package se.kth.isk.leffe.fileutil;

import java.io.IOException;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.nio.ByteBuffer;
import java.nio.MappedByteBuffer;
import java.nio.CharBuffer;
import java.nio.channels.FileChannel;
import java.nio.charset.Charset;
import java.nio.charset.CharsetDecoder;
import java.nio.charset.CharsetEncoder;


/**
 * This class handles replecement of one <code>String</code> with another in a file.
 */
public class Replace {
    private File file;
    private Charset charSet;

    /**
     * Constructs a <code>Replace</code> that handles replacements in the
     * specified file that is encoded with the specified charset.
     */
    public Replace(String fileName, String cs) {
	this.file = new File(fileName);
	charSet = Charset.forName(cs);
    }

    /**
     * Replaces all occurences of <code>from</code> with <code>to</code>. This simple version assumes that
     * <code>from</code> and <code>to</code> are of the same length.
     *
     * @param      from  The <code>String</code> that is replaced.
     * @param      to    The replacing <code>String</code>.
     * @return     The number of replacements that occured.
     * @exception  IOException if thrown by the underlying file io methods.
     * @exception  IllegalArgumentException if <code>from.length() != to.length</code>.
     */
    public int replace(String from, String to) throws IOException {
	if (from.length() != to.length()) {
	    throw new IllegalArgumentException();
	}
	CharBuffer content = openCharBuffer();
	int numOfHits = replace(content, from, to);
	saveCharBuffer(content);
	return numOfHits;
    }

    private CharBuffer openCharBuffer() throws IOException {
	FileChannel fc = new FileInputStream(file).getChannel();
	MappedByteBuffer mbb = fc.map(FileChannel.MapMode.READ_ONLY, 0, file.length());
	return charSet.decode(mbb);
    }

    private void saveCharBuffer(CharBuffer cb) throws IOException {
	ByteBuffer bb = charSet.encode(cb);
	FileChannel fc = new FileOutputStream(file).getChannel();
	fc.write(bb);
    }

    private int replace(CharBuffer content, String f, String to) {
	int numOfHits = 0;
	char[] from = f.toCharArray();

	for (int contPos=0; contPos<content.limit();) {
	    for (int fromPos=0; fromPos<from.length; fromPos++) {
		if (content.charAt(contPos++) != from[fromPos]) {
		    break;
		}
		if (fromPos == from.length - 1) {
		    numOfHits++;
		    for (int replacePos=contPos-from.length; replacePos<contPos; replacePos++) {
			content.put(replacePos, to.charAt(replacePos-contPos+from.length));
		    }
		}
	    }
	}
	return numOfHits;
    }

    public static void main(String[] args) {
	try {
	    new Replace(args[0], args[1]).replace(args[2], args[3]);
	} catch (IOException ioe) {
	    ioe.printStackTrace();
	}
    }
}