculateMorphedAttributeData(
					object,
					normalAttribute,
					morphNormal,
					morphTargetsRelative,
					a, b, c,
					modifiedNormal
				);

			}

		}

	}

	const morphedPositionAttribute = new Float32BufferAttribute( modifiedPosition, 3 );
	const morphedNormalAttribute = new Float32BufferAttribute( modifiedNormal, 3 );

	return {

		positionAttribute: positionAttribute,
		normalAttribute: normalAttribute,
		morphedPositionAttribute: morphedPositionAttribute,
		morphedNormalAttribute: morphedNormalAttribute

	};

}

function mergeGroups( geometry ) {

	if ( geometry.groups.length === 0 ) {

		console.warn( 'THREE.BufferGeometryUtils.mergeGroups(): No groups are defined. Nothing to merge.' );
		return geometry;

	}

	let groups = geometry.groups;

	// sort groups by material index

	groups = groups.sort( ( a, b ) => {

		if ( a.materialIndex !== b.materialIndex ) return a.materialIndex - b.materialIndex;

		return a.start - b.start;

	} );

	// create index for non-indexed geometries

	if ( geometry.getIndex() === null ) {

		const positionAttribute = geometry.getAttribute( 'position' );
		const indices = [];

		for ( let i = 0; i < positionAttribute.count; i += 3 ) {

			indices.push( i, i + 1, i + 2 );

		}

		geometry.setIndex( indices );

	}

	// sort index

	const index = geometry.getIndex();

	const newIndices = [];

	for ( let i = 0; i < groups.length; i ++ ) {

		const group = groups[ i ];

		const groupStart = group.start;
		const groupLength = groupStart + group.count;

		for ( let j = groupStart; j < groupLength; j ++ ) {

			newIndices.push( index.getX( j ) );

		}

	}

	geometry.dispose(); // Required to force buffer recreation
	geometry.setIndex( newIndices );

	// update groups indices

	let start = 0;

	for ( let i = 0; i < groups.length; i ++ ) {

		const group = groups[ i ];

		group.start = start;
		start += group.count;

	}

	// merge groups

	let currentGroup = groups[ 0 ];

	geometry.groups = [ currentGroup ];

	for ( let i = 1; i < groups.length; i ++ ) {

		const group = groups[ i ];

		if ( currentGroup.materialIndex === group.materialIndex ) {

			currentGroup.count += group.count;

		} else {

			currentGroup = group;
			geometry.groups.push( currentGroup );

		}

	}

	return geometry;

}


/**
 * Modifies the supplied geometry if it is non-indexed, otherwise creates a new,
 * non-indexed geometry. Returns the geometry with smooth normals everywhere except
 * faces that meet at an angle greater than the crease angle.
 *
 * @param {BufferGeometry} geometry
 * @param {number} [creaseAngle]
 * @return {BufferGeometry}
 */
function toCreasedNormals( geometry, creaseAngle = Math.PI / 3 /* 60 degrees */ ) {

	const creaseDot = Math.cos( creaseAngle );
	const hashMultiplier = ( 1 + 1e-10 ) * 1e2;

	// reusable vectors
	const verts = [ new Vector3(), new Vector3(), new Vector3() ];
	const tempVec1 = new Vector3();
	const tempVec2 = new Vector3();
	const tempNorm = new Vector3();
	const tempNorm2 = new Vector3();

	// hashes a vector
	function hashVertex( v ) {

		const x = ~ ~ ( v.x * hashMultiplier );
		const y = ~ ~ ( v.y * hashMultiplier );
		const z = ~ ~ ( v.z * hashMultiplier );
		return `${x},${y},${z}`;

	}

	// BufferGeometry.toNonIndexed() warns if the geometry is non-indexed
	// and returns the original geometry
	const resultGeometry = geometry.index ? geometry.toNonIndexed() : geometry;
	const posAttr = resultGeometry.attributes.position;
	const vertexMap = {};

	// find all the normals shared by commonly located vertices
	for ( let i = 0, l = posAttr.count / 3; i < l; i ++ ) {

		const i3 = 3 * i;
		const a = verts[ 0 ].fromBufferAttribute( posAttr, i3 + 0 );
		const b = verts[ 1 ].fromBufferAttribute( posAttr, i3 + 1 );
		const c = verts[ 2 ].fromBufferAttribute( posAttr, i3 + 2 );

		tempVec1.subVectors( c, b );
		tempVec2.subVectors( a, b );

		// add the normal to the map for all vertices
		const normal = new Vector3().crossVectors( tempVec1, tempVec2 ).normalize();
		for ( let n = 0; n < 3; n ++ ) {

			const vert = verts[ n ];
			const hash = hashVertex( vert );
			if ( ! ( hash in vertexMap ) ) {

				vertexMap[ hash ] = [];

			}

			vertexMap[ hash ].push( normal );

		}

	}

	// average normals from all vertices that share a common location if they are within the
	// provided crease threshold
	const normalArray = new Float32Array( posAttr.count * 3 );
	const normAttr = new BufferAttribute( normalArray, 3, false );
	for ( let i = 0, l = posAttr.count / 3; i < l; i ++ ) {

		// get the face normal for this vertex
		const i3 = 3 * i;
		const a = verts[ 0 ].fromBufferAttribute( posAttr, i3 + 0 );
		const b = verts[ 1 ].fromBufferAttribute( posAttr, i3 + 1 );
		const c = verts[ 2 ].fromBufferAttribute( posAttr, i3 + 2 );

		tempVec1.subVectors( c, b );
		tempVec2.subVectors( a, b );

		tempNorm.crossVectors( tempVec1, tempVec2 ).normalize();

		// average all normals that meet the threshold and set the normal value
		for ( let n = 0; n < 3; n ++ ) {

			const vert = verts[ n ];
			const hash = hashVertex( vert );
			const otherNormals = vertexMap[ hash ];
			tempNorm2.set( 0, 0, 0 );

			for ( let k = 0, lk = otherNormals.length; k < lk; k ++ ) {

				const otherNorm = otherNormals[ k ];
				if ( tempNorm.dot( otherNorm ) > creaseDot ) {

					tempNorm2.add( otherNorm );

				}

			}

			tempNorm2.normalize();
			normAttr.setXYZ( i3 + n, tempNorm2.x, tempNorm2.y, tempNorm2.z );

		}

	}

	resultGeometry.setAttribute( 'normal', normAttr );
	return resultGeometry;

}

export {
	computeMikkTSpaceTangents,
	mergeGeometries,
	mergeAttributes,
	interleaveAttributes,
	estimateBytesUsed,
	mergeVertices,
	toTrianglesDrawMode,
	computeMorphedAttributes,
	mergeGroups,
	toCreasedNormals
};