/*
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regarding copyright ownership.  The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License.  You may obtain a copy of the License at

  http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied.  See the License for the
specific language governing permissions and limitations
under the License.
*/
package org.apache.edgent.connectors.file;

import java.io.File;
import java.nio.file.WatchService;
import java.util.Comparator;

import org.apache.edgent.connectors.file.runtime.DirectoryWatcher;
import org.apache.edgent.connectors.file.runtime.IFileWriterPolicy;
import org.apache.edgent.connectors.file.runtime.TextFileReader;
import org.apache.edgent.connectors.file.runtime.TextFileWriter;
import org.apache.edgent.function.BiFunction;
import org.apache.edgent.function.Function;
import org.apache.edgent.function.Supplier;
import org.apache.edgent.topology.TSink;
import org.apache.edgent.topology.TStream;
import org.apache.edgent.topology.TopologyElement;

/**
 * {@code FileStreams} is a connector for integrating with file system objects.
 * <p>
 * File stream operations include:
 * <ul>
 * <li>Write tuples to text files - {@link #textFileWriter(TStream, Supplier, Supplier) textFileWriter}</li>
 * <li>Watch a directory for new files - {@link #directoryWatcher(TopologyElement, Supplier) directoryWatcher}</li>
 * <li>Create tuples from text files - {@link #textFileReader(TStream, Function, BiFunction) textFileReader}</li>
 * </ul>
 */
public class FileStreams {
    @SuppressWarnings("unused")
    private static final FileStreams forCodeCoverage = new FileStreams();
    private FileStreams() {};
    
    /**
     * Declare a stream containing the absolute pathname of 
     * newly created file names from watching {@code directory}.
     * <p>
     * This is the same as {@code directoryWatcher(t, () -> dir, null)}.
     * 
     * @param te org.apache.edgent.org.apache.edgent.topology element whose org.apache.edgent.org.apache.edgent.topology the watcher will be added to
     * @param directory
     *            Name of the directory to watch.
     * @return Stream containing absolute pathnames of newly created files in
     *            {@code directory}.
     */
    public static TStream<String> directoryWatcher(TopologyElement te,
            Supplier<String> directory) {
        return directoryWatcher(te, directory, null);
    }
    
    /**
     * Declare a stream containing the absolute pathname of 
     * newly created file names from watching {@code directory}.
     * <p>
     * Hidden files (java.io.File.isHidden()==true) are ignored.
     * This is compatible with {@code textFileWriter}.
     * <p>
     * Sample use:
     * <pre>{@code
     * String dir = "/some/directory/path";
     * Topology t = ...
     * TStream<String> pathnames = FileStreams.directoryWatcher(t, () -> dir, null);
     * }</pre>
     * <p>
     * The order of the files in the stream is dictated by a {@link Comparator}.
     * The default comparator orders files by {@link File#lastModified()} values.
     * There are no guarantees on the processing order of files that
     * have the same lastModified value.
     * Note, lastModified values are subject to filesystem timestamp
     * quantization - e.g., 1second.
     * <p>
     * Note: due to the asynchronous nature of things, if files in the
     * directory may be removed, the receiver of a tuple with a "new" file
     * pathname may need to be prepared for the pathname to no longer be
     * valid when it receives the tuple or during its processing of the tuple.
     * <p>
     * The behavior on MacOS may be unsavory, even as recent as Java8, as
     * MacOs Java lacks a native implementation of {@link WatchService}.
     * The result can be a delay in detecting newly created files (e.g., 10sec)
     * as well not detecting rapid deletion and recreation of a file.
     *
     * @param te org.apache.edgent.org.apache.edgent.topology element whose org.apache.edgent.org.apache.edgent.topology the watcher will be added to
     * @param directory
     *            Name of the directory to watch.
     * @param comparator
     *            Comparator to use to order newly seen file pathnames.
     *            May be null.
     * @return Stream containing absolute pathnames of newly created files in
     *            {@code directory}.
     */
    public static TStream<String> directoryWatcher(TopologyElement te,
            Supplier<String> directory, Comparator<File> comparator) {
        return te.topology().source(() -> new DirectoryWatcher(directory, comparator));
    }
    
    /**
     * Declare a stream containing the lines read from the files
     * whose pathnames correspond to each tuple on the {@code pathnames}
     * stream.
     * <p>
     * This is the same as {@code textFileReader(pathnames, null, null)}
     * <p>
     * Sample uses:
     * 
     * <pre>{@code
     * // continuously watch a directory for new files and process each one once
     * String dir = "/some/directory/path";
     * Topology t = ...
     * TStream<String> pathnames = FileStreams.directoryWatcher(t, () -> dir);
     * TStream<String> contents = FileStreams.textFileReader(pathnames);
     * contents.print();
     * }</pre>
     * 
     * <pre>{@code
     * // periodically process one or more files
     * Supplier<List<String>> myPathnamesSupplier = () -> } {
     * {@code
     *    // implementation of List<String> get() - return a list of pathnames
     *      //  return Arrays.asList("/some/pathname"); // a fixed list
     *      //  return Arrays.asList(new File("/some/dir").list()); // query a directory
     *      //  or query some other object(s) for a list ...
     * }
     *    };
     * {@code
     * Topology t = ...
     * TStream<String> pathnames = t.poll(myPathnamesSupplier, 30, TimeUnit.SECONDS)
     *                              .flatMap(tuple -> tuple);
     * TStream<String> contents = FileStreams.textFileReader(pathnames);
     * contents.print();
     * }</pre>
     * 
     * @param pathnames
     *            Stream containing pathnames of files to read.
     * @return Stream containing lines from the files.
     */
    public static TStream<String> textFileReader(TStream<String> pathnames) {
        return textFileReader(pathnames, null, null);
    }

