/*
 * Copyright 2012 The Netty Project
 *
 * The Netty Project 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.
 */
package org.jboss.netty.util;

import org.jboss.netty.logging.InternalLogger;
import org.jboss.netty.logging.InternalLoggerFactory;

import java.net.Inet4Address;
import java.net.Inet6Address;
import java.net.InetAddress;
import java.net.UnknownHostException;
import java.util.ArrayList;
import java.util.StringTokenizer;

/**
 * A class that holds a number of network-related constants.
 * <p/>
 * This class borrowed some of its methods from a  modified fork of the
 * <a href="http://svn.apache.org/repos/asf/harmony/enhanced/java/branches/java6/classlib/modules/luni/
 * src/main/java/org/apache/harmony/luni/util/Inet6Util.java">Inet6Util class</a> which was part of Apache Harmony.
 */
public final class NetUtil {

    /**
     * This defines how many words (represented as ints) are needed to represent an IPv6 address
     */
    private static final int IPV6_WORD_COUNT = 8;

    /**
     * The maximum number of characters for an IPV6 string with no scope
     */
    private static final int IPV6_MAX_CHAR_COUNT = 39;

    /**
     * Number of bytes needed to represent and IPV6 value
     */
    private static final int IPV6_BYTE_COUNT = 16;

    /**
     * Maximum amount of value adding characters in between IPV6 separators
     */
    private static final int IPV6_MAX_CHAR_BETWEEN_SEPARATOR = 4;

    /**
     * Minimum number of separators that must be present in an IPv6 string
     */
    private static final int IPV6_MIN_SEPARATORS = 2;

    /**
     * Maximum number of separators that must be present in an IPv6 string
     */
    private static final int IPV6_MAX_SEPARATORS = 8;

    /**
     * Number of bytes needed to represent and IPV4 value
     */
    private static final int IPV4_BYTE_COUNT = 4;

    /**
     * Maximum amount of value adding characters in between IPV4 separators
     */
    private static final int IPV4_MAX_CHAR_BETWEEN_SEPARATOR = 3;

    /**
     * Number of separators that must be present in an IPv4 string
     */
    private static final int IPV4_SEPARATORS = 3;

    /**
     * The logger being used by this class
     */
    private static final InternalLogger logger = InternalLoggerFactory.getInstance(NetUtil.class);

    /**
     * Creates an byte[] based on an ipAddressString. No error handling is
     * performed here.
     */
    public static byte[] createByteArrayFromIpAddressString(String ipAddressString) {

        if (isValidIpV4Address(ipAddressString)) {
            StringTokenizer tokenizer = new StringTokenizer(ipAddressString, ".");
            String token;
            int tempInt;
            byte[] byteAddress = new byte[IPV4_BYTE_COUNT];
            for (int i = 0; i < IPV4_BYTE_COUNT; i ++) {
                token = tokenizer.nextToken();
                tempInt = Integer.parseInt(token);
                byteAddress[i] = (byte) tempInt;
            }

            return byteAddress;
        }

        if (isValidIpV6Address(ipAddressString)) {
            if (ipAddressString.charAt(0) == '[') {
                ipAddressString = ipAddressString.substring(1, ipAddressString.length() - 1);
            }

            int percentPos = ipAddressString.indexOf('%');
            if (percentPos >= 0) {
                ipAddressString = ipAddressString.substring(0, percentPos);
            }

            StringTokenizer tokenizer = new StringTokenizer(ipAddressString, ":.", true);
            ArrayList<String> hexStrings = new ArrayList<String>();
            ArrayList<String> decStrings = new ArrayList<String>();
            String token = "";
            String prevToken = "";
            int doubleColonIndex = -1; // If a double colon exists, we need to
            // insert 0s.

