/*******************************************************************************
 * Copyright (c) 2005-2014 Obeo
 *  
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 * 
 * Contributors:
 *    Obeo - initial API and implementation
 *******************************************************************************/
package org.eclipse.sirius.query.legacy.tools.resources;

import java.io.IOException;
import java.io.InputStream;
import java.io.PushbackInputStream;

/**
 * The <code>UnicodeBOMInputStream</code> class wraps any
 * <code>InputStream</code> and detects the presence of any Unicode BOM (Byte
 * Order Mark) at its beginning, as defined by <a
 * href="http://www.faqs.org/rfcs/rfc3629.html">RFC 3629 - UTF-8, a
 * transformation format of ISO 10646</a>
 * 
 * <p>
 * The <a href="http://www.unicode.org/unicode/faq/utf_bom.html">Unicode FAQ</a>
 * defines 5 types of BOMs:
 * <ul>
 * <li>
 * 
 * <pre>
 * 00 00 FE FF  = UTF-32, big-endian
 * </pre>
 * 
 * </li>
 * <li>
 * 
 * <pre>
 * FF FE 00 00  = UTF-32, little-endian
 * </pre>
 * 
 * </li>
 * <li>
 * 
 * <pre>
 * FE FF        = UTF-16, big-endian
 * </pre>
 * 
 * </li>
 * <li>
 * 
 * <pre>
 * FF FE        = UTF-16, little-endian
 * </pre>
 * 
 * </li>
 * <li>
 * 
 * <pre>
 * EF BB BF     = UTF-8
 * </pre>
 * 
 * </li>
 * </ul>
 * </p>
 * 
 * <p>
 * Use the {@link #getBOM()} method to know whether a BOM has been detected or
 * not.
 * </p>
 * <p>
 * Use the {@link #skipBOM()} method to remove the detected BOM from the wrapped
 * <code>InputStream</code> object.
 * </p>
 */
public class UnicodeBOMInputStream extends InputStream {
    /**
     * Type safe enumeration class that describes the different types of Unicode
     * BOMs.
     */
    public static final class BOM {
        /**
         * NONE.
         */
        public static final BOM NONE = new BOM(new byte[] {}, "NONE");

        /**
         * UTF-8 BOM (EF BB BF).
         */
        public static final BOM UTF_8 = new BOM(new byte[] { (byte) 0xEF, (byte) 0xBB, (byte) 0xBF }, "UTF-8");

        /**
         * UTF-16, little-endian (FF FE).
         */
        public static final BOM UTF_16_LE = new BOM(new byte[] { (byte) 0xFF, (byte) 0xFE }, "UTF-16 little-endian");

        /**
         * UTF-16, big-endian (FE FF).
         */
        public static final BOM UTF_16_BE = new BOM(new byte[] { (byte) 0xFE, (byte) 0xFF }, "UTF-16 big-endian");

        /**
         * UTF-32, little-endian (FF FE 00 00).
         */
        public static final BOM UTF_32_LE = new BOM(new byte[] { (byte) 0xFF, (byte) 0xFE, (byte) 0x00, (byte) 0x00 }, "UTF-32 little-endian");

        /**
         * UTF-32, big-endian (00 00 FE FF).
         */
        public static final BOM UTF_32_BE = new BOM(new byte[] { (byte) 0x00, (byte) 0x00, (byte) 0xFE, (byte) 0xFF }, "UTF-32 big-endian");

        /**
         * Returns a <code>String</code> representation of this <code>BOM</code>
         * value.
         */
        @Override
        public final String toString() {
            return description;
        }

        /**
         * Returns the bytes corresponding to this <code>BOM</code> value.
         */
        public final byte[] getBytes() {
            final int length = bytes.length;
            final byte[] result = new byte[length];