    /**
     * Declare a stream containing the lines read from the files
     * whose pathnames correspond to each tuple on the {@code pathnames}
     * stream.
     * <p>
     * All files are assumed to be encoded in UTF-8.  The lines are
     * output in the order they appear in each file, with the first line of
     * a file appearing first.  A file is not subsequently monitored for
     * additional lines.
     * <p>
     * If a file can not be read, e.g., a file doesn't exist at that pathname
     * or the pathname is for a directory,
     * an error will be logged.
     * <p>
     * Optional {@code preFn} and {@code postFn} functions may be supplied.
     * These are called prior to processing a tuple (pathname) and after
     * respectively.  They provide a way to encode markers in the generated
     * stream.
     * <p>
     * Sample use:
     * <pre>{@code
     * // watch a directory for files, creating a stream with the contents of
     * // each file.  Use a preFn to include a file separator marker in the
     * // stream. Use a postFn to delete a file once it's been processed.
     * String dir = "/some/directory/path";
     * Topology t = ...
     * TStream<String> pathnames = FileStreams.directoryWatcher(t, () -> dir);
     * TStream<String> contents = FileStreams.textFileReader(
     *              pathnames,
     *              path -> { return "###<PATH-MARKER>### " + path },
     *              (path,exception) -> { new File(path).delete(), return null; }
     *              );
     * contents.print();
     * }</pre>
     * 
     * @param pathnames
     *            Stream containing pathnames of files to read.
     * @param preFn
     *            Pre-visit {@code Function<String,String>}.
     *            The input is the pathname.  
     *            The result, when non-null, is added to the output stream.
     *            The function may be null.
     * @param postFn
     *            Post-visit {@code BiFunction<String,Exception,String>}.
     *            The input is the pathname and an exception.  The exception
     *            is null if there were no errors.
     *            The result, when non-null, is added to the output stream.
     *            The function may be null.
     * @return Stream containing lines from the files.
     */
    public static TStream<String> textFileReader(TStream<String> pathnames,
        Function<String,String> preFn, BiFunction<String,Exception,String> postFn) {
        
        TextFileReader reader = new TextFileReader();
        reader.setPre(preFn);
        reader.setPost(postFn);
        return pathnames.pipe(reader);
    }
    
    /**
     * Write the contents of a stream to files.
     * <p>
     * The default {@link FileWriterPolicy} is used.
     * <p>
     * This is the same as {@code textFileWriter(contents, basePathname, null)}.
     * <p>
     * Sample use:
     * <pre>{@code
     * // write a stream of LogEvent to files, using the default
     * // file writer policy
     * String basePathname = "/myLogDir/LOG"; // yield LOG_YYYYMMDD_HHMMSS
     * TStream<MyLogEvent> events = ...
     * TStream<String> stringEvents = events.map(event -> event.toString()); 
     * FileStreams.textFileWriter(stringEvents, () -> basePathname);
     * }</pre>
     * @param contents the lines to write
     * @param basePathname the base pathname of the created files
     * @return a TSink
     */
    public static TSink<String> textFileWriter(TStream<String> contents,
            Supplier<String> basePathname) {
        return textFileWriter(contents, basePathname, null);
    }
    
    /**
     * Write the contents of a stream to files subject to the control
     * of a file writer policy.
     * <p>
     * A separate policy instance must be used for invocation.
     * A default {@link FileWriterPolicy} is used if a policy is not specified.
     * <p>
     * Sample use:
     * <pre>{@code
     * // write a stream of LogEvent to files using a policy of:
     * // no additional flush, 100 events per file, retain 5 files
     * IFileWriterPolicy<String> policy = new FileWriterPolicy<String>(
     *           FileWriterFlushConfig.newImplicitConfig(),
     *           FileWriterCycleConfig.newCountBasedConfig(100),
     *           FileWriterRetentionConfig.newFileCountBasedConfig(5)
     *           );
     * String basePathname = "/myLogDir/LOG"; // yield LOG_YYYYMMDD_HHMMSS
     * TStream<MyLogEvent> events = ...
     * TStream<String> stringEvents = events.map(event -> event.toString()); 
     * FileStreams.textFileWriter(stringEvents, () -> basePathname, () -> policy);
     * }</pre>
     * @param contents the lines to write
     * @param basePathname the base pathname of the created files
     * @param policy the policy to use.  may be null.
     * @return a TSink
     * @see FileWriterPolicy
     */
    public static TSink<String> textFileWriter(TStream<String> contents,
            Supplier<String> basePathname, Supplier<IFileWriterPolicy<String>> policy) {
        if (policy == null) {
            IFileWriterPolicy<String> defaultPolicy = new FileWriterPolicy<String>(){};
            policy = () -> defaultPolicy;
        }
        return contents.sink(new TextFileWriter(basePathname, policy));
    }
}