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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.UnsafeUtil.UNSAFE;
import static ai.vespa.airlift.zstd.Util.highestBit;
import static ai.vespa.airlift.zstd.Util.verify;

/**
 * Bit streams are encoded as a byte-aligned little-endian stream. Thus, bits are laid out
 * in the following manner, and the stream is read from right to left.
 * 
 * 
 * ... [16 17 18 19 20 21 22 23] [8 9 10 11 12 13 14 15] [0 1 2 3 4 5 6 7]
 */
class BitInputStream
{
    private BitInputStream()
    {
    }

    public static boolean isEndOfStream(long startAddress, long currentAddress, int bitsConsumed)
    {
        return startAddress == currentAddress && bitsConsumed == Long.SIZE;
    }

    @SuppressWarnings("fallthrough")
    static long readTail(Object inputBase, long inputAddress, int inputSize)
    {
        long bits = UNSAFE.getByte(inputBase, inputAddress) & 0xFF;

        switch (inputSize) {
            case 7:
                bits |= (UNSAFE.getByte(inputBase, inputAddress + 6) & 0xFFL) << 48;
            case 6:
                bits |= (UNSAFE.getByte(inputBase, inputAddress + 5) & 0xFFL) << 40;
            case 5:
                bits |= (UNSAFE.getByte(inputBase, inputAddress + 4) & 0xFFL) << 32;
            case 4:
                bits |= (UNSAFE.getByte(inputBase, inputAddress + 3) & 0xFFL) << 24;
            case 3:
                bits |= (UNSAFE.getByte(inputBase, inputAddress + 2) & 0xFFL) << 16;
            case 2:
                bits |= (UNSAFE.getByte(inputBase, inputAddress + 1) & 0xFFL) << 8;
        }

        return bits;
    }

    /**
     * @return numberOfBits in the low order bits of a long
     */
    public static long peekBits(int bitsConsumed, long bitContainer, int numberOfBits)
    {
        return (((bitContainer << bitsConsumed) >>> 1) >>> (63 - numberOfBits));
    }

    /**
     * numberOfBits must be > 0
     *
     * @return numberOfBits in the low order bits of a long
     */
    public static long peekBitsFast(int bitsConsumed, long bitContainer, int numberOfBits)
    {
        return ((bitContainer << bitsConsumed) >>> (64 - numberOfBits));
    }

    static class Initializer
    {
        private final Object inputBase;
        private final long startAddress;
        private final long endAddress;
        private long bits;
        private long currentAddress;
        private int bitsConsumed;

        public Initializer(Object inputBase, long startAddress, long endAddress)
        {
            this.inputBase = inputBase;
            this.startAddress = startAddress;
            this.endAddress = endAddress;
        }

        public long getBits()
        {
            return bits;
        }

        public long getCurrentAddress()
        {
            return currentAddress;
        }

        public int getBitsConsumed()
        {
            return bitsConsumed;
        }

        public void initialize()
        {
            verify(endAddress - startAddress >= 1, startAddress, "Bitstream is empty");

            int lastByte = UNSAFE.getByte(inputBase, endAddress - 1) & 0xFF;
            verify(lastByte != 0, endAddress, "Bitstream end mark not present");

            bitsConsumed = SIZE_OF_LONG - highestBit(lastByte);

            int inputSize = (int) (endAddress - startAddress);
            if (inputSize >= SIZE_OF_LONG) {  /* normal case */
                currentAddress = endAddress - SIZE_OF_LONG;
                bits = UNSAFE.getLong(inputBase, currentAddress);
            }
            else {
                currentAddress = startAddress;
                bits = readTail(inputBase, startAddress, inputSize);

                bitsConsumed += (SIZE_OF_LONG - inputSize) * 8;
            }
        }
    }

    static final class Loader
    {
        private final Object inputBase;
        private final long startAddress;
        private long bits;
        private long currentAddress;
        private int bitsConsumed;
        private boolean overflow;

        public Loader(Object inputBase, long startAddress, long currentAddress, long bits, int bitsConsumed)
        {
            this.inputBase = inputBase;
            this.startAddress = startAddress;
            this.bits = bits;
            this.currentAddress = currentAddress;
            this.bitsConsumed = bitsConsumed;
        }

        public long getBits()
        {
            return bits;
        }

        public long getCurrentAddress()
        {
            return currentAddress;
        }

        public int getBitsConsumed()
        {
            return bitsConsumed;
        }

        public boolean isOverflow()
        {
            return overflow;
        }

        public boolean load()
        {
            if (bitsConsumed > 64) {
                overflow = true;
                return true;
            }

            else if (currentAddress == startAddress) {
                return true;
            }

            int bytes = bitsConsumed >>> 3; // divide by 8
            if (currentAddress >= startAddress + SIZE_OF_LONG) {
                if (bytes > 0) {
                    currentAddress -= bytes;
                    bits = UNSAFE.getLong(inputBase, currentAddress);
                }
                bitsConsumed &= 0b111;
            }
            else if (currentAddress - bytes < startAddress) {
                bytes = (int) (currentAddress - startAddress);
                currentAddress = startAddress;
                bitsConsumed -= bytes * SIZE_OF_LONG;
                bits = UNSAFE.getLong(inputBase, startAddress);
                return true;
            }
            else {
                currentAddress -= bytes;
                bitsConsumed -= bytes * SIZE_OF_LONG;
                bits = UNSAFE.getLong(inputBase, currentAddress);
            }

            return false;
        }
    }
}