NodeIdMap.java
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	package org.softwareheritage.graph.maps;

	import it.unimi.dsi.fastutil.io.BinIO;
	import it.unimi.dsi.fastutil.longs.LongBigList;
	import it.unimi.dsi.fastutil.objects.Object2LongFunction;
	import it.unimi.dsi.util.ByteBufferLongBigList;
	import org.softwareheritage.graph.Graph;
	import org.softwareheritage.graph.SWHID;

	import java.io.FileInputStream;
	import java.io.IOException;
	import java.nio.charset.StandardCharsets;

	/**
	* Mapping between internal long node id and external SWHID.
	* <p>
	* The SWHID -> node mapping is obtained from hashing the SWHID with a MPH,
	* then permuting it using an mmap()-ed .order file containing the graph
	* permutation.
	*
	* The node -> SWHID reverse mapping is pre-computed and dumped on disk in the
	* {@link NodeMapBuilder} class, then it is loaded here using mmap().
	*
	* @author The Software Heritage developers
	* @see NodeMapBuilder
	*/

	public class NodeIdMap {
	/** Fixed length of binary SWHID buffer */
	public static final int SWHID_BIN_SIZE = 22;

	/** Graph path and basename */
	String graphPath;
	/** Number of ids to map */
	long nbIds;

	/** mmap()-ed NODE_TO_SWHID file */
	MapFile nodeToSwhMap;

	/** Minimal perfect hash (MPH) function SWHID -> initial order */
	Object2LongFunction<byte[]> mph;
	/** mmap()-ed long list with the permutation initial order -> graph order */
	LongBigList orderMap;
	/** FileInputStream containing the permutation */
	FileInputStream orderInputStream;

	/**
	* Constructor.
	*
	* @param graphPath full graph path
	* @param nbNodes number of nodes in the graph
	*/
	public NodeIdMap(String graphPath, long nbNodes) throws IOException {
	this.graphPath = graphPath;
	this.nbIds = nbNodes;

	// node -> SWHID
	this.nodeToSwhMap = new MapFile(graphPath + Graph.NODE_TO_SWHID, SWHID_BIN_SIZE);

	// SWHID -> node
	this.mph = loadMph(graphPath + ".mph");
	this.orderInputStream = new FileInputStream(graphPath + ".order");
	this.orderMap = ByteBufferLongBigList.map(orderInputStream.getChannel());
	}

	@SuppressWarnings("unchecked")
	private Object2LongFunction<byte[]> loadMph(String path) throws IOException {
	try {
	return (Object2LongFunction<byte[]>) BinIO.loadObject(path);
	} catch (ClassNotFoundException e) {
	throw new IOException(e.getMessage());
	}
	}

	/**
	* Converts SWHID to corresponding long node id.
	*
	* @param swhid node represented as a {@link SWHID}
	* @param checkExists if true, error if the SWHID is not present in the graph, if false the check
	* will be skipped and invalid data will be returned for non-existing SWHIDs.
	* @return corresponding node as a long id
	* @see SWHID
	*/
	public long getNodeId(SWHID swhid, boolean checkExists) {
	// 1. Hash the SWHID with the MPH to get its original ID
	long origNodeId = mph.getLong(swhid.toString().getBytes(StandardCharsets.US_ASCII));

	// 2. Use the order permutation to get the position in the permuted graph
	long nodeId = this.orderMap.getLong(origNodeId);

	// 3. Check that the position effectively corresponds to a real node using the reverse map.
	// This is necessary because the MPH makes no guarantees on whether the input SWHID is valid.
	if (!checkExists \|\| getSWHID(nodeId).equals(swhid)) {
	return nodeId;
	} else {
	throw new IllegalArgumentException("Unknown SWHID: " + swhid);
	}
	}

	public long getNodeId(SWHID swhid) {
	return getNodeId(swhid, true);
	}

	/**
	* Converts a node long id to corresponding SWHID.
	*
	* @param nodeId node as a long id
	* @return corresponding node as a {@link SWHID}
	* @see SWHID
	*/
	public SWHID getSWHID(long nodeId) {
	/*
	* Each line in NODE_TO_SWHID is formatted as: swhid The file is ordered by nodeId, meaning node0's
	* swhid is at line 0, hence we can read the nodeId-th line to get corresponding swhid
	*/
	if (nodeId < 0 \|\| nodeId >= nbIds) {
	throw new IllegalArgumentException("Node id " + nodeId + " should be between 0 and " + nbIds);
	}

	return SWHID.fromBytes(nodeToSwhMap.readAtLine(nodeId));
	}

	/**
	* Closes the mapping files.
	*/
	public void close() throws IOException {
	orderInputStream.close();
	nodeToSwhMap.close();
	}
	}

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