            // Go through the tokens, including the seperators ':' and '.'
            // When we hit a : or . the previous token will be added to either
            // the hex list or decimal list. In the case where we hit a ::
            // we will save the index of the hexStrings so we can add zeros
            // in to fill out the string
            while (tokenizer.hasMoreTokens()) {
                prevToken = token;
                token = tokenizer.nextToken();

                if (":".equals(token)) {
                    if (":".equals(prevToken)) {
                        doubleColonIndex = hexStrings.size();
                    } else if (prevToken.length() != 0) {
                        hexStrings.add(prevToken);
                    }
                } else if (".".equals(token)) {
                    decStrings.add(prevToken);
                }
            }

            if (":".equals(prevToken)) {
                if (":".equals(token)) {
                    doubleColonIndex = hexStrings.size();
                } else {
                    hexStrings.add(token);
                }
            } else if (".".equals(prevToken)) {
                decStrings.add(token);
            }

            // figure out how many hexStrings we should have
            // also check if it is a IPv4 address
            int hexStringsLength = 8;

            // If we have an IPv4 address tagged on at the end, subtract
            // 4 bytes, or 2 hex words from the total
            if (!decStrings.isEmpty()) {
                hexStringsLength -= 2;
            }

            // if we hit a double Colon add the appropriate hex strings
            if (doubleColonIndex != -1) {
                int numberToInsert = hexStringsLength - hexStrings.size();
                for (int i = 0; i < numberToInsert; i ++) {
                    hexStrings.add(doubleColonIndex, "0");
                }
            }

            byte[] ipByteArray = new byte[IPV6_BYTE_COUNT];

            // Finally convert these strings to bytes...
            for (int i = 0; i < hexStrings.size(); i ++) {
                convertToBytes(hexStrings.get(i), ipByteArray, i << 1);
            }

            // Now if there are any decimal values, we know where they go...
            for (int i = 0; i < decStrings.size(); i ++) {
                ipByteArray[i + 12] = (byte) (Integer.parseInt(decStrings.get(i)) & 255);
            }
            return ipByteArray;
        }
        return null;
    }

    /**
     * Converts a 4 character hex word into a 2 byte word equivalent
     */
    private static void convertToBytes(String hexWord, byte[] ipByteArray, int byteIndex) {

        int hexWordLength = hexWord.length();
        int hexWordIndex = 0;
        ipByteArray[byteIndex] = 0;
        ipByteArray[byteIndex + 1] = 0;
        int charValue;

        // high order 4 bits of first byte
        if (hexWordLength > 3) {
            charValue = getIntValue(hexWord.charAt(hexWordIndex ++));
            ipByteArray[byteIndex] |= charValue << 4;
        }

        // low order 4 bits of the first byte
        if (hexWordLength > 2) {
            charValue = getIntValue(hexWord.charAt(hexWordIndex ++));
            ipByteArray[byteIndex] |= charValue;
        }

        // high order 4 bits of second byte
        if (hexWordLength > 1) {
            charValue = getIntValue(hexWord.charAt(hexWordIndex ++));
            ipByteArray[byteIndex + 1] |= charValue << 4;
        }

        // low order 4 bits of the first byte
        charValue = getIntValue(hexWord.charAt(hexWordIndex));
        ipByteArray[byteIndex + 1] |= charValue & 15;
    }

    static int getIntValue(char c) {

        switch (c) {
        case '0':
            return 0;
        case '1':
            return 1;
        case '2':
            return 2;
        case '3':
            return 3;
        case '4':
            return 4;
        case '5':
            return 5;
        case '6':
            return 6;
        case '7':
            return 7;
        case '8':
            return 8;
        case '9':
            return 9;
        }

        c = Character.toLowerCase(c);
        switch (c) {
        case 'a':
            return 10;
        case 'b':
            return 11;
        case 'c':
            return 12;
        case 'd':
            return 13;
        case 'e':
            return 14;
        case 'f':
            return 15;
        }
        return 0;
    }

    public static boolean isValidIpV6Address(String ipAddress) {
        int length = ipAddress.length();
        boolean doubleColon = false;
        int numberOfColons = 0;
        int numberOfPeriods = 0;
        StringBuilder word = new StringBuilder();
        char c = 0;
        char prevChar;
        int startOffset = 0; // offset for [] ip addresses
        int endOffset = ipAddress.length();

        if (endOffset < 2) {
            return false;
        }

        // Strip []
        if (ipAddress.charAt(0) == '[') {
            if (ipAddress.charAt(endOffset - 1) != ']') {
                return false; // must have a close ]
            }

