add SplitLongs function

virtdata-lib-basics/src/main/java/io/nosqlbench/virtdata/library/basics/shared/from_long/to_long/PartitionLongs.java

@@ -0,0 +1,55 @@
+package io.nosqlbench.virtdata.library.basics.shared.from_long.to_long;
+
+import io.nosqlbench.virtdata.api.annotations.Categories;
+import io.nosqlbench.virtdata.api.annotations.Category;
+import io.nosqlbench.virtdata.api.annotations.ThreadSafeMapper;
+
+import java.util.function.LongUnaryOperator;
+
+/**
+ * Split the value range of Java longs into a number of offsets,
+ * starting with Long.MIN_VALUE. This method makes it easy to construct
+ * a set of offsets for testing, or to limit the values used a subset.
+ * The outputs will range from Long.MIN_VALUE (-2^63) up.
+ *
+ * This is not an exactly emulation of token range splits in Apache Cassandra.
+ */
+@ThreadSafeMapper
+@Categories({Category.general})
+public class PartitionLongs implements LongUnaryOperator {
+
+    // Number of partitions to split the value range into
+    private final int partitions;
+    private final long[] starts;
+
+    public PartitionLongs(int partitions) {
+        this.partitions = partitions;
+        if (partitions < 1) {
+            throw new RuntimeException("partitions must be at least 1");
+        } else if (partitions == 1) {
+            this.starts = new long[]{Long.MIN_VALUE};
+        } else {
+            this.starts = new long[partitions];
+
+            // because the full interval is signed and Long.MIN_VALUE is 0x8000.0000.0000.0000
+            long half_interval = Long.MIN_VALUE / partitions;
+
+            for (int i = 0; i < starts.length; i++) {
+                // scaling the interval to whole interval would overflow for 2 partitions
+                starts[i] = Long.MIN_VALUE + ((i * half_interval) + (i * half_interval));
+            }
+        }
+    }
+
+    /**
+     * This method uses a divisor value with a maximum value of 2^63
+     *
+     * @param operand
+     * @return
+     */
+    @Override
+    public long applyAsLong(long operand) {
+        int idx = (int) (operand % starts.length);
+        return starts[idx];
+    }
+}

virtdata-lib-basics/src/test/java/io/nosqlbench/virtdata/library/basics/shared/from_long/to_long/PartitionLongsTest.java

@@ -0,0 +1,26 @@
+package io.nosqlbench.virtdata.library.basics.shared.from_long.to_long;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import java.util.HashSet;
+import java.util.Set;
+
+public class PartitionLongsTest {
+
+    @Test
+    void testInvalidInitializerValues() {
+        Assertions.assertThrows(RuntimeException.class, () -> new PartitionLongs(-3));
+        Assertions.assertThrows(RuntimeException.class, () -> new PartitionLongs(0));
+    }
+
+    @Test
+    public void testValueCardinality() {
+        PartitionLongs f = new PartitionLongs(15);
+        Set<Long> values = new HashSet<Long>();
+        for (int i = 0; i < 100; i++) {
+            values.add(f.applyAsLong(i));
+        }
+        Assertions.assertEquals(15, values.size());
+    }
+}