Add unrolled bit-packing kernels for longs (widths 1..63)

Branch-free unrolled fastpackwithoutmask/fastunpack for all widths,
mirroring the 32-bit BitPacking.

Measured on Graviton (aarch64), Corretto 21, median over widths 1..63:
pack 1.93x, unpack 2.66x.

Author: raunaqmorarka

src/main/java/me/lemire/longcompression/BenchmarkLongBitPacking.java

@@ -0,0 +1,87 @@
+/**
+ * This code is released under the
+ * Apache License Version 2.0 http://www.apache.org/licenses/.
+ *
+ * (c) Daniel Lemire, http://lemire.me/en/
+ */
+package me.lemire.longcompression;
+
+import java.text.DecimalFormat;
+import java.util.Random;
+
+/**
+ * Class used to benchmark the speed of long bit packing, comparing the
+ * generic and unrolled {@link LongBitPacking} kernels. (For expert use.)
+ *
+ * @author Daniel Lemire
+ *
+ */
+public class BenchmarkLongBitPacking {
+
+        private static void test(boolean verbose) {
+                DecimalFormat dfspeed = new DecimalFormat("0");
+                final int N = 64;
+                final int times = 100000;
+                Random r = new Random(0);
+                long[] data = new long[N];
+                long[] compressed = new long[N];
+                long[] uncompressed = new long[N];
+                for (int bit = 1; bit < 64; ++bit) {
+                        long mask = (1L << bit) - 1;
+                        long slowcomp = 0;
+                        long slowdecomp = 0;
+                        long fastcomp = 0;
+                        long fastdecomp = 0;
+                        for (int t = 0; t < times; ++t) {
+                                for (int k = 0; k < N; ++k) {
+                                        data[k] = r.nextLong() & mask;
+                                }
+                                long time1 = System.nanoTime();
+                                LongBitPacking.slowpackwithoutmask(data, 0,
+                                        compressed, 0, bit);
+                                long time2 = System.nanoTime();
+                                LongBitPacking.slowunpack(compressed, 0,
+                                        uncompressed, 0, bit);
+                                long time3 = System.nanoTime();
+                                LongBitPacking.fastpackwithoutmask(data, 0,
+                                        compressed, 0, bit);
+                                long time4 = System.nanoTime();
+                                LongBitPacking.fastunpack(compressed, 0,
+                                        uncompressed, 0, bit);
+                                long time5 = System.nanoTime();
+                                slowcomp += time2 - time1;
+                                slowdecomp += time3 - time2;
+                                fastcomp += time4 - time3;
+                                fastdecomp += time5 - time4;
+                        }
+                        if (verbose)
+                                System.out.println("bit = "
+                                        + bit
+                                        + " slow comp. speed = "
+                                        + dfspeed.format(N * times * 1000.0
+                                                / (slowcomp))
+                                        + " slow decomp. speed = "
+                                        + dfspeed.format(N * times * 1000.0
+                                                / (slowdecomp))
+                                        + " unrolled comp. speed = "
+                                        + dfspeed.format(N * times * 1000.0
+                                                / (fastcomp))
+                                        + " unrolled decomp. speed = "
+                                        + dfspeed.format(N * times * 1000.0
+                                                / (fastdecomp)));
+                }
+        }
+
+        /**
+         * Main method
+         *
+         * @param args
+         *                command-line arguments
+         */
+        public static void main(String[] args) {
+                System.out.println("Testing long packing (slow vs unrolled) ");
+                test(false);
+                test(true);
+        }
+
+}