            // Make a defensive copy
            System.arraycopy(bytes, 0, result, 0, length);

            return result;
        }

        private BOM(final byte bom[], final String description) {
            assert (bom != null) : "invalid BOM: null is not allowed";
            assert (description != null) : "invalid description: null is not allowed";
            assert (description.length() != 0) : "invalid description: empty string is not allowed";

            this.bytes = bom;
            this.description = description;
        }

        final byte bytes[];

        private final String description;

    } // BOM

    /**
     * Constructs a new <code>UnicodeBOMInputStream</code> that wraps the
     * specified <code>InputStream</code>.
     * 
     * @param inputStream
     *            an <code>InputStream</code>.
     * 
     * @throws NullPointerException
     *             when <code>inputStream</code> is <code>null</code>.
     * @throws IOException
     *             on reading from the specified <code>InputStream</code> when
     *             trying to detect the Unicode BOM.
     */
    public UnicodeBOMInputStream(final InputStream inputStream) throws NullPointerException, IOException

    {
        if (inputStream == null) {
            throw new NullPointerException("invalid input stream: null is not allowed");
        }

        in = new PushbackInputStream(inputStream, 4);

        final byte bom[] = new byte[4];
        final int read = in.read(bom);

        switch (read) {
        case 4:
            if ((bom[0] == (byte) 0xFF) && (bom[1] == (byte) 0xFE) && (bom[2] == (byte) 0x00) && (bom[3] == (byte) 0x00)) {
                this.bom = BOM.UTF_32_LE;
                break;
            } else if ((bom[0] == (byte) 0x00) && (bom[1] == (byte) 0x00) && (bom[2] == (byte) 0xFE) && (bom[3] == (byte) 0xFF)) {
                this.bom = BOM.UTF_32_BE;
                break;
            }

        case 3:
            if ((bom[0] == (byte) 0xEF) && (bom[1] == (byte) 0xBB) && (bom[2] == (byte) 0xBF)) {
                this.bom = BOM.UTF_8;
                break;
            }

        case 2:
            if ((bom[0] == (byte) 0xFF) && (bom[1] == (byte) 0xFE)) {
                this.bom = BOM.UTF_16_LE;
                break;
            } else if ((bom[0] == (byte) 0xFE) && (bom[1] == (byte) 0xFF)) {
                this.bom = BOM.UTF_16_BE;
                break;
            }

        default:
            this.bom = BOM.NONE;
            break;
        }

        if (read > 0) {
            in.unread(bom, 0, read);
        }
    }

    /**
     * Returns the <code>BOM</code> that was detected in the wrapped
     * <code>InputStream</code> object.
     * 
     * @return a <code>BOM</code> value.
     */
    public final BOM getBOM() {
        // BOM type is immutable.
        return bom;
    }

    /**
     * Skips the <code>BOM</code> that was found in the wrapped
     * <code>InputStream</code> object.
     * 
     * @return this <code>UnicodeBOMInputStream</code>.
     * 
     * @throws IOException
     *             when trying to skip the BOM from the wrapped
     *             <code>InputStream</code> object.
     */
    public final synchronized UnicodeBOMInputStream skipBOM() throws IOException {
        if (!skipped) {
            in.skip(bom.bytes.length);
            skipped = true;
        }
        return this;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public int read() throws IOException {
        return in.read();
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public int read(final byte b[]) throws IOException, NullPointerException {
        return in.read(b, 0, b.length);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public int read(final byte b[], final int off, final int len) throws IOException, NullPointerException {
        return in.read(b, off, len);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public long skip(final long n) throws IOException {
        return in.skip(n);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public int available() throws IOException {
        return in.available();
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void close() throws IOException {
        in.close();
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public synchronized void mark(final int readlimit) {
        in.mark(readlimit);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public synchronized void reset() throws IOException {
        in.reset();
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public boolean markSupported() {
        return in.markSupported();
    }

    private final PushbackInputStream in;

    private final BOM bom;

    private boolean skipped = false;

} // UnicodeBOMInputStream