/*
    * Copyright (C) 2010, Google Inc.
    * and other copyright owners as documented in the project's IP log.
    *
    * This program and the accompanying materials are made available
    * under the terms of the Eclipse Distribution License v1.0 which
    * accompanies this distribution, is reproduced below, and is
    * available at http://www.eclipse.org/org/documents/edl-v10.php
    *
   * All rights reserved.
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   * without modification, are permitted provided that the following
   * conditions are met:
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   *
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   *   products derived from this software without specific prior
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  package org.eclipse.jgit.internal.storage.pack;
  
  import java.io.IOException;
  import java.io.OutputStream;
  
  import org.eclipse.jgit.lib.Constants;
  
  /**
   * Encodes an instruction stream for
   * {@link org.eclipse.jgit.internal.storage.pack.BinaryDelta}.
   */
  public class DeltaEncoder {
  	/**
  	 * Maximum number of bytes to be copied in pack v2 format.
  	 * <p>
  	 * Historical limitations have this at 64k, even though current delta
  	 * decoders recognize larger copy instructions.
  	 */
  	private static final int MAX_V2_COPY = 0x10000;
  
  	/*
  	 * Maximum number of bytes to be copied in pack v3 format.
  	 *
  	 * Current delta decoders can recognize a copy instruction with a count that
  	 * is this large, but the historical limitation of {@link MAX_V2_COPY} is
  	 * still used.
  	 */
  	// private static final int MAX_V3_COPY = (0xff << 16) | (0xff << 8) | 0xff;
  
  	/** Maximum number of bytes used by a copy instruction. */
  	private static final int MAX_COPY_CMD_SIZE = 8;
  
  	/** Maximum length that an an insert command can encode at once. */
  	private static final int MAX_INSERT_DATA_SIZE = 127;
  
  	private final OutputStream out;
  
  	private final byte[] buf = new byte[MAX_COPY_CMD_SIZE * 4];
  
  	private final int limit;
  
  	private int size;
  
  	/**
  	 * Create an encoder with no upper bound on the instruction stream size.
  	 *
  	 * @param out
  	 *            buffer to store the instructions written.
  	 * @param baseSize
  	 *            size of the base object, in bytes.
  	 * @param resultSize
  	 *            size of the resulting object, after applying this instruction
  	 *            stream to the base object, in bytes.
  	 * @throws java.io.IOException
  	 *             the output buffer cannot store the instruction stream's
  	 *             header with the size fields.
 	 */
 	public DeltaEncoder(OutputStream out, long baseSize, long resultSize)
 			throws IOException {
 		this(out, baseSize, resultSize, 0);
 	}
 
 	/**
 	 * Create an encoder with an upper limit on the instruction size.
 	 *
 	 * @param out
 	 *            buffer to store the instructions written.
 	 * @param baseSize
 	 *            size of the base object, in bytes.
 	 * @param resultSize
 	 *            size of the resulting object, after applying this instruction
 	 *            stream to the base object, in bytes.
 	 * @param limit
 	 *            maximum number of bytes to write to the out buffer declaring
 	 *            the stream is over limit and should be discarded. May be 0 to
 	 *            specify an infinite limit.
 	 * @throws java.io.IOException
 	 *             the output buffer cannot store the instruction stream's
 	 *             header with the size fields.
 	 */
 	public DeltaEncoder(OutputStream out, long baseSize, long resultSize,
 			int limit) throws IOException {
 		this.out = out;
 		this.limit = limit;
 		writeVarint(baseSize);
 		writeVarint(resultSize);
 	}
 
 	private void writeVarint(long sz) throws IOException {
 		int p = 0;
 		while (sz >= 0x80) {
 			buf[p++] = (byte) (0x80 | (((int) sz) & 0x7f));
 			sz >>>= 7;
 		}
 		buf[p++] = (byte) (((int) sz) & 0x7f);
 		size += p;
 		if (limit == 0 || size < limit)
 			out.write(buf, 0, p);
 	}
 
 	/**
 	 * Get current size of the delta stream, in bytes.
 	 *
 	 * @return current size of the delta stream, in bytes.
 	 */
 	public int getSize() {
 		return size;
 	}
 
 	/**
 	 * Insert a literal string of text, in UTF-8 encoding.
 	 *
 	 * @param text
 	 *            the string to insert.
 	 * @return true if the insert fits within the limit; false if the insert
 	 *         would cause the instruction stream to exceed the limit.
 	 * @throws java.io.IOException
 	 *             the instruction buffer can't store the instructions.
 	 */
 	public boolean insert(String text) throws IOException {
 		return insert(Constants.encode(text));
 	}
 
