/*
 *  Licensed to the Apache Software Foundation (ASF) under one
 *  or more contributor license agreements.  See the NOTICE file
 *  distributed with this work for additional information
 *  regarding copyright ownership.  The ASF 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 groovy.util;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.LineNumberReader;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.util.Collection;

/**
 * Utility class to guess the encoding of a given text file.
 * <p>
 * Unicode files encoded in UTF-16 (low or big endian) or UTF-8 files
 * with a Byte Order Marker are correctly discovered. For UTF-8 files with no BOM, if the buffer
 * is wide enough, the charset should also be discovered.
 * <p>
 * A byte buffer of 4KB is used to be able to guess the encoding.
 * <p>
 * Usage:
 * <pre>
 * CharsetToolkit toolkit = new CharsetToolkit(file);
 *
 * // guess the encoding
 * Charset guessedCharset = toolkit.getCharset();
 *
 * // create a reader with the correct charset
 * BufferedReader reader = toolkit.getReader();
 *
 * // read the file content
 * String line;
 * while ((line = br.readLine())!= null)
 * {
 *     System.out.println(line);
 * }
 * </pre>
 */
public class CharsetToolkit {
    private static final Charset[] EMPTY_CHARSET_ARRAY = new Charset[0];
    private final byte[] buffer;
    private Charset defaultCharset;
    private Charset charset;
    private boolean enforce8Bit = true;
    private final File file;
    private static final byte[] EMPTY_BYTE_ARRAY = new byte[0];

    /**
     * Constructor of the <code>CharsetToolkit</code> utility class.
     *
     * @param file of which we want to know the encoding.
     */
    public CharsetToolkit(File file) throws IOException {
        this.file = file;
        this.defaultCharset = getDefaultSystemCharset();
        this.charset = null;
        try (InputStream input = new FileInputStream(file)) {
            byte[] bytes = new byte[4096];
            int bytesRead = input.read(bytes);
            if (bytesRead == -1) {
                this.buffer = EMPTY_BYTE_ARRAY;
            } else if (bytesRead < 4096) {
                byte[] bytesToGuess = new byte[bytesRead];
                System.arraycopy(bytes, 0, bytesToGuess, 0, bytesRead);
                this.buffer = bytesToGuess;
            } else {
                this.buffer = bytes;
            }
        }
    }

    /**
     * Defines the default <code>Charset</code> used in case the buffer represents
     * an 8-bit <code>Charset</code>.
     *
     * @param defaultCharset the default <code>Charset</code> to be returned
     * if an 8-bit <code>Charset</code> is encountered.
     */
    public void setDefaultCharset(Charset defaultCharset) {
        if (defaultCharset != null)
            this.defaultCharset = defaultCharset;
        else
            this.defaultCharset = getDefaultSystemCharset();
    }

    public Charset getCharset() {
        if (this.charset == null)
            this.charset = guessEncoding();
        return charset;
    }

    /**
     * If US-ASCII is recognized, enforce to return the default encoding, rather than US-ASCII.
     * It might be a file without any special character in the range 128-255, but that may be or become
     * a file encoded with the default <code>charset</code> rather than US-ASCII.
     *
     * @param enforce a boolean specifying the use or not of US-ASCII.
     */
    public void setEnforce8Bit(boolean enforce) {
        this.enforce8Bit = enforce;
    }

    /**
     * Gets the enforce8Bit flag, in case we do not want to ever get a US-ASCII encoding.
     *
     * @return a boolean representing the flag of use of US-ASCII.
     */
    public boolean getEnforce8Bit() {
        return this.enforce8Bit;
    }

    /**
     * Retrieves the default Charset
     */
    public Charset getDefaultCharset() {
        return defaultCharset;
    }

