/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
package org.mozilla.jss.asn1;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.math.BigInteger;
import java.util.Vector;

/**
 * The portion of a BER encoding that precedes the contents octets. Consists
 * of the tag, form, and length octets.
 */
public class ASN1Header {

    // This is set by the the decoding constructor, and by the encode()
    // method.  If it is set by the decoding constructor, it is supposed
    // to represent what was actually read from the input stream, so it
    // must not be overwritten later by the output of encode(), which could
    // be a different encoding (DER vs. BER, for example).
    private byte[] cachedEncoding = null;

    /**
     * Returns the length of the header plus the length of the contents;
     * the total length of the DER encoding of an ASN1 value. Returns
     * -1 if indefinite length encoding was used.
     * 
     * @return Total length.
     */
    public long getTotalLength() {
        if (contentLength == -1) {
            return -1;
        } else {
            return encode().length + contentLength;
        }
    }

    private Tag tag;

    public Tag getTag() {
        return tag;
    }

    // -1 means indefinite length encoding
    private long contentLength;

    /**
     * Returns -1 for indefinite length encoding.
     * 
     * @return Content length.
     */
    public long getContentLength() {
        return contentLength;
    }

    // PRIMITIVE or CONSTRUCTED
    public static final Form PRIMITIVE = Form.PRIMITIVE;
    public static final Form CONSTRUCTED = Form.CONSTRUCTED;
    private Form form;

    /**
     * Returns the Form, PRIMITIVE or CONSTRUCTED.
     * 
     * @return The form.
     */
    public Form getForm() {
        return form;
    }

    // This is the maximum size of ASN1 Header we support.
    // 32 bytes is pretty huge, I've never seen anything bigger than 7.
    private static final int MAX_LOOK_AHEAD = 32;

    /**
     * Returns information about the next item in the stream, but does not
     * consume any octets.
     * 
     * @param derStream DER stream.
     * @return ASN.1 header.
     * @throws InvalidBERException If there is an invalid BER encoding.
     * @exception IOException If the input stream does not support look ahead.
     */
    public static ASN1Header lookAhead(InputStream derStream)
            throws IOException, InvalidBERException {
        if (!derStream.markSupported()) {
            throw new IOException("Mark not supported on this input stream");
        }

        derStream.mark(MAX_LOOK_AHEAD);
        ASN1Header info = new ASN1Header(derStream);
        derStream.reset();

        return info;
    }

    /**
     * Gets info about the next item in the DER stream, consuming the
     * identifier and length octets.
     * 
     * @param istream Input stream.
     * @throws InvalidBERException If there is an invalid BER encoding.
     * @throws IOException If other error occurred.
     */
    public ASN1Header(InputStream istream)
            throws InvalidBERException, IOException {
        // default BAOS size is 32 bytes, which is plenty
        ByteArrayOutputStream encoding = new ByteArrayOutputStream();
        int inInt = istream.read();
        if (inInt == -1) {
            throw new InvalidBERException("End-of-file reached while " +
                    "decoding ASN.1 header");
        }
        encoding.write(inInt);
        byte byte1 = (byte) inInt;
        Tag.Class tagClass;

        //
        // Get Tag Class
        //
        tagClass = Tag.Class.fromInt((byte1 & 0xff) >>> 6);

        //
        // Get form
        //
        if ((byte1 & 0x20) == 0x20) {
            form = CONSTRUCTED;
        } else {
            form = PRIMITIVE;
        }

        //
        // Get Tag Number
        //
        long tagNum;
        if ((byte1 & 0x1f) == 0x1f) {
            // long form

            //
            // read all octets into a Vector of Bytes
            //
            byte next;
            Vector<Byte> bV = new Vector<>();

            // last byte has MSB == 0.
            do {
                inInt = istream.read();
                if (inInt == -1) {
                    throw new InvalidBERException("End-of-file reached while"
                            + " decoding ASN.1 header");
                }
                encoding.write(inInt);
                next = (byte) inInt;
                bV.addElement(Byte.valueOf(next));
            } while ((next & 0x80) == 0x80);
            assert (bV.size() > 0);

            //
            // Copy Vector of 7-bit bytes into array of 8-bit bytes.
            //
            byte[] bA = new byte[((bV.size() * 7) + 7) / 8];
            int v; // vector index
            int a; // array index

            // clear the target array
            for (a = 0; a < bA.length; a++) {
                bA[a] = 0;
            }
            int shift = 0; // the amount the Vector is shifted from the array

            // copy bits from the Vector to the array, going from the
            // end (LSB) to the beginning (MSB).
            a = bA.length - 1;
            for (v = bV.size() - 1; v >= 0; v--) {
                assert (v >= 0);
                assert (v < bV.size());
                assert (a >= 0);
                assert (a < bA.length);

                // MSB is not part of the number
                byte b = (byte) (bV.elementAt(v).byteValue() & 0x7f);
                bA[a] |= b << shift;
                if (shift > 1) {
                    // The byte from the Vector falls across a byte boundary
                    // in the array.  We've already got the less-significant
                    // bits, now copy the more-significant bits into
                    // the next element of the array.
                    assert (a > 0);
                    --a;
                    bA[a] |= b >>> (8 - shift);
                }

                shift = (shift + 7) % 8; // update shift
            }

            // Create a new unsigned BigInteger from the byte array
            tagNum = (new BigInteger(1, bA)).longValue();

        } else {
            // short form
            tagNum = byte1 & 0x1f;
        }

        tag = new Tag(tagClass, tagNum);

