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Fork of https://github.com/airlift/aircompressor (zstd only). This module is temporary until we get an official release that includes the ZstdInputStream API (which is already implemented by two different people but neither PR shows any progress).
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/*
 * Licensed 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 ai.vespa.airlift.zstd;

import static ai.vespa.airlift.zstd.Constants.SIZE_OF_LONG;
import static ai.vespa.airlift.zstd.Constants.SIZE_OF_SHORT;
import static ai.vespa.airlift.zstd.UnsafeUtil.UNSAFE;

class HuffmanCompressor
{
    private HuffmanCompressor()
    {
    }

    public static int compress4streams(Object outputBase, long outputAddress, int outputSize, Object inputBase, long inputAddress, int inputSize, HuffmanCompressionTable table)
    {
        long input = inputAddress;
        long inputLimit = inputAddress + inputSize;
        long output = outputAddress;
        long outputLimit = outputAddress + outputSize;

        int segmentSize = (inputSize + 3) / 4;

        if (outputSize < 6 /* jump table */ + 1 /* first stream */ + 1 /* second stream */ + 1 /* third stream */ + 8 /* 8 bytes minimum needed by the bitstream encoder */) {
            return 0; // minimum space to compress successfully
        }

        if (inputSize <= 6 + 1 + 1 + 1) { // jump table + one byte per stream
            return 0;  // no saving possible: input too small
        }

        output += SIZE_OF_SHORT + SIZE_OF_SHORT + SIZE_OF_SHORT; // jump table

        int compressedSize;

        // first segment
        compressedSize = compressSingleStream(outputBase, output, (int) (outputLimit - output), inputBase, input, segmentSize, table);
        if (compressedSize == 0) {
            return 0;
        }
        UNSAFE.putShort(outputBase, outputAddress, (short) compressedSize);
        output += compressedSize;
        input += segmentSize;

        // second segment
        compressedSize = compressSingleStream(outputBase, output, (int) (outputLimit - output), inputBase, input, segmentSize, table);
        if (compressedSize == 0) {
            return 0;
        }
        UNSAFE.putShort(outputBase, outputAddress + SIZE_OF_SHORT, (short) compressedSize);
        output += compressedSize;
        input += segmentSize;

        // third segment
        compressedSize = compressSingleStream(outputBase, output, (int) (outputLimit - output), inputBase, input, segmentSize, table);
        if (compressedSize == 0) {
            return 0;
        }
        UNSAFE.putShort(outputBase, outputAddress + SIZE_OF_SHORT + SIZE_OF_SHORT, (short) compressedSize);
        output += compressedSize;
        input += segmentSize;

        // fourth segment
        compressedSize = compressSingleStream(outputBase, output, (int) (outputLimit - output), inputBase, input, (int) (inputLimit - input), table);
        if (compressedSize == 0) {
            return 0;
        }
        output += compressedSize;

        return (int) (output - outputAddress);
    }

    @SuppressWarnings("fallthrough")
    public static int compressSingleStream(Object outputBase, long outputAddress, int outputSize, Object inputBase, long inputAddress, int inputSize, HuffmanCompressionTable table)
    {
        if (outputSize < SIZE_OF_LONG) {
            return 0;
        }

        BitOutputStream bitstream = new BitOutputStream(outputBase, outputAddress, outputSize);
        long input = inputAddress;

        int n = inputSize & ~3; // join to mod 4

        switch (inputSize & 3) {
            case 3:
                table.encodeSymbol(bitstream, UNSAFE.getByte(inputBase, input + n + 2) & 0xFF);
                if (SIZE_OF_LONG * 8 < Huffman.MAX_TABLE_LOG * 4 + 7) {
                    bitstream.flush();
                }
                // fall-through
            case 2:
                table.encodeSymbol(bitstream, UNSAFE.getByte(inputBase, input + n + 1) & 0xFF);
                if (SIZE_OF_LONG * 8 < Huffman.MAX_TABLE_LOG * 2 + 7) {
                    bitstream.flush();
                }
                // fall-through
            case 1:
                table.encodeSymbol(bitstream, UNSAFE.getByte(inputBase, input + n + 0) & 0xFF);
                bitstream.flush();
                // fall-through
            case 0: /* fall-through */
            default:
                break;
        }

        for (; n > 0; n -= 4) {  // note: n & 3 == 0 at this stage
            table.encodeSymbol(bitstream, UNSAFE.getByte(inputBase, input + n - 1) & 0xFF);
            if (SIZE_OF_LONG * 8 < Huffman.MAX_TABLE_LOG * 2 + 7) {
                bitstream.flush();
            }
            table.encodeSymbol(bitstream, UNSAFE.getByte(inputBase, input + n - 2) & 0xFF);
            if (SIZE_OF_LONG * 8 < Huffman.MAX_TABLE_LOG * 4 + 7) {
                bitstream.flush();
            }
            table.encodeSymbol(bitstream, UNSAFE.getByte(inputBase, input + n - 3) & 0xFF);
            if (SIZE_OF_LONG * 8 < Huffman.MAX_TABLE_LOG * 2 + 7) {
                bitstream.flush();
            }
            table.encodeSymbol(bitstream, UNSAFE.getByte(inputBase, input + n - 4) & 0xFF);
            bitstream.flush();
        }

        return bitstream.close();
    }
}