            startOffset = 1;
            endOffset --;
        }

        // Strip the interface name/index after the percent sign.
        int percentIdx = ipAddress.indexOf('%', startOffset);
        if (percentIdx >= 0) {
            endOffset = percentIdx;
        }

        for (int i = startOffset; i < endOffset; i ++) {
            prevChar = c;
            c = ipAddress.charAt(i);
            switch (c) {
            // case for the last 32-bits represented as IPv4 x:x:x:x:x:x:d.d.d.d
            case '.':
                numberOfPeriods ++;
                if (numberOfPeriods > 3) {
                    return false;
                }
                if (!isValidIp4Word(word.toString())) {
                    return false;
                }
                if (numberOfColons != 6 && !doubleColon) {
                    return false;
                }
                // a special case ::1:2:3:4:5:d.d.d.d allows 7 colons with an
                // IPv4 ending, otherwise 7 :'s is bad
                if (numberOfColons == 7 && ipAddress.charAt(startOffset) != ':' &&
                    ipAddress.charAt(1 + startOffset) != ':') {
                    return false;
                }
                word.delete(0, word.length());
                break;

            case ':':
                // FIX "IP6 mechanism syntax #ip6-bad1"
                // An IPV6 address cannot start with a single ":".
                // Either it can starti with "::" or with a number.
                if (i == startOffset && (ipAddress.length() <= i || ipAddress.charAt(i + 1) != ':')) {
                    return false;
                }
                // END FIX "IP6 mechanism syntax #ip6-bad1"
                numberOfColons ++;
                if (numberOfColons > 7) {
                    return false;
                }
                if (numberOfPeriods > 0) {
                    return false;
                }
                if (prevChar == ':') {
                    if (doubleColon) {
                        return false;
                    }
                    doubleColon = true;
                }
                word.delete(0, word.length());
                break;

            default:
                if (word != null && word.length() > 3) {
                    return false;
                }
                if (!isValidHexChar(c)) {
                    return false;
                }
                word.append(c);
            }
        }

        // Check if we have an IPv4 ending
        if (numberOfPeriods > 0) {
            // There is a test case with 7 colons and valid ipv4 this should resolve it
            if (numberOfPeriods != 3 || !(isValidIp4Word(word.toString()) && numberOfColons < 7)) {
                return false;
            }
        } else {
            // If we're at then end and we haven't had 7 colons then there is a
            // problem unless we encountered a doubleColon
            if (numberOfColons != 7 && !doubleColon) {
                return false;
            }

            // If we have an empty word at the end, it means we ended in either
            // a : or a .
            // If we did not end in :: then this is invalid
            if (word.length() == 0 && ipAddress.charAt(length - 1 - startOffset) == ':' &&
                ipAddress.charAt(length - 2 - startOffset) != ':') {
                return false;
            }
        }

        return true;
    }

    public static boolean isValidIp4Word(String word) {
        char c;
        if (word.length() < 1 || word.length() > 3) {
            return false;
        }
        for (int i = 0; i < word.length(); i ++) {
            c = word.charAt(i);
            if (!(c >= '0' && c <= '9')) {
                return false;
            }
        }
        return Integer.parseInt(word) <= 255;
    }

    private static boolean isValidHexChar(char c) {
        return c >= '0' && c <= '9' || c >= 'A' && c <= 'F' || c >= 'a' && c <= 'f';
    }

    private static boolean isValidNumericChar(char c) {
        return c >= '0' && c <= '9';
    }

    /**
     * Takes a string and parses it to see if it is a valid IPV4 address.
     *
     * @return true, if the string represents an IPV4 address in dotted
     *         notation, false otherwise
     */
    public static boolean isValidIpV4Address(String value) {

        int periods = 0;
        int i;
        int length = value.length();

        if (length > 15) {
            return false;
        }
        char c;
        StringBuilder word = new StringBuilder();
        for (i = 0; i < length; i ++) {
            c = value.charAt(i);
            if (c == '.') {
                periods ++;
                if (periods > 3) {
                    return false;
                }
                if (word.length() == 0) {
                    return false;
                }
                if (Integer.parseInt(word.toString()) > 255) {
                    return false;
                }
                word.delete(0, word.length());
            } else if (!Character.isDigit(c)) {
                return false;
            } else {
                if (word.length() > 2) {
                    return false;
                }
                word.append(c);
            }
        }

