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Zero-allocation implementations of fast non-cryptographic hash functions for byte sequences or blocks of memory

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package net.openhft.hashing;

import java.nio.ByteBuffer;
import sun.nio.ch.DirectBuffer;

import org.jetbrains.annotations.NotNull;

import static java.nio.ByteOrder.*;

final class Util {
    static final boolean NATIVE_LITTLE_ENDIAN = nativeOrder() == LITTLE_ENDIAN;

    /* Known java.vm.name list:
     *
     *   HotSpot:
     *   - Java HotSpot(TM) xx-Bit Server VM
     *   - OpenJDK xx-Bit Server VM
     *
     *   J9:
     *   - Eclipse OpenJ9 VM
     *   - IBM J9 VM
     */
    static private boolean isHotSpotVM(@NotNull final String name) {
        return name.contains("HotSpot") || name.contains("OpenJDK");
    }
    static private boolean isJ9VM(@NotNull final String name) {
        return name.contains("Eclipse OpenJ9") || name.contains("IBM J9");
    }

    @NotNull
    static final StringHash VALID_STRING_HASH;
    static  {
        StringHash stringHash = null;
        try {
            final String vmName = System.getProperty("java.vm.name");
            if (isHotSpotVM(vmName) || isJ9VM(vmName)) {
                final String javaVersion = System.getProperty("java.version");
                if (javaVersion.compareTo("1.7.0_06") >= 0) {
                    if (javaVersion.compareTo("1.9") >= 0) {
                        // JDK 9+
                        stringHash = ModernCompactStringHash.INSTANCE;
                    } else {
                        // JDK [1.7.0_06, 9)
                        stringHash = ModernHotSpotStringHash.INSTANCE;
                    }
                } else {
                    // JDK [1.7, 1.7.0_06)
                    stringHash = HotSpotPrior7u6StringHash.INSTANCE;
                }
            } else {
                // try to initialize this version anyway
                stringHash = HotSpotPrior7u6StringHash.INSTANCE;
            }
        } catch (final Throwable ignore) {
        } finally {
            if (null == stringHash) {
                VALID_STRING_HASH = UnknownJvmStringHash.INSTANCE;
            } else {
                VALID_STRING_HASH = stringHash;
            }
        }
    }

    static void checkArrayOffs(final int arrayLength, final int off, final int len) {
        if (len < 0 || off < 0 || off + len > arrayLength || off + len < 0)
            throw new IndexOutOfBoundsException();
    }

    static long getDirectBufferAddress(@NotNull final ByteBuffer buff) {
        return ((DirectBuffer)buff).address();
    }
}