/*
 * Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package jdk.dns.client.internal;

import jdk.dns.client.ex.DnsResolverException;

class Header {

    static final int HEADER_SIZE = 12;  // octets in a DNS header

    // Masks and shift amounts for DNS header flag fields.
    static final short QR_BIT = (short) 0x8000;
    static final short OPCODE_MASK = (short) 0x7800;
    static final int OPCODE_SHIFT = 11;
    static final short AA_BIT = (short) 0x0400;
    static final short AD_BIT = (short) 0x0020;
    static final short CD_BIT = (short) 0x0010;
    static final short TC_BIT = (short) 0x0200;
    static final short RD_BIT = (short) 0x0100;
    static final short RA_BIT = (short) 0x0080;
    static final short RCODE_MASK = (short) 0x000F;

    int xid;                    // ID:  16-bit query identifier
    boolean query;              // QR:  true if query, false if response
    int opcode;                 // OPCODE:  4-bit opcode
    boolean authoritative;      // AA
    boolean truncated;          // TC
    boolean recursionDesired;   // RD
    boolean recursionAvail;     // RA
    boolean authenticData;      // AD
    boolean checkingDisabled;   // CD
    int rcode;                  // RCODE:  4-bit response code
    int numQuestions;
    int numAnswers;
    int numAuthorities;
    int numAdditionals;

    /*
     * Returns a representation of a decoded DNS message header.
     * Does not modify or store a reference to the msg array.
     */
    Header(byte[] msg, int msgLen) throws DnsResolverException {
        decode(msg, msgLen);
    }

    /*
     * Decodes a DNS message header.  Does not modify or store a
     * reference to the msg array.
     */
    private void decode(byte[] msg, int msgLen) throws DnsResolverException {

        try {
            int pos = 0;        // current offset into msg

            if (msgLen < HEADER_SIZE) {
                throw new DnsResolverException(
                        "DNS error: corrupted message header");
            }

            xid = getShort(msg, pos);
            pos += 2;

            // Flags
            short flags = (short) getShort(msg, pos);
            pos += 2;
            query = (flags & QR_BIT) == 0;
            opcode = (flags & OPCODE_MASK) >>> OPCODE_SHIFT;
            authoritative = (flags & AA_BIT) != 0;
            truncated = (flags & TC_BIT) != 0;
            recursionDesired = (flags & RD_BIT) != 0;
            recursionAvail = (flags & RA_BIT) != 0;
            authenticData = (flags & AD_BIT) != 0;
            checkingDisabled = (flags & CD_BIT) != 0;
            rcode = (flags & RCODE_MASK);

            // RR counts
            numQuestions = getShort(msg, pos);
            pos += 2;
            numAnswers = getShort(msg, pos);
            pos += 2;
            numAuthorities = getShort(msg, pos);
            pos += 2;
            numAdditionals = getShort(msg, pos);
            pos += 2;

        } catch (IndexOutOfBoundsException e) {
            throw new DnsResolverException("DNS error: corrupted message header");
        }
    }

    /*
     * Returns the 2-byte unsigned value at msg[pos].  The high
     * order byte comes first.
     */
    private static int getShort(byte[] msg, int pos) {
        return (((msg[pos] & 0xFF) << 8) |
                (msg[pos + 1] & 0xFF));
    }
}