        if (word.length() == 0 || Integer.parseInt(word.toString()) > 255) {
            return false;
        }

        return periods == 3;
    }

    /**
     * Returns the {@link Inet6Address} representation of a {@link CharSequence} IP address.
     * <p>
     * This method will treat all IPv4 type addresses as "IPv4 mapped" (see {@link #getByName(CharSequence, boolean)})
     * @param ip {@link CharSequence} IP address to be converted to a {@link Inet6Address}
     * @return {@link Inet6Address} representation of the {@code ip} or {@code null} if not a valid IP address.
     */
    public static Inet6Address getByName(CharSequence ip) {
        return getByName(ip, true);
    }

    /**
     * Returns the {@link Inet6Address} representation of a {@link CharSequence} IP address.
     * <p>
     * The {@code ipv4Mapped} parameter specifies how IPv4 addresses should be treated.
     * "IPv4 mapped" format as
     * defined in <a href="http://tools.ietf.org/html/rfc4291#section-2.5.5">rfc 4291 section 2</a> is supported.
     * @param ip {@link CharSequence} IP address to be converted to a {@link Inet6Address}
     * @param ipv4Mapped
     * <ul>
     * <li>{@code true} To allow IPv4 mapped inputs to be translated into {@link Inet6Address}</li>
     * <li>{@code false} Don't turn IPv4 addressed to mapped addresses</li>
     * </ul>
     * @return {@link Inet6Address} representation of the {@code ip} or {@code null} if not a valid IP address.
     */
    public static Inet6Address getByName(CharSequence ip, boolean ipv4Mapped) {
        final byte[] bytes = new byte[IPV6_BYTE_COUNT];
        final int ipLength = ip.length();
        int compressBegin = 0;
        int compressLength = 0;
        int currentIndex = 0;
        int value = 0;
        int begin = -1;
        int i = 0;
        int ipv6Seperators = 0;
        int ipv4Seperators = 0;
        int tmp;
        boolean needsShift = false;
        for (; i < ipLength; ++i) {
            final char c = ip.charAt(i);
            switch (c) {
            case ':':
                ++ipv6Seperators;
                if (i - begin > IPV6_MAX_CHAR_BETWEEN_SEPARATOR ||
                    ipv4Seperators > 0 || ipv6Seperators > IPV6_MAX_SEPARATORS ||
                    currentIndex + 1 >= bytes.length) {
                    return null;
                }
                value <<= (IPV6_MAX_CHAR_BETWEEN_SEPARATOR - (i - begin)) << 2;

                if (compressLength > 0) {
                    compressLength -= 2;
                }

                // The value integer holds at most 4 bytes from right (most significant) to left (least significant).
                // The following bit shifting is used to extract and re-order the individual bytes to achieve a
                // left (most significant) to right (least significant) ordering.
                bytes[currentIndex++] = (byte) (((value & 0xf) << 4) | ((value >> 4) & 0xf));
                bytes[currentIndex++] = (byte) ((((value >> 8) & 0xf) << 4) | ((value >> 12) & 0xf));
                tmp = i + 1;
                if (tmp < ipLength && ip.charAt(tmp) == ':') {
                    ++tmp;
                    if (compressBegin != 0 || (tmp < ipLength && ip.charAt(tmp) == ':')) {
                        return null;
                    }
                    ++ipv6Seperators;
                    needsShift = ipv6Seperators == 2 && value == 0;
                    compressBegin = currentIndex;
                    compressLength = bytes.length - compressBegin - 2;
                    ++i;
                }
                value = 0;
                begin = -1;
                break;
            case '.':
                ++ipv4Seperators;
                if (i - begin > IPV4_MAX_CHAR_BETWEEN_SEPARATOR
                    || ipv4Seperators > IPV4_SEPARATORS
                    || (ipv6Seperators > 0 && (currentIndex + compressLength < 12))
                    || i + 1 >= ipLength
                    || currentIndex >= bytes.length
                    || begin < 0
                    || (begin == 0 && (i == 3 && (!isValidNumericChar(ip.charAt(2)) ||
                                                  !isValidNumericChar(ip.charAt(1)) ||
                                                  !isValidNumericChar(ip.charAt(0))) ||
                                       i == 2 && (!isValidNumericChar(ip.charAt(1)) ||
                                                  !isValidNumericChar(ip.charAt(0))) ||
                                       i == 1 && !isValidNumericChar(ip.charAt(0))))) {
                    return null;
                }
                value <<= (IPV4_MAX_CHAR_BETWEEN_SEPARATOR - (i - begin)) << 2;

