Java bencode - BDecoder.java

package bencode;

import java.io.InputStream;
import java.io.IOException;
import java.nio.charset.Charset;
import java.util.List;
import java.util.ArrayList;
import java.util.Map;
import java.util.HashMap;

public class BDecoder {

    private InputStream in;
    private Charset charset;

    public BDecoder(InputStream in) {
        this(in, Charset.forName("UTF-8"));
    }

    public BDecoder(InputStream in, Charset charset) {
        this.in = in;
        this.charset = charset;
    }

    public Object decode() throws IOException, BencodeFormatException {

        int indicator;
        int strLen;

        while ((indicator = in.read()) != -1) {
            if (indicator == 'i') {
                return readNumber();
            } else if (Character.isDigit(indicator)) {
                strLen = readStringLength((char) indicator);
                return readString(strLen);
            } else if (indicator == 'l') {
                return readList();
            } else if (indicator == 'd') {
                return readMap();
            } else if (indicator == 'e') {
                return null;
            } else {
                throw new BencodeFormatException(
                        "Incorrect indicator character: " + (char) indicator);
            }
        }

        return null;

    }

    public Map decodeMap() throws IOException, BencodeFormatException {

        int indicator = in.read();

        if (indicator == 'd') {
            return readMap();
        } else if (indicator == -1) {
            throw new BencodeFormatException("Unexpected end of stream.");
        } else {
            throw new BencodeFormatException("Incorrect indicator character: "
                    + (char) indicator);
        }

    }

    private String readString(int len) throws IOException {

        byte[] result = new byte[len];
        int read = 0;
        int num = 0;

        while (read < len) {
            num = in.read(result, read, len - read);
            read += num;
        }

        return new String(result, charset);

    }

    private int readStringLength(char startChar)
            throws IOException, BencodeFormatException {

        StringBuilder lenStr = new StringBuilder(64);
        int c;

        lenStr.append(startChar);

        while (true) {
            c = in.read();
            if (Character.isDigit(c)) {
                lenStr.append((char) c);
            } else if (c == ':') {
                break;
            } else if (c == -1) {
                throw new BencodeFormatException("Unexpected end of stream.");
            } else {
                throw new BencodeFormatException("Character ('" + (char) c
                        + "') not allowed in integer declaration.");
            }
        }

        return Integer.parseInt(lenStr.toString());

    }

    private Long readNumber() throws IOException, BencodeFormatException {

        StringBuilder numStr = new StringBuilder(64);
        int c = in.read();

        if (Character.isDigit(c) || c == '-') {
            numStr.append((char) c);
        } else {
            throw new BencodeFormatException(
                    "Integer must begin with digit or '-' character.");
        }

        while (true) {
            c = in.read();
            if (Character.isDigit(c)) {
                numStr.append((char) c);
            } else if (c == 'e') {
                break;
            } else if (c == -1) {
                throw new BencodeFormatException("Unexpected end of stream.");
            } else {
                throw new BencodeFormatException("Character ('" + (char) c
                        + "') not allowed in integer declaration.");
            }
        }

        return new Long(numStr.toString());

    }

    private List readList() throws IOException, BencodeFormatException {

        List list = new ArrayList();
        Object o;

        while ((o = decode()) != null) {
            list.add(o);
        }

        return list;

    }

    private Map readMap() throws IOException, BencodeFormatException {

        Map map = new HashMap<String, Object>();
        Object key;
        Object value;

        while ((key =  decode()) != null) {
            value = decode();
            if (value == null) {
                throw new BencodeFormatException(
                        "Bad dictionary format: value for key \"" + key
                        + "\" not found.");
            }
            map.put(key, value);
        }

        return map;

    }

}