/*
 * Copyright (c) 2010-2013 TMate Software Ltd
 *  
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; version 2 of the License.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * For information on how to redistribute this software under
 * the terms of a license other than GNU General Public License
 * contact TMate Software at support@hg4j.com
 */
package org.tmatesoft.hg.internal;

import java.io.IOException;
import java.nio.ByteBuffer;

/**
 * relevant parts of DataInput, non-stream nature (seek operation), explicit check for end of data.
 * convenient skip (+/- bytes)
 * Primary goal - effective file read, so that clients don't need to care whether to call few 
 * distinct getInt() or readBytes(totalForFewInts) and parse themselves instead in an attempt to optimize.  
 * 
 * @author Artem Tikhomirov
 * @author TMate Software Ltd.
 */
public class DataAccess {
	private byte[] longBuffer;

	public boolean isEmpty() throws IOException {
		return true;
	}
	// IOException due to few subclasses that have non-trivial length() operation
	// long length and offset are needed only in RevlogStream, makes no sense elsewhere
	// because chunks Mercurial operates with fit into int (4 bytes actualLength field)
	// For those that may face large pieces of data (actual data streams) there are #longLength 
	// and #longSeek() to implement
	public int length() throws IOException {
		return 0;
	}
	
	public long longLength() throws IOException {
		return length();
	}
	
	public void longSeek(long offset) throws IOException {
		seek(Internals.ltoi(offset));
	}
	
	/**
	 * get this instance into initial state
	 * @throws IOException
	 * @return <code>this</code> for convenience
	 */
	public DataAccess reset() throws IOException {
		// nop, empty instance is always in the initial state
		return this;
	}
	// absolute positioning
	public void seek(int offset) throws IOException {
		if (offset == 0) {
			// perfectly OK for the "empty slice" instance
			return;
		}
		throw new IOException(String.format("No data, can't seek %d bytes", offset));
	}
	// relative positioning
	public void skip(int bytes) throws IOException {
		if (bytes == 0) {
			return;
		}
		throw new IOException(String.format("No data, can't skip %d bytes", bytes));
	}
	// shall be called once this object no longer needed
	public void done() {
		// no-op in this empty implementation
	}
	public int readInt() throws IOException {
		if (longBuffer == null) {
			longBuffer = new byte[8];
		}
		byte[] b = longBuffer;
		readBytes(b, 0, 4);
		return b[0] << 24 | (b[1] & 0xFF) << 16 | (b[2] & 0xFF) << 8 | (b[3] & 0xFF);
	}

	/**
	 * Read 8 bytes as long value, big-endian.
	 */
	public long readLong() throws IOException {
		if (longBuffer == null) {
			longBuffer = new byte[8];
		}
		byte[] b = longBuffer;
		readBytes(b, 0, 8);
		int i1 = b[0] << 24 | (b[1] & 0xFF) << 16 | (b[2] & 0xFF) << 8 | (b[3] & 0xFF);
		int i2 = b[4] << 24 | (b[5] & 0xFF) << 16 | (b[6] & 0xFF) << 8 | (b[7] & 0xFF);
		return ((long) i1) << 32 | ((long) i2 & 0xFFFFFFFFl);
	}
	public void readBytes(byte[] buf, int offset, int length) throws IOException {
		if (length == 0) {
			return;
		}
		throw new IOException(String.format("No data, can't read %d bytes", length));
	}
	/**
	 * reads bytes into ByteBuffer, up to its limit or total data length, whichever smaller.
	 * XXX perhaps, in DataAccess paradigm (when we read known number of bytes, we shall pass specific byte count to read)
	 */
	public void readBytes(ByteBuffer buf) throws IOException {
//		int toRead = Math.min(buf.remaining(), (int) length());
//		if (buf.hasArray()) {
//			readBytes(buf.array(), buf.arrayOffset(), toRead);
//		} else {
//			byte[] bb = new byte[toRead];
//			readBytes(bb, 0, bb.length);
//			buf.put(bb);
//		}
		// TODO [post-1.1] optimize to read as much as possible at once
		while (!isEmpty() && buf.hasRemaining()) {
			buf.put(readByte());
		}
	}
	public byte readByte() throws IOException {
		throw new UnsupportedOperationException();
	}

	/**
	 * Content of this DataAccess as byte array.
	 * Note, likely changes position in the DataAccess.
	 * Might provide direct access to underlying data structure in certain cases, do not alter.
	 * 
	 * @return byte array of {@link #length()} size, filled with data   
	 * @throws IOException
	 */
	public byte[] byteArray() throws IOException {
		reset();
		byte[] rv = new byte[length()];
		readBytes(rv, 0, rv.length);
		return rv;
	}
}