import java.io.BufferedReader;
import java.io.IOException;
import java.io.Writer;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Iterator;
import java.util.stream.Stream;

import static java.nio.charset.StandardCharsets.ISO_8859_1;
import static java.nio.file.Files.createDirectories;
import static java.nio.file.Files.exists;
import static java.nio.file.Files.newBufferedReader;
import static java.nio.file.Files.newBufferedWriter;

public class Main {

    private static final String DELIMETER_TO_START_FROM = "R41-5000";
    private static final int COUNT_LINES_TO_WRITE_AFTER_R41 = 32;

    public static void main(String[] args) throws IOException {
        Path
                dirCache = Paths.get("cache"),
                fileSrc  = Paths.get("MSG.DAT");

        if (!exists(dirCache)) {
            createDirectories(dirCache);
        }

        try (BufferedReader bufferedReader = newBufferedReader(fileSrc, ISO_8859_1)) {
            Iterable<String> lines = asIterable(bufferedReader.lines());

            //<editor-fold defaultstate="collapsed" desc="search for firstLine">
            String firstLine = null;
            for (String line : lines) {
                int indexOfR41 = line.indexOf(DELIMETER_TO_START_FROM);
                if (indexOfR41 != -1) {
                    firstLine = line.substring(indexOfR41);
                    break;
                }
            }
            //</editor-fold>

            if (firstLine != null) {
                Path fileOut = dirCache.resolve(
                        firstLine.substring(0, firstLine.indexOf('.') + 4)
                );

                try (Writer writer = newBufferedWriter(fileOut, ISO_8859_1)) {
                    writer.write(firstLine);
                    //<editor-fold defaultstate="collapsed" desc="оставшиеся 32 строки">
                    int cntLinesWritedAfterR41 = 0;
                    for (String line : lines) {
                        writer.write("\n"); //в исходном файле могут быть несколько разделителей или только \r, -> возможно искажение
                        writer.write(line);
                        if (++cntLinesWritedAfterR41 >= COUNT_LINES_TO_WRITE_AFTER_R41) {
                            break;
                        }
                    }
                    //</editor-fold>
                }
            }
        }
    }

    /** !Инстанс Iterable при каждом вызове метода iterator() будет возвращать один и тотже итератор. */
    public static <T> Iterable<T> asIterable(Stream<T> stream) {
        Iterator<T> iterator = stream.iterator();
        return () -> iterator;
    }
}