        //
        // Get Length
        //
        inInt = istream.read();
        if (inInt == -1) {
            throw new InvalidBERException("End-of-file reached while " +
                    "decoding ASN.1 header");
        }
        encoding.write(inInt);
        byte lenByte = (byte) inInt;

        if ((lenByte & 0x80) == 0) {
            // short form
            contentLength = lenByte;
        } else {
            // long form
            if ((lenByte & 0x7f) == 0) {
                // indefinite
                contentLength = -1;
            } else {
                // definite
                byte[] lenBytes = new byte[lenByte & 0x7f];
                ASN1Util.readFully(lenBytes, istream);
                encoding.write(lenBytes);
                contentLength = (new BigInteger(1, lenBytes)).longValue();
            }
        }

        // save our encoding so we don't have to recompute it later
        cachedEncoding = encoding.toByteArray();
    }

    /**
     * This constructor is to be called when we are constructing an ASN1Value
     * rather than decoding it.
     * 
     * @param tag Tag.
     * @param form Form.
     * @param contentLength Must be &ge;0. Although indefinite length
     *            <i>decoding</i> is supported, indefinite length <i>encoding</i>
     *            is not.
     */
    public ASN1Header(Tag tag, Form form, long contentLength) {
        this.tag = tag;
        this.form = form;
        assert (contentLength >= 0);
        this.contentLength = contentLength;
    }

    public void encode(OutputStream ostream)
            throws IOException {
        ostream.write(encode());
    }

    public byte[] encode() {
        // It's important that we not recompute the encoding if it was
        // set by ASN1Header(InputStream), since in that case it represents
        // the encoding that was actually read from the InputStream.
        if (cachedEncoding != null) {
            return cachedEncoding;
        }

        ByteArrayOutputStream cache = new ByteArrayOutputStream();

        //
        // Identifier octet(s)
        //

        byte idOctet = 0;
        idOctet |= tag.getTagClass().toInt() << 6;

        if (form == CONSTRUCTED) {
            idOctet |= 0x20;
        }

        if (tag.getNum() <= 30) {
            // short form
            idOctet |= (tag.getNum() & 0x1f);

            cache.write(idOctet);
        } else {
            // long form
            idOctet |= 0x1f;
            BigInteger tagNum = BigInteger.valueOf(tag.getNum());

            cache.write(idOctet);

            int bitlength = tagNum.bitLength();
            int reps = (bitlength + 6) / 7;

            for (reps = reps - 1; reps > 0; reps--) {
                long shifted = tag.getNum() >>> (7 * reps);
                cache.write((((byte) shifted) & 0x7f) | 0x80);
            }

            cache.write(((byte) tag.getNum()) & 0x7f);
        }

        //
        // Length Octets
        //
        if (contentLength == -1) {
            // indefinite form
            cache.write((byte) 0x80);
        } else if (contentLength <= 127) {
            // short form
            cache.write((byte) contentLength);
        } else {
            // long form
            byte[] val = unsignedBigIntToByteArray(
                    BigInteger.valueOf(contentLength));
            cache.write(((byte) val.length) | 0x80);
            cache.write(val, 0, val.length);
        }

        cachedEncoding = cache.toByteArray();

        return cachedEncoding;
    }

    /**
     * Converts an unsigned BigInteger to a minimal-length byte array.
     * This is necessary because BigInteger.toByteArray() attaches an extra
     * sign bit, which could cause the size of the byte representation to
     * be bumped up by an extra byte.
     * 
     * @param bi Input BigInteger.
     * @return Byte array.
     */
    public static byte[] unsignedBigIntToByteArray(BigInteger bi) {
        // make sure it is not negative
        assert (bi.compareTo(BigInteger.valueOf(0)) != -1);

        // find minimal number of bytes to hold this value
        int bitlen = bi.bitLength(); // minimal number of bits, without sign
        int bytelen;
        if (bitlen == 0) {
            // special case, since bitLength() returns 0
            bytelen = 1;
        } else {
            bytelen = (bitlen + 7) / 8;
        }

        byte[] withSign = bi.toByteArray();

        if (bytelen == withSign.length) {
            return withSign;
        } else {
            // trim off extra byte at the beginning
            assert (bytelen == withSign.length - 1);
            assert (withSign[0] == 0);
            byte[] without = new byte[bytelen];
            System.arraycopy(withSign, 1, without, 0, bytelen);
            return without;
        }
    }

    /**
     * Verifies that this header has the given tag and form.
     * 
     * @param expectedTag Expected tag.
     * @param expectedForm Expected form.
     * @exception InvalidBERException If the header's tag or form
     *                differ from those passed in.
     */
    public void validate(Tag expectedTag, Form expectedForm)
            throws InvalidBERException {
        validate(expectedTag);
        if (getForm() != expectedForm) {
            throw new InvalidBERException("Incorrect form: expected [" +
                    expectedForm + "], found [" + getForm());
        }
    }

    /**
     * Verifies that this head has the given tag.
     * 
     * @param expectedTag Expected tag.
     * @exception InvalidBERException If the header's tag differs from that
     *                passed in.
     */
    public void validate(Tag expectedTag) throws InvalidBERException {
        if (!getTag().equals(expectedTag)) {
            throw new InvalidBERException("Incorrect tag: expected [" +
                    expectedTag + "], found [" + getTag() + "]");
        }
    }

    /**
     * @return <code>true</code> if this is a BER end-of-contents marker.
     */
    public boolean isEOC() {
        return (tag.equals(Tag.EOC));
    }

}
