Refactor common parts from the Rounding class into a separate 'round'…

… package (opensearch-project#11023)

* Refactor common parts from the Rounding class into a separate 'round' package

Signed-off-by: Ketan Verma <ketan9495@gmail.com>

* Move RoundableTests from :server to :libs:opensearch-common module

Signed-off-by: Ketan Verma <ketan9495@gmail.com>

* Address PR comments

Signed-off-by: Ketan Verma <ketan9495@gmail.com>

* Replace assert with IllegalArgumentException for size checks

Signed-off-by: Ketan Verma <ketan9495@gmail.com>

---------

Signed-off-by: Ketan Verma <ketan9495@gmail.com>

CHANGELOG.md

@@ -130,6 +130,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Add telemetry tracer/metric enable flag and integ test. ([#10395](https://github.com/opensearch-project/OpenSearch/pull/10395))
 - Add instrumentation for indexing in transport bulk action and transport shard bulk action. ([#10273](https://github.com/opensearch-project/OpenSearch/pull/10273))
 - [BUG] Disable sort optimization for HALF_FLOAT ([#10999](https://github.com/opensearch-project/OpenSearch/pull/10999))
+- Refactor common parts from the Rounding class into a separate 'round' package ([#11023](https://github.com/opensearch-project/OpenSearch/issues/11023))
 - Performance improvement for MultiTerm Queries on Keyword fields ([#7057](https://github.com/opensearch-project/OpenSearch/issues/7057))
 - Disable concurrent aggs for Diversified Sampler and Sampler aggs ([#11087](https://github.com/opensearch-project/OpenSearch/issues/11087))
 - Made leader/follower check timeout setting dynamic ([#10528](https://github.com/opensearch-project/OpenSearch/pull/10528))

benchmarks/src/main/java/org/opensearch/common/ArrayRoundingBenchmark.java → benchmarks/src/main/java/org/opensearch/common/round/RoundableBenchmark.java

@@ -6,7 +6,7 @@
  * compatible open source license.
  */
 
-package org.opensearch.common;
+package org.opensearch.common.round;
 
 import org.openjdk.jmh.annotations.Benchmark;
 import org.openjdk.jmh.annotations.BenchmarkMode;
@@ -27,13 +27,13 @@
 @Warmup(iterations = 3, time = 1)
 @Measurement(iterations = 1, time = 1)
 @BenchmarkMode(Mode.Throughput)
-public class ArrayRoundingBenchmark {
+public class RoundableBenchmark {
 
     @Benchmark
-    public void round(Blackhole bh, Options opts) {
-        Rounding.Prepared rounding = opts.supplier.get();
+    public void floor(Blackhole bh, Options opts) {
+        Roundable roundable = opts.supplier.get();
         for (long key : opts.queries) {
-            bh.consume(rounding.round(key));
+            bh.consume(roundable.floor(key));
         }
     }
 
@@ -90,7 +90,7 @@ public static class Options {
         public String distribution;
 
         public long[] queries;
-        public Supplier<Rounding.Prepared> supplier;
+        public Supplier<Roundable> supplier;
 
         @Setup
         public void setup() {
@@ -130,10 +130,10 @@ public void setup() {
 
             switch (type) {
                 case "binary":
-                    supplier = () -> new Rounding.BinarySearchArrayRounding(values, size, null);
+                    supplier = () -> new BinarySearcher(values, size);
                     break;
                 case "linear":
-                    supplier = () -> new Rounding.BidirectionalLinearSearchArrayRounding(values, size, null);
+                    supplier = () -> new BidirectionalLinearSearcher(values, size);
                     break;
                 default:
                     throw new IllegalArgumentException("invalid type: " + type);

libs/common/src/main/java/org/opensearch/common/round/BidirectionalLinearSearcher.java

@@ -0,0 +1,59 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.common.round;
+
+import org.opensearch.common.annotation.InternalApi;
+
+/**
+ * It uses linear search on a sorted array of pre-computed round-down points.
+ * For small inputs (≤ 64 elements), this can be much faster than binary search as it avoids the penalty of
+ * branch mispredictions and pipeline stalls, and accesses memory sequentially.
+ *
+ * <p>
+ * It uses "meet in the middle" linear search to avoid the worst case scenario when the desired element is present
+ * at either side of the array. This is helpful for time-series data where velocity increases over time, so more
+ * documents are likely to find a greater timestamp which is likely to be present on the right end of the array.
+ *
+ * @opensearch.internal
+ */
+@InternalApi
+class BidirectionalLinearSearcher implements Roundable {
+    private final long[] ascending;
+    private final long[] descending;
+
+    BidirectionalLinearSearcher(long[] values, int size) {
+        if (size <= 0) {
+            throw new IllegalArgumentException("at least one value must be present");
+        }
+
+        int len = (size + 1) >>> 1; // rounded-up to handle odd number of values
+        ascending = new long[len];
+        descending = new long[len];
+
+        for (int i = 0; i < len; i++) {
+            ascending[i] = values[i];
+            descending[i] = values[size - i - 1];
+        }
+    }
+
+    @Override
+    public long floor(long key) {
+        int i = 0;
+        for (; i < ascending.length; i++) {
+            if (descending[i] <= key) {
+                return descending[i];
+            }
+            if (ascending[i] > key) {
+                assert i > 0 : "key must be greater than or equal to " + ascending[0];
+                return ascending[i - 1];
+            }
+        }
+        return ascending[i - 1];
+    }
+}