    /**
     * Guess the encoding of the provided buffer.
     * If Byte Order Markers are encountered at the beginning of the buffer, we immediately
     * return the charset implied by this BOM. Otherwise, the file would not be a human
     * readable text file.
     * <p>
     * If there is no BOM, this method tries to discern whether the file is UTF-8 or not.
     * If it is not UTF-8, we assume the encoding is the default system encoding
     * (of course, it might be any 8-bit charset, but usually, an 8-bit charset is the default one).
     * <p>
     * It is possible to discern UTF-8 thanks to the pattern of characters with a multi-byte sequence.
     * <pre>
     * UCS-4 range (hex.)        UTF-8 octet sequence (binary)
     * 0000 0000-0000 007F       0xxxxxxx
     * 0000 0080-0000 07FF       110xxxxx 10xxxxxx
     * 0000 0800-0000 FFFF       1110xxxx 10xxxxxx 10xxxxxx
     * 0001 0000-001F FFFF       11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
     * 0020 0000-03FF FFFF       111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
     * 0400 0000-7FFF FFFF       1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
     * </pre>
     * With UTF-8, 0xFE and 0xFF never appear.
     *
     * @return the Charset recognized.
     */
    private Charset guessEncoding() {
        // if the file has a Byte Order Marker, we can assume the file is in UTF-xx
        // otherwise, the file would not be human readable
        if (hasUTF8Bom())
            return StandardCharsets.UTF_8;
        if (hasUTF16LEBom())
            return StandardCharsets.UTF_16LE;
        if (hasUTF16BEBom())
            return StandardCharsets.UTF_16BE;

        // if a byte has its most significant bit set, the file is in UTF-8 or in the default encoding
        // otherwise, the file is in US-ASCII
        boolean highOrderBit = false;

        // if the file is in UTF-8, high order bytes must have a certain value, in order to be valid
        // if it's not the case, we can assume the encoding is the default encoding of the system
        boolean validU8Char = true;

        // TODO the buffer is not read up to the end, but up to length - 6

        int length = buffer.length;
        int i = 0;
        while (i < length - 6) {
            byte b0 = buffer[i];
            byte b1 = buffer[i + 1];
            byte b2 = buffer[i + 2];
            byte b3 = buffer[i + 3];
            byte b4 = buffer[i + 4];
            byte b5 = buffer[i + 5];
            if (b0 < 0) {
                // a high order bit was encountered, thus the encoding is not US-ASCII
                // it may be either an 8-bit encoding or UTF-8
                highOrderBit = true;
                // a two-bytes sequence was encountered
                if (isTwoBytesSequence(b0)) {
                    // there must be one continuation byte of the form 10xxxxxx,
                    // otherwise the following character is is not a valid UTF-8 construct
                    if (!isContinuationChar(b1))
                        validU8Char = false;
                    else
                        i++;
                }
                // a three-bytes sequence was encountered
                else if (isThreeBytesSequence(b0)) {
                    // there must be two continuation bytes of the form 10xxxxxx,
                    // otherwise the following character is is not a valid UTF-8 construct
                    if (!(isContinuationChar(b1) && isContinuationChar(b2)))
                        validU8Char = false;
                    else
                        i += 2;
                }
                // a four-bytes sequence was encountered
                else if (isFourBytesSequence(b0)) {
                    // there must be three continuation bytes of the form 10xxxxxx,
                    // otherwise the following character is is not a valid UTF-8 construct
                    if (!(isContinuationChar(b1) && isContinuationChar(b2) && isContinuationChar(b3)))
                        validU8Char = false;
                    else
                        i += 3;
                }
                // a five-bytes sequence was encountered
                else if (isFiveBytesSequence(b0)) {
                    // there must be four continuation bytes of the form 10xxxxxx,
                    // otherwise the following character is is not a valid UTF-8 construct
                    if (!(isContinuationChar(b1)
                        && isContinuationChar(b2)
                        && isContinuationChar(b3)
                        && isContinuationChar(b4)))
                        validU8Char = false;
                    else
                        i += 4;
                }
                // a six-bytes sequence was encountered
                else if (isSixBytesSequence(b0)) {
                    // there must be five continuation bytes of the form 10xxxxxx,
                    // otherwise the following character is is not a valid UTF-8 construct
                    if (!(isContinuationChar(b1)
                        && isContinuationChar(b2)
                        && isContinuationChar(b3)
                        && isContinuationChar(b4)
                        && isContinuationChar(b5)))
                        validU8Char = false;
                    else
                        i += 5;
                }
                else
                    validU8Char = false;
            }
            if (!validU8Char)
                break;
            i++;
        }
        // if no byte with an high order bit set, the encoding is US-ASCII
        // (it might have been UTF-7, but this encoding is usually internally used only by mail systems)
        if (!highOrderBit) {
            // returns the default charset rather than US-ASCII if the enforce8Bit flag is set.
            if (this.enforce8Bit)
                return this.defaultCharset;
            else
                return StandardCharsets.US_ASCII;
        }
        // if no invalid UTF-8 were encountered, we can assume the encoding is UTF-8,
        // otherwise the file would not be human readable
        if (validU8Char)
            return StandardCharsets.UTF_8;
        // finally, if it's not UTF-8 nor US-ASCII, let's assume the encoding is the default encoding
        return this.defaultCharset;
    }