                // The value integer holds at most 3 bytes from right (most significant) to left (least significant).
                // The following bit shifting is to restructure the bytes to be left (most significant) to
                // right (least significant) while also accounting for each IPv4 digit is base 10.
                begin = (value & 0xf) * 100 + ((value >> 4) & 0xf) * 10 + ((value >> 8) & 0xf);
                if (begin < 0 || begin > 255) {
                    return null;
                }
                bytes[currentIndex++] = (byte) begin;
                value = 0;
                begin = -1;
                break;
            default:
                if (!isValidHexChar(c) || (ipv4Seperators > 0 && !isValidNumericChar(c))) {
                    return null;
                }
                if (begin < 0) {
                    begin = i;
                } else if (i - begin > IPV6_MAX_CHAR_BETWEEN_SEPARATOR) {
                    return null;
                }
                // The value is treated as a sort of array of numbers because we are dealing with
                // at most 4 consecutive bytes we can use bit shifting to accomplish this.
                // The most significant byte will be encountered first, and reside in the right most
                // position of the following integer
                value += getIntValue(c) << ((i - begin) << 2);
                break;
            }
        }

        final boolean isCompressed = compressBegin > 0;
        // Finish up last set of data that was accumulated in the loop (or before the loop)
        if (ipv4Seperators > 0) {
            if (begin > 0 && i - begin > IPV4_MAX_CHAR_BETWEEN_SEPARATOR ||
                ipv4Seperators != IPV4_SEPARATORS ||
                currentIndex >= bytes.length) {
                return null;
            }
            if (ipv6Seperators == 0) {
                compressLength = 12;
            } else if (ipv6Seperators >= IPV6_MIN_SEPARATORS &&
                       ip.charAt(ipLength - 1) != ':' &&
                       (!isCompressed && (ipv6Seperators == 6 && ip.charAt(0) != ':') ||
                        isCompressed && (ipv6Seperators + 1 < IPV6_MAX_SEPARATORS &&
                                         (ip.charAt(0) != ':' || compressBegin <= 2)))) {
                compressLength -= 2;
            } else {
                return null;
            }
            value <<= (IPV4_MAX_CHAR_BETWEEN_SEPARATOR - (i - begin)) << 2;

            // The value integer holds at most 3 bytes from right (most significant) to left (least significant).
            // The following bit shifting is to restructure the bytes to be left (most significant) to
            // right (least significant) while also accounting for each IPv4 digit is base 10.
            begin = (value & 0xf) * 100 + ((value >> 4) & 0xf) * 10 + ((value >> 8) & 0xf);
            if (begin < 0 || begin > 255) {
                return null;
            }
            bytes[currentIndex++] = (byte) begin;
        } else {
            tmp = ipLength - 1;
            if (begin > 0 && i - begin > IPV6_MAX_CHAR_BETWEEN_SEPARATOR ||
                ipv6Seperators < IPV6_MIN_SEPARATORS ||
                !isCompressed && (ipv6Seperators + 1 != IPV6_MAX_SEPARATORS  ||
                                  ip.charAt(0) == ':' || ip.charAt(tmp) == ':') ||
                isCompressed && (ipv6Seperators > IPV6_MAX_SEPARATORS ||
                                 (ipv6Seperators == IPV6_MAX_SEPARATORS &&
                                  (compressBegin <= 2 && ip.charAt(0) != ':' ||
                                   compressBegin >= 14 && ip.charAt(tmp) != ':'))) ||
                currentIndex + 1 >= bytes.length) {
                return null;
            }
            if (begin >= 0 && i - begin <= IPV6_MAX_CHAR_BETWEEN_SEPARATOR) {
                value <<= (IPV6_MAX_CHAR_BETWEEN_SEPARATOR - (i - begin)) << 2;
            }
            // The value integer holds at most 4 bytes from right (most significant) to left (least significant).
            // The following bit shifting is used to extract and re-order the individual bytes to achieve a
            // left (most significant) to right (least significant) ordering.
            bytes[currentIndex++] = (byte) (((value & 0xf) << 4) | ((value >> 4) & 0xf));
            bytes[currentIndex++] = (byte) ((((value >> 8) & 0xf) << 4) | ((value >> 12) & 0xf));
        }

