package org.apache.lucene.store;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information 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.
 */

import java.io.IOException;

/** Abstract base class for input from a file in a {@link Directory}.  A
 * random-access input stream.  Used for all Lucene index input operations.
 * @see Directory
 */
public abstract class IndexInput implements Cloneable {
  private char[] chars;                           // used by readString()

  /** Reads and returns a single byte.
   * @see IndexOutput#writeByte(byte)
   */
  public abstract byte readByte() throws IOException;

  /** Reads a specified number of bytes into an array at the specified offset.
   * @param b the array to read bytes into
   * @param offset the offset in the array to start storing bytes
   * @param len the number of bytes to read
   * @see IndexOutput#writeBytes(byte[],int)
   */
  public abstract void readBytes(byte[] b, int offset, int len)
    throws IOException;

  /** Reads four bytes and returns an int.
   * @see IndexOutput#writeInt(int)
   */
  public int readInt() throws IOException {
    return ((readByte() & 0xFF) << 24) | ((readByte() & 0xFF) << 16)
         | ((readByte() & 0xFF) <<  8) |  (readByte() & 0xFF);
  }

  /** Reads an int stored in variable-length format.  Reads between one and
   * five bytes.  Smaller values take fewer bytes.  Negative numbers are not
   * supported.
   * @see IndexOutput#writeVInt(int)
   */
  public int readVInt() throws IOException {
    byte b = readByte();
    int i = b & 0x7F;
    for (int shift = 7; (b & 0x80) != 0; shift += 7) {
      b = readByte();
      i |= (b & 0x7F) << shift;
    }
    return i;
  }

  /** Reads eight bytes and returns a long.
   * @see IndexOutput#writeLong(long)
   */
  public long readLong() throws IOException {
    return (((long)readInt()) << 32) | (readInt() & 0xFFFFFFFFL);
  }

  /** Reads a long stored in variable-length format.  Reads between one and
   * nine bytes.  Smaller values take fewer bytes.  Negative numbers are not
   * supported. */
  public long readVLong() throws IOException {
    byte b = readByte();
    long i = b & 0x7F;
    for (int shift = 7; (b & 0x80) != 0; shift += 7) {
      b = readByte();
      i |= (b & 0x7FL) << shift;
    }
    return i;
  }

  /** Reads a string.
   * @see IndexOutput#writeString(String)
   */
  public String readString() throws IOException {
    int length = readVInt();
    if (chars == null || length > chars.length)
      chars = new char[length];
    readChars(chars, 0, length);
    return new String(chars, 0, length);
  }

  /** Reads UTF-8 encoded characters into an array.
   * @param buffer the array to read characters into
   * @param start the offset in the array to start storing characters
   * @param length the number of characters to read
   * @see IndexOutput#writeChars(String,int,int)
   */
  public void readChars(char[] buffer, int start, int length)
       throws IOException {
    final int end = start + length;
    for (int i = start; i < end; i++) {
      byte b = readByte();
      if ((b & 0x80) == 0)
	buffer[i] = (char)(b & 0x7F);
      else if ((b & 0xE0) != 0xE0) {
	buffer[i] = (char)(((b & 0x1F) << 6)
		 | (readByte() & 0x3F));
      } else
	buffer[i] = (char)(((b & 0x0F) << 12)
		| ((readByte() & 0x3F) << 6)
	        |  (readByte() & 0x3F));
    }
  }

  /**
   * Expert
   * 
   * Similar to {@link #readChars(char[], int, int)} but does not do any conversion operations on the bytes it is reading in.  It still
   * has to invoke {@link #readByte()} just as {@link #readChars(char[], int, int)} does, but it does not need a buffer to store anything
   * and it does not have to do any of the bitwise operations, since we don't actually care what is in the byte except to determine
   * how many more bytes to read
   * @param length The number of chars to read
   */
  public void skipChars(int length) throws IOException{
    for (int i = 0; i < length; i++) {
      byte b = readByte();
      if ((b & 0x80) == 0){
        //do nothing, we only need one byte
      }
      else if ((b & 0xE0) != 0xE0) {
        readByte();//read an additional byte
      } else{      
        //read two additional bytes.
        readByte();
        readByte();
      }
    }
  }
  

  /** Closes the stream to futher operations. */
  public abstract void close() throws IOException;

  /** Returns the current position in this file, where the next read will
   * occur.
   * @see #seek(long)
   */
  public abstract long getFilePointer();

  /** Sets current position in this file, where the next read will occur.
   * @see #getFilePointer()
   */
  public abstract void seek(long pos) throws IOException;

  /** The number of bytes in the file. */
  public abstract long length();

  /** Returns a clone of this stream.
   *
   * <p>Clones of a stream access the same data, and are positioned at the same
   * point as the stream they were cloned from.
   *
   * <p>Expert: Subclasses must ensure that clones may be positioned at
   * different points in the input from each other and from the stream they
   * were cloned from.
   */
  public Object clone() {
    IndexInput clone = null;
    try {
      clone = (IndexInput)super.clone();
    } catch (CloneNotSupportedException e) {}

    clone.chars = null;

    return clone;
  }

}