/*
    * Copyright (C) 2017, Google Inc. and others
    *
    * This program and the accompanying materials are made available under the
    * terms of the Eclipse Distribution License v. 1.0 which is available at
    * https://www.eclipse.org/org/documents/edl-v10.php.
    *
    * SPDX-License-Identifier: BSD-3-Clause
    */
  
  package org.eclipse.jgit.internal.storage.dfs;
  
  import static org.eclipse.jgit.internal.storage.pack.PackExt.REFTABLE;
  
  import java.io.IOException;
  import java.nio.ByteBuffer;
  
  import org.eclipse.jgit.internal.storage.io.BlockSource;
  import org.eclipse.jgit.internal.storage.reftable.ReftableReader;
  
  /**
   * A reftable stored in {@link org.eclipse.jgit.internal.storage.dfs.DfsBlockCache}.
   */
  public class DfsReftable extends BlockBasedFile {
  	/**
  	 * Construct a reader for an existing reftable.
  	 *
  	 * @param desc
  	 *            description of the reftable within the DFS.
  	 */
  	public DfsReftable(DfsPackDescription desc) {
  		this(DfsBlockCache.getInstance(), desc);
  	}
  
  	/**
  	 * Construct a reader for an existing reftable.
  	 *
  	 * @param cache
  	 *            cache that will store the reftable data.
  	 * @param desc
  	 *            description of the reftable within the DFS.
  	 */
  	public DfsReftable(DfsBlockCache cache, DfsPackDescription desc) {
  		super(cache, desc, REFTABLE);
  
  		int bs = desc.getBlockSize(REFTABLE);
  		if (bs > 0) {
  			setBlockSize(bs);
  		}
  
  		long sz = desc.getFileSize(REFTABLE);
  		length = sz > 0 ? sz : -1;
  	}
  
  	/**
  	 * Get description that was originally used to configure this file.
  	 *
  	 * @return description that was originally used to configure this file.
  	 */
  	public DfsPackDescription getPackDescription() {
  		return desc;
  	}
  
  	/**
  	 * Open reader on the reftable.
  	 * <p>
  	 * The returned reader is not thread safe.
  	 *
  	 * @param ctx
  	 *            reader to access the DFS storage.
  	 * @return cursor to read the table; caller must close.
  	 * @throws java.io.IOException
  	 *             table cannot be opened.
  	 */
  	public ReftableReader open(DfsReader ctx) throws IOException {
  		return new ReftableReader(new CacheSource(this, cache, ctx));
  	}
  
  	private static final class CacheSource extends BlockSource {
  		private final DfsReftable file;
  		private final DfsBlockCache cache;
  		private final DfsReader ctx;
  		private ReadableChannel ch;
  		private int readAhead;
  
  		CacheSource(DfsReftable file, DfsBlockCache cache, DfsReader ctx) {
  			this.file = file;
  			this.cache = cache;
  			this.ctx = ctx;
  		}
  
  		@Override
  		public ByteBuffer read(long pos, int cnt) throws IOException {
  			if (ch == null && readAhead > 0 && notInCache(pos)) {
  				open().setReadAheadBytes(readAhead);
  			}
  
  			DfsBlock block = cache.getOrLoad(file, pos, ctx, () -> open());
  			if (block.start == pos && block.size() >= cnt) {
 				return block.zeroCopyByteBuffer(cnt);
 			}
 
 			byte[] dst = new byte[cnt];
 			ByteBuffer buf = ByteBuffer.wrap(dst);
 			buf.position(ctx.copy(file, pos, dst, 0, cnt));
 			return buf;
 		}
 
 		private boolean notInCache(long pos) {
 			return cache.get(file.key, file.alignToBlock(pos)) == null;
 		}
 
 		@Override
 		public long size() throws IOException {
 			long n = file.length;
 			if (n < 0) {
 				n = open().size();
 				file.length = n;
 			}
 			return n;
 		}
 
 		@Override
 		public void adviseSequentialRead(long start, long end) {
 			int sz = ctx.getOptions().getStreamPackBufferSize();
 			if (sz > 0) {
 				readAhead = (int) Math.min(sz, end - start);
 			}
 		}
 
 		private ReadableChannel open() throws IOException {
 			if (ch == null) {
 				ch = ctx.db.openFile(file.desc, file.ext);
 			}
 			return ch;
 		}
 
 		@Override
 		public void close() {
 			if (ch != null) {
 				try {
 					ch.close();
 				} catch (IOException e) {
 					// Ignore read close failures.
 				} finally {
 					ch = null;
 				}
 			}
 		}
 	}
 }