        i = currentIndex + compressLength;
        if (needsShift || i >= bytes.length) {
            // Right shift array
            if (i >= bytes.length) {
                ++compressBegin;
            }
            for (i = currentIndex; i < bytes.length; ++i) {
                for (begin = bytes.length - 1; begin >= compressBegin; --begin) {
                    bytes[begin] = bytes[begin - 1];
                }
                bytes[begin] = 0;
                ++compressBegin;
            }
        } else {
            // Selectively move elements
            for (i = 0; i < compressLength; ++i) {
                begin = i + compressBegin;
                currentIndex = begin + compressLength;
                if (currentIndex < bytes.length) {
                    bytes[currentIndex] = bytes[begin];
                    bytes[begin] = 0;
                } else {
                    break;
                }
            }
        }

        if (ipv4Mapped && ipv4Seperators > 0 &&
            bytes[0] == 0 && bytes[1] == 0 && bytes[2] == 0 && bytes[3] == 0 && bytes[4] == 0 &&
            bytes[5] == 0 && bytes[6] == 0 && bytes[7] == 0 && bytes[8] == 0 && bytes[9] == 0) {
            bytes[10] = bytes[11] = (byte) 0xff;
        }

        try {
            return Inet6Address.getByAddress(null, bytes, -1);
        } catch (UnknownHostException e) {
            throw new RuntimeException(e); // Should never happen
        }
    }

    /**
     * Returns the {@link String} representation of an {@link InetAddress}.
     * <ul>
     * <li>Inet4Address results are identical to {@link InetAddress#getHostAddress()}</li>
     * <li>Inet6Address results adhere to
     * <a href="http://tools.ietf.org/html/rfc5952#section-4">rfc 5952 section 4</a></li>
     * </ul>
     * <p>
     * The output does not include Scope ID.
     * @param ip {@link InetAddress} to be converted to an address string
     * @return {@code String} containing the text-formatted IP address
     */
    public static String toAddressString(InetAddress ip) {
        return toAddressString(ip, false);
    }

    /**
     * Returns the {@link String} representation of an {@link InetAddress}.
     * <ul>
     * <li>Inet4Address results are identical to {@link InetAddress#getHostAddress()}</li>
     * <li>Inet6Address results adhere to
     * <a href="http://tools.ietf.org/html/rfc5952#section-4">rfc 5952 section 4</a> if
     * {@code ipv4Mapped} is false.  If {@code ipv4Mapped} is true then "IPv4 mapped" format
     * from <a href="http://tools.ietf.org/html/rfc4291#section-2.5.5">rfc 4291 section 2</a> will be supported.
     * The compressed result will always obey the compression rules defined in
     * <a href="http://tools.ietf.org/html/rfc5952#section-4">rfc 5952 section 4</a></li>
     * </ul>
     * <p>
     * The output does not include Scope ID.
     * @param ip {@link InetAddress} to be converted to an address string
     * @param ipv4Mapped
     * <ul>
     * <li>{@code true} to stray from strict rfc 5952 and support the "IPv4 mapped" format
     * defined in <a href="http://tools.ietf.org/html/rfc4291#section-2.5.5">rfc 4291 section 2</a> while still
     * following the updated guidelines in
     * <a href="http://tools.ietf.org/html/rfc5952#section-4">rfc 5952 section 4</a></li>
     * <li>{@code false} to strictly follow rfc 5952</li>
     * </ul>
     * @return {@code String} containing the text-formatted IP address
     */
    public static String toAddressString(InetAddress ip, boolean ipv4Mapped) {
        if (ip instanceof Inet4Address) {
            return ip.getHostAddress();
        }
        if (!(ip instanceof Inet6Address)) {
            throw new IllegalArgumentException("Unhandled type: " + ip.getClass());
        }

