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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 java.util.Arrays;

import static ai.vespa.airlift.zstd.UnsafeUtil.UNSAFE;
import static sun.misc.Unsafe.ARRAY_BYTE_BASE_OFFSET;

class Histogram
{
    private Histogram()
    {
    }

    // TODO: count parallel heuristic for large inputs
    private static void count(Object inputBase, long inputAddress, int inputSize, int[] counts)
    {
        long input = inputAddress;

        Arrays.fill(counts, 0);

        for (int i = 0; i < inputSize; i++) {
            int symbol = UNSAFE.getByte(inputBase, input) & 0xFF;
            input++;
            counts[symbol]++;
        }
    }

    public static int findLargestCount(int[] counts, int maxSymbol)
    {
        int max = 0;
        for (int i = 0; i <= maxSymbol; i++) {
            if (counts[i] > max) {
                max = counts[i];
            }
        }

        return max;
    }

    public static int findMaxSymbol(int[] counts, int maxSymbol)
    {
        while (counts[maxSymbol] == 0) {
            maxSymbol--;
        }
        return maxSymbol;
    }

    public static void count(byte[] input, int length, int[] counts)
    {
        count(input, ARRAY_BYTE_BASE_OFFSET, length, counts);
    }
}