/*
 * Copyright 2012-2017 the original author or authors.
 *
 * Licensed 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.springframework.boot.loader.data;

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.util.Queue;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.Semaphore;

/**
 * {@link RandomAccessData} implementation backed by a {@link RandomAccessFile}.
 *
 * @author Phillip Webb
 */
public class RandomAccessDataFile implements RandomAccessData {

	private static final int DEFAULT_CONCURRENT_READS = 4;

	private final File file;

	private final FilePool filePool;

	private final long offset;

	private final long length;

	/**
	 * Create a new {@link RandomAccessDataFile} backed by the specified file.
	 * @param file the underlying file
	 * @throws IllegalArgumentException if the file is null or does not exist
	 * @see #RandomAccessDataFile(File, int)
	 */
	public RandomAccessDataFile(File file) {
		this(file, DEFAULT_CONCURRENT_READS);
	}

	/**
	 * Create a new {@link RandomAccessDataFile} backed by the specified file.
	 * @param file the underlying file
	 * @param concurrentReads the maximum number of concurrent reads allowed on the
	 * underlying file before blocking
	 * @throws IllegalArgumentException if the file is null or does not exist
	 * @see #RandomAccessDataFile(File)
	 */
	public RandomAccessDataFile(File file, int concurrentReads) {
		if (file == null) {
			throw new IllegalArgumentException("File must not be null");
		}
		if (!file.exists()) {
			throw new IllegalArgumentException("File must exist");
		}
		this.file = file;
		this.filePool = new FilePool(concurrentReads);
		this.offset = 0L;
		this.length = file.length();
	}

	/**
	 * Private constructor used to create a {@link #getSubsection(long, long) subsection}.
	 * @param file the underlying file
	 * @param pool the underlying pool
	 * @param offset the offset of the section
	 * @param length the length of the section
	 */
	private RandomAccessDataFile(File file, FilePool pool, long offset, long length) {
		this.file = file;
		this.filePool = pool;
		this.offset = offset;
		this.length = length;
	}

	/**
	 * Returns the underlying File.
	 * @return the underlying file
	 */
	public File getFile() {
		return this.file;
	}

	@Override
	public InputStream getInputStream(ResourceAccess access) throws IOException {
		return new DataInputStream(access);
	}

	@Override
	public RandomAccessData getSubsection(long offset, long length) {
		if (offset < 0 || length < 0 || offset + length > this.length) {
			throw new IndexOutOfBoundsException();
		}
		return new RandomAccessDataFile(this.file, this.filePool, this.offset + offset,
				length);
	}

	@Override
	public long getSize() {
		return this.length;
	}

	public void close() throws IOException {
		this.filePool.close();
	}

	/**
	 * {@link RandomAccessDataInputStream} implementation for the
	 * {@link RandomAccessDataFile}.
	 */
	private class DataInputStream extends InputStream {

		private RandomAccessFile file;

		private int position;

		DataInputStream(ResourceAccess access) throws IOException {
			if (access == ResourceAccess.ONCE) {
				this.file = new RandomAccessFile(RandomAccessDataFile.this.file, "r");
				this.file.seek(RandomAccessDataFile.this.offset);
			}
		}

		@Override
		public int read() throws IOException {
			return doRead(null, 0, 1);
		}

		@Override
		public int read(byte[] b) throws IOException {
			return read(b, 0, b == null ? 0 : b.length);
		}

		@Override
		public int read(byte[] b, int off, int len) throws IOException {
			if (b == null) {
				throw new NullPointerException("Bytes must not be null");
			}
			return doRead(b, off, len);
		}

		/**
		 * Perform the actual read.
		 * @param b the bytes to read or {@code null} when reading a single byte
		 * @param off the offset of the byte array
		 * @param len the length of data to read
		 * @return the number of bytes read into {@code b} or the actual read byte if
		 * {@code b} is {@code null}. Returns -1 when the end of the stream is reached
		 * @throws IOException in case of I/O errors
		 */
		public int doRead(byte[] b, int off, int len) throws IOException {
			if (len == 0) {
				return 0;
			}
			int cappedLen = cap(len);
			if (cappedLen <= 0) {
				return -1;
			}
			RandomAccessFile file = this.file;
			if (file == null) {
				file = RandomAccessDataFile.this.filePool.acquire();
				file.seek(RandomAccessDataFile.this.offset + this.position);
			}
			try {
				if (b == null) {
					int rtn = file.read();
					moveOn(rtn == -1 ? 0 : 1);
					return rtn;
				}
				else {
					return (int) moveOn(file.read(b, off, cappedLen));
				}
			}
			finally {
				if (this.file == null) {
					RandomAccessDataFile.this.filePool.release(file);
				}
			}
		}

		@Override
		public long skip(long n) throws IOException {
			return (n <= 0 ? 0 : moveOn(cap(n)));
		}

		@Override
		public void close() throws IOException {
			if (this.file != null) {
				this.file.close();
			}
		}

		/**
		 * Cap the specified value such that it cannot exceed the number of bytes
		 * remaining.
		 * @param n the value to cap
		 * @return the capped value
		 */
		private int cap(long n) {
			return (int) Math.min(RandomAccessDataFile.this.length - this.position, n);
		}

		/**
		 * Move the stream position forwards the specified amount.
		 * @param amount the amount to move
		 * @return the amount moved
		 */
		private long moveOn(int amount) {
			this.position += amount;
			return amount;
		}

	}

	/**
	 * Manage a pool that can be used to perform concurrent reads on the underlying
	 * {@link RandomAccessFile}.
	 */
	private class FilePool {

		private final int size;

		private final Semaphore available;

		private final Queue<RandomAccessFile> files;

		FilePool(int size) {
			this.size = size;
			this.available = new Semaphore(size);
			this.files = new ConcurrentLinkedQueue<>();
		}

		public RandomAccessFile acquire() throws IOException {
			this.available.acquireUninterruptibly();
			RandomAccessFile file = this.files.poll();
			if (file != null) {
				return file;
			}
			return new RandomAccessFile(RandomAccessDataFile.this.file, "r");
		}

		public void release(RandomAccessFile file) {
			this.files.add(file);
			this.available.release();
		}

		public void close() throws IOException {
			this.available.acquireUninterruptibly(this.size);
			try {
				RandomAccessFile pooledFile = this.files.poll();
				while (pooledFile != null) {
					pooledFile.close();
					pooledFile = this.files.poll();
				}
			}
			finally {
				this.available.release(this.size);
			}
		}

	}

}
