package org.xbib.elasticsearch.plugin.bundle.index.analysis.icu;

/**
 * Provides support for converting byte sequences to Strings and back again.
 * The resulting Strings preserve the original byte sequences' sort order.
 *
 * The Strings are constructed using a Base 8000h encoding of the original
 * binary data - each char of an encoded String represents a 15-bit chunk
 * from the byte sequence.  Base 8000h was chosen because it allows for all
 * lower 15 bits of char to be used without restriction; the surrogate range
 * [U+D8000-U+DFFF] does not represent valid chars, and would require
 * complicated handling to avoid them and allow use of char's high bit.
 *
 * Although unset bits are used as padding in the final char, the original
 * byte sequence could contain trailing bytes with no set bits (null bytes):
 * padding is indistinguishable from valid information.  To overcome this
 * problem, a char is appended, indicating the number of encoded bytes in the
 * final content char.
 */
public final class IndexableBinaryStringTools {

    private static final CodingCase[] CODING_CASES = {
            // CodingCase(int initialShift, int finalShift)
            new CodingCase( 7, 1   ),
            // CodingCase(int initialShift, int middleShift, int finalShift)
            new CodingCase(14, 6, 2),
            new CodingCase(13, 5, 3),
            new CodingCase(12, 4, 4),
            new CodingCase(11, 3, 5),
            new CodingCase(10, 2, 6),
            new CodingCase( 9, 1, 7),
            new CodingCase( 8, 0   )
    };

    // Export only static methods
    private IndexableBinaryStringTools() {}

    /**
     * Returns the number of chars required to encode the given bytes.
     *
     * @param inputArray byte sequence to be encoded
     * @param inputOffset initial offset into inputArray
     * @param inputLength number of bytes in inputArray
     * @return The number of chars required to encode the number of bytes.
     */
    public static int getEncodedLength(byte[] inputArray, int inputOffset,
                                       int inputLength) {
        // Use long for intermediaries to protect against overflow
        return (int)((8L * inputLength + 14L) / 15L) + 1;
    }

    /**
     * Returns the number of bytes required to decode the given char sequence.
     *
     * @param encoded char sequence to be decoded
     * @param offset initial offset
     * @param length number of characters
     * @return The number of bytes required to decode the given char sequence
     */
    public static int getDecodedLength(char[] encoded, int offset, int length) {
        final int numChars = length - 1;
        if (numChars <= 0) {
            return 0;
        } else {
            // Use long for intermediaries to protect against overflow
            final long numFullBytesInFinalChar = encoded[offset + length - 1];
            final long numEncodedChars = numChars - 1;
            return (int)((numEncodedChars * 15L + 7L) / 8L + numFullBytesInFinalChar);
        }
    }

    /**
     * Encodes the input byte sequence into the output char sequence.  Before
     * calling this method, ensure that the output array has sufficient
     * capacity by calling {@link #getEncodedLength(byte[], int, int)}.
     *
     * @param inputArray byte sequence to be encoded
     * @param inputOffset initial offset into inputArray
     * @param inputLength number of bytes in inputArray
     * @param outputArray char sequence to store encoded result
     * @param outputOffset initial offset into outputArray
     * @param outputLength length of output, must be getEncodedLength
     */
    public static void encode(byte[] inputArray, int inputOffset,
                              int inputLength, char[] outputArray, int outputOffset, int outputLength) {
        assert (outputLength == getEncodedLength(inputArray, inputOffset,
                inputLength));
        if (inputLength > 0) {
            int inputByteNum = inputOffset;
            int caseNum = 0;
            int outputCharNum = outputOffset;
            CodingCase codingCase;
            for (; inputByteNum + CODING_CASES[caseNum].numBytes <= inputLength; ++outputCharNum) {
                codingCase = CODING_CASES[caseNum];
                if (2 == codingCase.numBytes) {
                    outputArray[outputCharNum] = (char) (((inputArray[inputByteNum] & 0xFF) << codingCase.initialShift)
                            + (((inputArray[inputByteNum + 1] & 0xFF) >>> codingCase.finalShift) &
                            codingCase.finalMask) & (short) 0x7FFF);
                } else { // numBytes is 3
                    outputArray[outputCharNum] = (char) (((inputArray[inputByteNum] & 0xFF) << codingCase.initialShift)
                            + ((inputArray[inputByteNum + 1] & 0xFF) << codingCase.middleShift)
                            + (((inputArray[inputByteNum + 2] & 0xFF) >>> codingCase.finalShift) &
                            codingCase.finalMask) & (short) 0x7FFF);
                }
                inputByteNum += codingCase.advanceBytes;
                if (++caseNum == CODING_CASES.length) {
                    caseNum = 0;
                }
            }
            // Produce final char (if any) and trailing count chars.
            codingCase = CODING_CASES[caseNum];