 	/**
 	 * Insert a literal binary sequence.
 	 *
 	 * @param text
 	 *            the binary to insert.
 	 * @return true if the insert fits within the limit; false if the insert
 	 *         would cause the instruction stream to exceed the limit.
 	 * @throws java.io.IOException
 	 *             the instruction buffer can't store the instructions.
 	 */
 	public boolean insert(byte[] text) throws IOException {
 		return insert(text, 0, text.length);
 	}
 
 	/**
 	 * Insert a literal binary sequence.
 	 *
 	 * @param text
 	 *            the binary to insert.
 	 * @param off
 	 *            offset within {@code text} to start copying from.
 	 * @param cnt
 	 *            number of bytes to insert.
 	 * @return true if the insert fits within the limit; false if the insert
 	 *         would cause the instruction stream to exceed the limit.
 	 * @throws java.io.IOException
 	 *             the instruction buffer can't store the instructions.
 	 */
 	public boolean insert(byte[] text, int off, int cnt)
 			throws IOException {
 		if (cnt <= 0)
 			return true;
 		if (limit != 0) {
 			int hdrs = cnt / MAX_INSERT_DATA_SIZE;
 			if (cnt % MAX_INSERT_DATA_SIZE != 0)
 				hdrs++;
 			if (limit < size + hdrs + cnt)
 				return false;
 		}
 		do {
 			int n = Math.min(MAX_INSERT_DATA_SIZE, cnt);
 			out.write((byte) n);
 			out.write(text, off, n);
 			off += n;
 			cnt -= n;
 			size += 1 + n;
 		} while (0 < cnt);
 		return true;
 	}
 
 	/**
 	 * Create a copy instruction to copy from the base object.
 	 *
 	 * @param offset
 	 *            position in the base object to copy from. This is absolute,
 	 *            from the beginning of the base.
 	 * @param cnt
 	 *            number of bytes to copy.
 	 * @return true if the copy fits within the limit; false if the copy
 	 *         would cause the instruction stream to exceed the limit.
 	 * @throws java.io.IOException
 	 *             the instruction buffer cannot store the instructions.
 	 */
 	public boolean copy(long offset, int cnt) throws IOException {
 		if (cnt == 0)
 			return true;
 
 		int p = 0;
 
 		// We cannot encode more than MAX_V2_COPY bytes in a single
 		// command, so encode that much and start a new command.
 		// This limit is imposed by the pack file format rules.
 		//
 		while (MAX_V2_COPY < cnt) {
 			p = encodeCopy(p, offset, MAX_V2_COPY);
 			offset += MAX_V2_COPY;
 			cnt -= MAX_V2_COPY;
 
 			if (buf.length < p + MAX_COPY_CMD_SIZE) {
 				if (limit != 0 && limit < size + p)
 					return false;
 				out.write(buf, 0, p);
 				size += p;
 				p = 0;
 			}
 		}
 
 		p = encodeCopy(p, offset, cnt);
 		if (limit != 0 && limit < size + p)
 			return false;
 		out.write(buf, 0, p);
 		size += p;
 		return true;
 	}
 
 	private int encodeCopy(int p, long offset, int cnt) {
 		int cmd = 0x80;
 		final int cmdPtr = p++; // save room for the command
 		byte b;
 
 		if ((b = (byte) (offset & 0xff)) != 0) {
 			cmd |= 0x01;
 			buf[p++] = b;
 		}
 		if ((b = (byte) ((offset >>> 8) & 0xff)) != 0) {
 			cmd |= 0x02;
 			buf[p++] = b;
 		}
 		if ((b = (byte) ((offset >>> 16) & 0xff)) != 0) {
 			cmd |= 0x04;
 			buf[p++] = b;
 		}
 		if ((b = (byte) ((offset >>> 24) & 0xff)) != 0) {
 			cmd |= 0x08;
 			buf[p++] = b;
 		}
 
 		if (cnt != MAX_V2_COPY) {
 			if ((b = (byte) (cnt & 0xff)) != 0) {
 				cmd |= 0x10;
 				buf[p++] = b;
 			}
 			if ((b = (byte) ((cnt >>> 8) & 0xff)) != 0) {
 				cmd |= 0x20;
 				buf[p++] = b;
 			}
 			if ((b = (byte) ((cnt >>> 16) & 0xff)) != 0) {
 				cmd |= 0x40;
 				buf[p++] = b;
 			}
 		}
 
 		buf[cmdPtr] = (byte) cmd;
 		return p;
 	}
 }