    /**
     * If the byte has the form 10xxxxx, then it's a continuation byte of a multiple byte character;
     *
     * @param b a byte.
     * @return true if it's a continuation char.
     */
    private static boolean isContinuationChar(byte b) {
        return b <= -65;
    }

    /**
     * If the byte has the form 110xxxx, then it's the first byte of a two-bytes sequence character.
     *
     * @param b a byte.
     * @return true if it's the first byte of a two-bytes sequence.
     */
    private static boolean isTwoBytesSequence(byte b) {
        return -64 <= b && b <= -33;
    }

    /**
     * If the byte has the form 1110xxx, then it's the first byte of a three-bytes sequence character.
     *
     * @param b a byte.
     * @return true if it's the first byte of a three-bytes sequence.
     */
    private static boolean isThreeBytesSequence(byte b) {
        return -32 <= b && b <= -17;
    }

    /**
     * If the byte has the form 11110xx, then it's the first byte of a four-bytes sequence character.
     *
     * @param b a byte.
     * @return true if it's the first byte of a four-bytes sequence.
     */
    private static boolean isFourBytesSequence(byte b) {
        return -16 <= b && b <= -9;
    }

    /**
     * If the byte has the form 11110xx, then it's the first byte of a five-bytes sequence character.
     *
     * @param b a byte.
     * @return true if it's the first byte of a five-bytes sequence.
     */
    private static boolean isFiveBytesSequence(byte b) {
        return -8 <= b && b <= -5;
    }

    /**
     * If the byte has the form 1110xxx, then it's the first byte of a six-bytes sequence character.
     *
     * @param b a byte.
     * @return true if it's the first byte of a six-bytes sequence.
     */
    private static boolean isSixBytesSequence(byte b) {
        return -4 <= b && b <= -3;
    }

    /**
     * Retrieve the default charset of the system.
     *
     * @return the default <code>Charset</code>.
     */
    public static Charset getDefaultSystemCharset() {
        return Charset.forName(System.getProperty("file.encoding"));
    }

    /**
     * Has a Byte Order Marker for UTF-8 (Used by Microsoft's Notepad and other editors).
     *
     * @return true if the buffer has a BOM for UTF8.
     */
    public boolean hasUTF8Bom() {
        if (buffer.length >= 3)
            return (buffer[0] == -17 && buffer[1] == -69 && buffer[2] == -65);
        else
            return false;
    }

    /**
     * Has a Byte Order Marker for UTF-16 Low Endian
     * (ucs-2le, ucs-4le, and ucs-16le).
     *
     * @return true if the buffer has a BOM for UTF-16 Low Endian.
     */
    public boolean hasUTF16LEBom() {
        if (buffer.length >= 2)
            return (buffer[0] == -1 && buffer[1] == -2);
        else
            return false;
    }

    /**
     * Has a Byte Order Marker for UTF-16 Big Endian
     * (utf-16 and ucs-2).
     *
     * @return true if the buffer has a BOM for UTF-16 Big Endian.
     */
    public boolean hasUTF16BEBom() {
        if (buffer.length >= 2)
            return (buffer[0] == -2 && buffer[1] == -1);
        else
            return false;
    }

    /**
     * Gets a <code>BufferedReader</code> (indeed a <code>LineNumberReader</code>) from the <code>File</code>
     * specified in the constructor of <code>CharsetToolkit</code> using the charset discovered or the default
     * charset if an 8-bit <code>Charset</code> is encountered.
     *
     * @return a <code>BufferedReader</code>
     * @throws FileNotFoundException if the file is not found.
     */
    public BufferedReader getReader() throws FileNotFoundException {
        LineNumberReader reader = new LineNumberReader(new InputStreamReader(new FileInputStream(file), getCharset()));
        if (hasUTF8Bom() || hasUTF16LEBom() || hasUTF16BEBom()) {
            try {
                reader.read();
            }
            catch (IOException e) {
                // should never happen, as a file with no content
                // but with a BOM has at least one char
            }
        }
        return reader;
    }

    /**
     * Retrieves all the available <code>Charset</code>s on the platform,
     * among which the default <code>charset</code>.
     *
     * @return an array of <code>Charset</code>s.
     */
    public static Charset[] getAvailableCharsets() {
        Collection collection = Charset.availableCharsets().values();
        return (Charset[]) collection.toArray(EMPTY_CHARSET_ARRAY);
    }
}