            if (inputByteNum + 1 < inputLength) { // codingCase.numBytes must be 3
                outputArray[outputCharNum++] = (char) ((((inputArray[inputByteNum] & 0xFF) << codingCase.initialShift) +
                        ((inputArray[inputByteNum + 1] & 0xFF) << codingCase.middleShift)) & (short) 0x7FFF);
                // Add trailing char containing the number of full bytes in final char
                outputArray[outputCharNum] = (char) 1;
            } else if (inputByteNum < inputLength) {
                outputArray[outputCharNum++] = (char) (((inputArray[inputByteNum] & 0xFF) << codingCase.initialShift) &
                        (short) 0x7FFF);
                // Add trailing char containing the number of full bytes in final char
                outputArray[outputCharNum] = caseNum == 0 ? (char) 1 : (char) 0;
            } else { // No left over bits - last char is completely filled.
                // Add trailing char containing the number of full bytes in final char
                outputArray[outputCharNum] = (char) 1;
            }
        }
    }

    /**
     * Decodes the input char sequence into the output byte sequence. Before
     * calling this method, ensure that the output array has sufficient capacity
     * by calling {@link #getDecodedLength(char[], int, int)}.
     *
     * @param inputArray char sequence to be decoded
     * @param inputOffset initial offset into inputArray
     * @param inputLength number of chars in inputArray
     * @param outputArray byte sequence to store encoded result
     * @param outputOffset initial offset into outputArray
     * @param outputLength length of output, must be
     *        getDecodedLength(inputArray, inputOffset, inputLength)
     */
    public static void decode(char[] inputArray, int inputOffset,
                              int inputLength, byte[] outputArray, int outputOffset, int outputLength) {
        assert (outputLength == getDecodedLength(inputArray, inputOffset,
                inputLength));
        final int numInputChars = inputLength - 1;

        if (outputLength > 0) {
            int caseNum = 0;
            int outputByteNum = outputOffset;
            int inputCharNum = inputOffset;
            short inputChar;
            CodingCase codingCase;
            for (; inputCharNum < numInputChars - 1; ++inputCharNum) {
                codingCase = CODING_CASES[caseNum];
                inputChar = (short) inputArray[inputCharNum];
                if (2 == codingCase.numBytes) {
                    if (0 == caseNum) {
                        outputArray[outputByteNum] = (byte) (inputChar >>> codingCase.initialShift);
                    } else {
                        outputArray[outputByteNum] += (byte) (inputChar >>> codingCase.initialShift);
                    }
                    outputArray[outputByteNum + 1] = (byte) ((inputChar & codingCase.finalMask) << codingCase.finalShift);
                } else { // numBytes is 3
                    outputArray[outputByteNum] += (byte) (inputChar >>> codingCase.initialShift);
                    outputArray[outputByteNum + 1] = (byte) ((inputChar & codingCase.middleMask) >>> codingCase.middleShift);
                    outputArray[outputByteNum + 2] = (byte) ((inputChar & codingCase.finalMask) << codingCase.finalShift);
                }
                outputByteNum += codingCase.advanceBytes;
                if (++caseNum == CODING_CASES.length) {
                    caseNum = 0;
                }
            }
            // Handle final char
            inputChar = (short) inputArray[inputCharNum];
            codingCase = CODING_CASES[caseNum];
            if (0 == caseNum) {
                outputArray[outputByteNum] = 0;
            }
            outputArray[outputByteNum] += (byte) (inputChar >>> codingCase.initialShift);
            final int bytesLeft = outputLength - outputByteNum;
            if (bytesLeft > 1) {
                if (2 == codingCase.numBytes) {
                    outputArray[outputByteNum + 1] = (byte) ((inputChar & codingCase.finalMask) >>> codingCase.finalShift);
                } else { // numBytes is 3
                    outputArray[outputByteNum + 1] = (byte) ((inputChar & codingCase.middleMask) >>> codingCase.middleShift);
                    if (bytesLeft > 2) {
                        outputArray[outputByteNum + 2] = (byte) ((inputChar & codingCase.finalMask) << codingCase.finalShift);
                    }
                }
            }
        }
    }

    static class CodingCase {
        int numBytes, initialShift, middleShift, finalShift, advanceBytes = 2;
        short middleMask, finalMask;

        CodingCase(int initialShift, int middleShift, int finalShift) {
            this.numBytes = 3;
            this.initialShift = initialShift;
            this.middleShift = middleShift;
            this.finalShift = finalShift;
            this.finalMask = (short)((short)0xFF >>> finalShift);
            this.middleMask = (short)((short)0xFF << middleShift);
        }

        CodingCase(int initialShift, int finalShift) {
            this.numBytes = 2;
            this.initialShift = initialShift;
            this.finalShift = finalShift;
            this.finalMask = (short)((short)0xFF >>> finalShift);
            if (finalShift != 0) {
                advanceBytes = 1;
            }
        }
    }
}