        final byte[] bytes = ip.getAddress();
        final int[] words = new int[IPV6_WORD_COUNT];
        int i;
        for (i = 0; i < words.length; ++i) {
            words[i] = ((bytes[i << 1] & 0xff) << 8) | (bytes[(i << 1) + 1] & 0xff);
        }

        // Find longest run of 0s, tie goes to first found instance
        int currentStart = -1;
        int currentLength = 0;
        int shortestStart = -1;
        int shortestLength = 0;
        for (i = 0; i < words.length; ++i) {
            if (words[i] == 0) {
                if (currentStart < 0) {
                    currentStart = i;
                }
            } else if (currentStart >= 0) {
                currentLength = i - currentStart;
                if (currentLength > shortestLength) {
                    shortestStart = currentStart;
                    shortestLength = currentLength;
                }
                currentStart = -1;
            }
        }
        // If the array ends on a streak of zeros, make sure we account for it
        if (currentStart >= 0) {
            currentLength = i - currentStart;
            if (currentLength > shortestLength) {
                shortestStart = currentStart;
                shortestLength = currentLength;
            }
        }
        // Ignore the longest streak if it is only 1 long
        if (shortestLength == 1) {
            shortestLength = 0;
            shortestStart = -1;
        }

        // Translate to string taking into account longest consecutive 0s
        final int shortestEnd = shortestStart + shortestLength;
        final StringBuilder b = new StringBuilder(IPV6_MAX_CHAR_COUNT);
        if (shortestEnd < 0) { // Optimization when there is no compressing needed
            b.append(Integer.toHexString(words[0]));
            for (i = 1; i < words.length; ++i) {
                b.append(':');
                b.append(Integer.toHexString(words[i]));
            }
        } else { // General case that can handle compressing (and not compressing)
            // Loop unroll the first index (so we don't constantly check i==0 cases in loop)
            final boolean isIpv4Mapped;
            if (inRangeEndExclusive(0, shortestStart, shortestEnd)) {
                b.append("::");
                isIpv4Mapped = ipv4Mapped && (shortestEnd == 5 && words[5] == 0xffff);
            } else {
                b.append(Integer.toHexString(words[0]));
                isIpv4Mapped = false;
            }
            for (i = 1; i < words.length; ++i) {
                if (!inRangeEndExclusive(i, shortestStart, shortestEnd)) {
                    if (!inRangeEndExclusive(i - 1, shortestStart, shortestEnd)) {
                        // If the last index was not part of the shortened sequence
                        if (!isIpv4Mapped || i == 6) {
                            b.append(':');
                        } else {
                            b.append('.');
                        }
                    }
                    if (isIpv4Mapped && i > 5) {
                        b.append(words[i] >> 8);
                        b.append('.');
                        b.append(words[i] & 0xff);
                    } else {
                        b.append(Integer.toHexString(words[i]));
                    }
                } else if (!inRangeEndExclusive(i - 1, shortestStart, shortestEnd)) {
                    // If we are in the shortened sequence and the last index was not
                    b.append("::");
                }
            }
        }

        return b.toString();
    }

    /**
     * Does a range check on {@code value} if is within {@code start} (inclusive) and {@code end} (exclusive).
     * @param value The value to checked if is within {@code start} (inclusive) and {@code end} (exclusive)
     * @param start The start of the range (inclusive)
     * @param end The end of the range (exclusive)
     * @return
     * <ul>
     * <li>{@code true} if {@code value} if is within {@code start} (inclusive) and {@code end} (exclusive)</li>
     * <li>{@code false} otherwise</li>
     * </ul>
     */
    private static boolean inRangeEndExclusive(int value, int start, int end) {
        return value >= start && value < end;
    }

    /**
     * A constructor to stop this class being constructed.
     */
    private NetUtil() {
        // Unused
    }
}