rollup/src/utils/collapseSourcemaps.js

import { encode, decode } from 'sourcemap-codec';
import { dirname, relative, resolve } from './path.js';

class Source {
	constructor ( filename, content ) {
		this.isOriginal = true;
		this.filename = filename;
		this.content = content;
	}

	traceSegment ( line, column, name ) {
		return { line, column, name, source: this };
	}
}

class Link {
	constructor ( map, sources ) {
		if ( !map ) throw new Error( 'Cannot generate a sourcemap if non-sourcemap-generating transformers are used' );

		this.sources = sources;
		this.names = map.names;
		this.mappings = decode( map.mappings );
	}

	traceMappings () {
		let sources = [], sourcesContent = [], names = [];

		const mappings = this.mappings.map( line => {
			let tracedLine = [];

			line.forEach( segment => {
				const source = this.sources[ segment[1] ];
				const traced = source.traceSegment( segment[2], segment[3], this.names[ segment[4] ] );

				if ( traced ) {
					let sourceIndex = null, nameIndex = null;
					segment = [
						segment[0],
						null,
						traced.line,
						traced.column
					];

					// newer sources are more likely to be used, so search backwards.
					sourceIndex = sources.lastIndexOf( traced.source.filename );
					if ( sourceIndex === -1 ) {
						sourceIndex = sources.length;
						sources.push( traced.source.filename );
						sourcesContent[ sourceIndex ] = traced.source.content;
					} else if ( sourcesContent[ sourceIndex ] == null ) {
						sourcesContent[ sourceIndex ] = traced.source.content;
					} else if ( traced.source.content != null && sourcesContent[ sourceIndex ] !== traced.source.content ) {
						throw new Error( `Multiple conflicting contents for sourcemap source ${source.filename}` );
					}

					segment[1] = sourceIndex;

					if ( traced.name ) {
						nameIndex = names.indexOf( traced.name );
						if ( nameIndex === -1 ) {
							nameIndex = names.length;
							names.push( traced.name );
						}

						segment[4] = nameIndex;
					}

					tracedLine.push( segment );
				}
			});

			return tracedLine;
		});

		return { sources, sourcesContent, names, mappings };
	}

	traceSegment ( line, column, name ) {
		const segments = this.mappings[ line ];

		if ( !segments ) return null;

		for ( let i = 0; i < segments.length; i += 1 ) {
			const segment = segments[i];

			if ( segment[0] > column ) return null;

			if ( segment[0] === column ) {
				const source = this.sources[ segment[1] ];
				if ( !source ) return null;

				return source.traceSegment( segment[2], segment[3], this.names[ segment[4] ] || name );
			}
		}

		return null;
	}
}

export default function collapseSourcemaps ( file, map, modules, bundleSourcemapChain ) {
	const moduleSources = modules.map( module => {
		let sourceMapChain = module.sourceMapChain;
		
		let source;
		if ( module.originalSourceMap == null ) {
			source = new Source( module.id, module.originalCode );
		} else {
			const sources = module.originalSourceMap.sources;

			if ( sources == null || ( sources.length <= 1 && sources[0] == null ) ) {
				source = new Source( module.id, module.originalCode );
				sourceMapChain = [ module.originalSourceMap ].concat( sourceMapChain );
			} else {
				const sourcesContent = module.originalSourceMap.sourcesContent || [];

				// TODO indiscriminately treating IDs and sources as normal paths is probably bad.
				const directory = dirname( module.id ) || '.';
				const sourceRoot = module.originalSourceMap.sourceRoot || '.';

				const baseSources = sources.map( (source, i) => {
					return new Source( resolve( directory, sourceRoot, source ), sourcesContent[i] );
				});

				source = new Link( module.originalSourceMap, baseSources );
			}
		}

		sourceMapChain.forEach( map => {
			source = new Link( map, [ source ]);
		});

		return source;
	});

	let source = new Link( map, moduleSources );

	bundleSourcemapChain.forEach( map => {
		source = new Link( map, [ source ] );
	});

	let { sources, sourcesContent, names, mappings } = source.traceMappings();

	if ( file ) {
		const directory = dirname( file );
		sources = sources.map( source => relative( directory, source ) );
	}

	// we re-use the `map` object because it has convenient toString/toURL methods
	map.sources = sources;
	map.sourcesContent = sourcesContent;
	map.names = names;
	map.mappings = encode( mappings );

	return map;
}