/*******************************************************************************
 * Copyright (c) 2000, 2003 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials 
 * are made available under the terms of the Common Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/cpl-v10.html
 * 
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *     MetaMatrix, Inc - repackaging and updates for use as a metadata store
 *******************************************************************************/
package org.teiid.designer.core.index;

import java.io.UTFDataFormatException;
import org.teiid.core.designer.util.StringUtilities;

/**
 * Uses prefix coding on words, and gamma coding of document numbers differences.
 *
 * @since 8.0
 */
public class GammaCompressedIndexBlock extends IndexBlock {
    CodeByteStream writeCodeStream = new CodeByteStream();
    CodeByteStream readCodeStream;
    char[] prevWord = null;

    /**
     * @param blockSize
     */
    public GammaCompressedIndexBlock(int blockSize) {
        super(blockSize);
        readCodeStream = new CodeByteStream(field.buffer());
    }

    /**
     * @see IndexBlock#addEntry
     */
    @Override
    public boolean addEntry(WordEntry entry) {
        writeCodeStream.reset();
        encodeEntry(entry, prevWord, writeCodeStream);

        if (getOffset() + writeCodeStream.byteLength() > this.blockSize - 2) {
            return false;
        }

        byte[] bytes = writeCodeStream.toByteArray();
        field.put(getOffset(), bytes);
        setOffset(getOffset() + bytes.length);
        prevWord = entry.getWord();
        return true;
    }

    protected void encodeEntry(WordEntry entry, char[] prevWord, CodeByteStream codeStream) {
        char[] word = entry.getWord();
        int prefixLen = prevWord == null ? 0 : Math.min(StringUtilities.prefixLength(prevWord, word), 255);
        codeStream.writeByte(prefixLen);
        codeStream.writeUTF(word, prefixLen, word.length);
        int n = entry.getNumRefs();
        codeStream.writeGamma(n);
        int prevRef = 0;
        for (int i = 0; i < n; ++i) {
            int ref = entry.getRef(i);
            if (ref <= prevRef)
                throw new IllegalArgumentException();
            codeStream.writeGamma(ref - prevRef);
            prevRef = ref;
        }
    }

    /**
     * @see IndexBlock#flush
     */
    @Override
    public void flush() {
        if (getOffset() > 0) {
            field.putInt2(getOffset(), 0);
            setOffset(0);
            prevWord = null;
        }
    }

    /**
     * @see IndexBlock#nextEntry
     */
    @Override
    public boolean nextEntry(WordEntry entry) {
        try {
            readCodeStream.reset(field.buffer(), getOffset());
            int prefixLength = readCodeStream.readByte();
            char[] word = readCodeStream.readUTF();
            if (prevWord != null && prefixLength > 0) {
                char[] temp = new char[prefixLength + word.length];
                System.arraycopy(prevWord, 0, temp, 0, Math.min(prefixLength, prevWord.length));
                System.arraycopy(word, 0, temp, Math.min(prefixLength, prevWord.length), word.length);
                word = temp;
            }
            if (word.length == 0) {
                return false;
            }
            entry.reset(word);
            int n = readCodeStream.readGamma();
            int prevRef = 0;
            for (int i = 0; i < n; ++i) {
                int ref = prevRef + readCodeStream.readGamma();
                if (ref < prevRef)
                    throw new InternalError();
                entry.addRef(ref);
                prevRef = ref;
            }
            setOffset(readCodeStream.byteLength());
            prevWord = word;
            return true;
        } catch (UTFDataFormatException e) {
            return false;
        }
    }

    /**
     * @see IndexBlock#reset
     */
    @Override
    public void reset() {
        super.reset();
        prevWord = null;
    }
}