libs/common/src/main/java/org/opensearch/common/round/BinarySearcher.java

@@ -0,0 +1,43 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.common.round;
+
+import org.opensearch.common.annotation.InternalApi;
+
+import java.util.Arrays;
+
+/**
+ * It uses binary search on a sorted array of pre-computed round-down points.
+ *
+ * @opensearch.internal
+ */
+@InternalApi
+class BinarySearcher implements Roundable {
+    private final long[] values;
+    private final int size;
+
+    BinarySearcher(long[] values, int size) {
+        if (size <= 0) {
+            throw new IllegalArgumentException("at least one value must be present");
+        }
+
+        this.values = values;
+        this.size = size;
+    }
+
+    @Override
+    public long floor(long key) {
+        int idx = Arrays.binarySearch(values, 0, size, key);
+        assert idx != -1 : "key must be greater than or equal to " + values[0];
+        if (idx < 0) {
+            idx = -2 - idx;
+        }
+        return values[idx];
+    }
+}

libs/common/src/main/java/org/opensearch/common/round/Roundable.java

@@ -0,0 +1,28 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.common.round;
+
+import org.opensearch.common.annotation.InternalApi;
+
+/**
+ * Interface to round-off values.
+ *
+ * @opensearch.internal
+ */
+@InternalApi
+@FunctionalInterface
+public interface Roundable {
+    /**
+     * Returns the greatest lower bound of the given key.
+     * In other words, it returns the largest value such that {@code value <= key}.
+     * @param key to floor
+     * @return the floored value
+     */
+    long floor(long key);
+}

libs/common/src/main/java/org/opensearch/common/round/RoundableFactory.java

@@ -0,0 +1,39 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.common.round;
+
+import org.opensearch.common.annotation.InternalApi;
+
+/**
+ * Factory class to create and return the fastest implementation of {@link Roundable}.
+ *
+ * @opensearch.internal
+ */
+@InternalApi
+public final class RoundableFactory {
+    /**
+     * The maximum limit up to which linear search is used, otherwise binary search is used.
+     * This is because linear search is much faster on small arrays.
+     * Benchmark results: <a href="https://github.com/opensearch-project/OpenSearch/pull/9727">PR #9727</a>
+     */
+    private static final int LINEAR_SEARCH_MAX_SIZE = 64;
+
+    private RoundableFactory() {}
+
+    /**
+     * Creates and returns the fastest implementation of {@link Roundable}.
+     */
+    public static Roundable create(long[] values, int size) {
+        if (size <= LINEAR_SEARCH_MAX_SIZE) {
+            return new BidirectionalLinearSearcher(values, size);
+        } else {
+            return new BinarySearcher(values, size);
+        }
+    }
+}

libs/common/src/main/java/org/opensearch/common/round/package-info.java

@@ -0,0 +1,12 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+/**
+ * Contains classes to round-off values.
+ */
+package org.opensearch.common.round;

libs/common/src/test/java/org/opensearch/common/round/RoundableTests.java

@@ -0,0 +1,57 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.common.round;
+
+import org.opensearch.test.OpenSearchTestCase;
+
+public class RoundableTests extends OpenSearchTestCase {
+
+    public void testFloor() {
+        int size = randomIntBetween(1, 256);
+        long[] values = new long[size];
+        for (int i = 1; i < values.length; i++) {
+            values[i] = values[i - 1] + (randomNonNegativeLong() % 200) + 1;
+        }
+
+        Roundable[] impls = { new BinarySearcher(values, size), new BidirectionalLinearSearcher(values, size) };
+
+        for (int i = 0; i < 100000; i++) {
+            // Index of the expected round-down point.
+            int idx = randomIntBetween(0, size - 1);
+
+            // Value of the expected round-down point.
+            long expected = values[idx];
+
+            // Delta between the expected and the next round-down point.
+            long delta = (idx < size - 1) ? (values[idx + 1] - values[idx]) : 200;
+
+            // Adding a random delta between 0 (inclusive) and delta (exclusive) to the expected
+            // round-down point, which will still floor to the same value.
+            long key = expected + (randomNonNegativeLong() % delta);
+
+            for (Roundable roundable : impls) {
+                assertEquals(expected, roundable.floor(key));
+            }
+        }
+    }
+
+    public void testFailureCases() {
+        Throwable throwable;
+
+        throwable = assertThrows(IllegalArgumentException.class, () -> new BinarySearcher(new long[0], 0));
+        assertEquals("at least one value must be present", throwable.getMessage());
+        throwable = assertThrows(IllegalArgumentException.class, () -> new BidirectionalLinearSearcher(new long[0], 0));
+        assertEquals("at least one value must be present", throwable.getMessage());
+
+        throwable = assertThrows(AssertionError.class, () -> new BinarySearcher(new long[] { 100 }, 1).floor(50));
+        assertEquals("key must be greater than or equal to 100", throwable.getMessage());
+        throwable = assertThrows(AssertionError.class, () -> new BidirectionalLinearSearcher(new long[] { 100 }, 1).floor(50));
+        assertEquals("key must be greater than or equal to 100", throwable.getMessage());
+    }
+}