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Provides common utility functions

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package org.conqat.lib.commons.filesystem;

import java.io.IOException;
import java.nio.file.FileVisitResult;
import java.nio.file.SimpleFileVisitor;
import java.nio.file.attribute.BasicFileAttributes;

import org.checkerframework.checker.nullness.qual.NonNull;
import org.conqat.lib.commons.assertion.CCSMAssert;
import org.conqat.lib.commons.collections.PairList;
import org.conqat.lib.commons.function.ConsumerWithException;

/**
 * {@link java.nio.file.FileVisitor} that does not throw any {@link IOException}. Instead, any
 * {@link IOException} is collected together with the respective file and can be retrieved using
 * {@link #getFailedFiles()}.
 *
 * @param 
 *            file type
 */
public class NonThrowingFileVisitor extends SimpleFileVisitor {

	/**
	 * Contains all files for which the traversal failed, together with the respective
	 * {@link IOException}.
	 */
	private final PairList failedFiles = new PairList<>();

	/**
	 * Actual consumer of the file in {@link #visitFile(Object, BasicFileAttributes)}.
	 */
	private final ConsumerWithException fileConsumer;

	public NonThrowingFileVisitor(@NonNull ConsumerWithException fileConsumer) {
		CCSMAssert.isNotNull(fileConsumer, () -> String.format("Expected \"%s\" to be not null", "fileConsumer"));
		this.fileConsumer = fileConsumer;
	}

	@Override
	public FileVisitResult visitFile(T file, BasicFileAttributes attrs) {
		try {
			fileConsumer.accept(file);
		} catch (IOException e) {
			// Do not throw this Exception, as this will abort the complete walk
			// (and will not be handled by visitFileFailed)
			handleIoException(file, e);
		}
		return FileVisitResult.CONTINUE;
	}

	@Override
	public FileVisitResult visitFileFailed(T file, IOException exc) {
		handleIoException(file, exc);
		return FileVisitResult.CONTINUE;
	}

	private void handleIoException(T file, IOException exception) {
		// Do not throw this (or any other) Exception, as this will abort the complete
		// file walk, and may confuse users (TS-30138).
		// Instead, accept that this may lead to slightly incorrect results and let the
		// caller decide how to proceed
		failedFiles.add(file, exception);
	}

	/** @see #failedFiles */
	public PairList getFailedFiles() {
		return failedFiles;
	}
}