/*
 * Copyright (c) 2011 Artem Tikhomirov 
 */
package com.tmate.hgkit.fs;

import java.io.IOException;

/**
 * XXX Perhaps, DataAccessSlice? Unlike FilterInputStream, we limit amount of data read from DataAccess being filtered.
 *   
 * @author artem
 */
public class FilterDataAccess extends DataAccess {
	private final DataAccess dataAccess;
	private final long offset;
	private final int length;
	private int count;

	public FilterDataAccess(DataAccess dataAccess, long offset, int length) {
		this.dataAccess = dataAccess;
		this.offset = offset;
		this.length = length;
		count = length;
	}

	protected int available() {
		return count;
	}

	@Override
	public void reset() throws IOException {
		count = length;
	}
	
	@Override
	public boolean isEmpty() {
		return count <= 0;
	}
	
	@Override
	public long length() {
		return length;
	}

	@Override
	public void seek(long localOffset) throws IOException {
		if (localOffset < 0 || localOffset > length) {
			throw new IllegalArgumentException();
		}
		dataAccess.seek(offset + localOffset);
		count = (int) (length - localOffset);
	}

	@Override
	public void skip(int bytes) throws IOException {
		int newCount = count - bytes;
		if (newCount < 0 || newCount > length) {
			throw new IllegalArgumentException();
		}
		seek(length - newCount);
		/*
		 can't use next code because don't want to rewind backing DataAccess on reset()
		 i.e. this.reset() modifies state of this instance only, while filtered DA may go further.
		 Only actual this.skip/seek/read would rewind it to desired position 
	  		dataAccess.skip(bytes);
			count = newCount;
		 */

	}

	@Override
	public byte readByte() throws IOException {
		if (count <= 0) {
			throw new IllegalArgumentException("Underflow"); // XXX be descriptive
		}
		if (count == length) {
			dataAccess.seek(offset);
		}
		count--;
		return dataAccess.readByte();
	}

	@Override
	public void readBytes(byte[] b, int off, int len) throws IOException {
		if (count <= 0 || len > count) {
			throw new IllegalArgumentException("Underflow"); // XXX be descriptive
		}
		if (count == length) {
			dataAccess.seek(offset);
		}
		dataAccess.readBytes(b, off, len);
		count -= len;
	}

	// done shall be no-op, as we have no idea what's going on with DataAccess we filter
}