Add JNI for strings::repeat_strings (#8491)

This PR adds JNI for `strings::repeat_strings`.

Authors:
  - Nghia Truong (https://github.com/ttnghia)

Approvers:
  - Robert (Bobby) Evans (https://github.com/revans2)
  - Gera Shegalov (https://github.com/gerashegalov)

URL: #8491

java/src/main/java/ai/rapids/cudf/ColumnView.java

@@ -2194,6 +2194,25 @@ public final ColumnVector stringConcatenateListElements(Scalar separator,
         emptyStringOutputIfEmptyList));
   }
 
+  /**
+   * Given a strings column, each string in the given column is repeated a number of times
+   * specified by the <code>repeatTimes</code> parameter. If the parameter has a non-positive value,
+   * all the rows of the output strings column will be an empty string. Any null row will result
+   * in a null row regardless of the value of <code>repeatTimes</code>.
+   *
+   * Note that this function cannot handle the cases when the size of the output column exceeds
+   * the maximum value that can be indexed by int type (i.e., {@link Integer#MAX_VALUE}).
+   * In such situations, the output result is undefined.
+   *
+   * @param repeatTimes The number of times each input string is copied to the output.
+   * @return A new java column vector containing repeated strings.
+   */
+  public final ColumnVector repeatStrings(int repeatTimes) {
+    assert type.equals(DType.STRING) : "column type must be a String";
+
+    return new ColumnVector(repeatStrings(getNativeView(), repeatTimes));
+  }
+
    /**
    * Apply a JSONPath string to all rows in an input strings column.
    *
@@ -2870,6 +2889,23 @@ private static native long stringConcatenationListElements(long listColumnHandle
                                                              boolean separateNulls,
                                                              boolean emptyStringOutputIfEmptyList);
 
+  /**
+   * Native method to repeat each string in the given strings column a number of times
+   * specified by the <code>repeatTimes</code> parameter. If the parameter has a non-positive value,
+   * all the rows of the output strings column will be an empty string. Any null row will result
+   * in a null row regardless of the value of <code>repeatTimes</code>.
+   *
+   * Note that this function cannot handle the cases when the size of the output column exceeds
+   * the maximum value that can be indexed by int type (i.e., {@link Integer#MAX_VALUE}).
+   * In such situations, the output result is undefined.
+   *
+   * @param viewHandle long holding the native handle of the column containing strings to repeat.
+   * @param repeatTimes The number of times each input string is copied to the output.
+   * @return native handle of the resulting cudf column containing repeated strings.
+   */
+  private static native long repeatStrings(long viewHandle, int repeatTimes);
+
+
   private static native long getJSONObject(long viewHandle, long scalarHandle) throws CudfException;
 
   /**

java/src/main/java/ai/rapids/cudf/Scalar.java

@@ -495,7 +495,7 @@ private static ColumnVector buildNullColumnVector(HostColumnVector.DataType host
   private static native long makeDecimal64Scalar(long value, int scale, boolean isValid);
   private static native long makeListScalar(long viewHandle, boolean isValid);
   private static native long makeStructScalar(long[] viewHandles, boolean isValid);
-
+  private static native long repeatString(long scalarHandle, int repeatTimes);
 
   Scalar(DType type, long scalarHandle) {
     this.type = type;
@@ -865,6 +865,20 @@ public String toString() {
     return sb.toString();
   }
 
+
+  /**
+   * Repeat the given string scalar a number of times specified by the <code>repeatTimes</code>
+   * parameter. If that parameter has a non-positive value, an empty (valid) string scalar will be
+   * returned. An invalid input scalar will always result in an invalid output scalar regardless
+   * of the value of <code>repeatTimes</code>.
+   *
+   * @param repeatTimes The number of times the input string is copied to the output.
+   * @return The resulting scalar containing repeated result of the current string.
+   */
+  public Scalar repeatString(int repeatTimes) {
+    return new Scalar(DType.STRING, repeatString(getScalarHandle(), repeatTimes));
+  }
+
   /**
    * Holds the off-heap state of the scalar so it can be cleaned up, even if it is leaked.
    */

java/src/main/native/src/ColumnViewJni.cpp

@@ -50,6 +50,7 @@
 #include <cudf/strings/extract.hpp>
 #include <cudf/strings/find.hpp>
 #include <cudf/strings/padding.hpp>
+#include <cudf/strings/repeat_strings.hpp>
 #include <cudf/strings/replace.hpp>
 #include <cudf/strings/replace_re.hpp>
 #include <cudf/strings/split/split.hpp>
@@ -1962,4 +1963,17 @@ JNIEXPORT jlong JNICALL Java_ai_rapids_cudf_ColumnView_stringConcatenationListEl
   CATCH_STD(env, 0);
 }
 
+JNIEXPORT jlong JNICALL Java_ai_rapids_cudf_ColumnView_repeatStrings(JNIEnv *env, jclass,
+                                                                     jlong column_handle,
+                                                                     jint repeat_times) {
+  JNI_NULL_CHECK(env, column_handle, "column handle is null", 0);
+  try {
+    cudf::jni::auto_set_device(env);
+    auto const cv = *reinterpret_cast<cudf::column_view *>(column_handle);
+    auto const strs_col = cudf::strings_column_view(cv);
+    return reinterpret_cast<jlong>(cudf::strings::repeat_strings(strs_col, repeat_times).release());
+  }
+  CATCH_STD(env, 0);
+}
+
 } // extern "C"

java/src/main/native/src/ScalarJni.cpp

@@ -17,6 +17,7 @@
 #include <cudf/binaryop.hpp>
 #include <cudf/fixed_point/fixed_point.hpp>
 #include <cudf/scalar/scalar_factories.hpp>
+#include <cudf/strings/repeat_strings.hpp>
 
 #include "cudf_jni_apis.hpp"
 #include "dtype_utils.hpp"
@@ -512,4 +513,15 @@ JNIEXPORT jlong JNICALL Java_ai_rapids_cudf_Scalar_makeStructScalar(JNIEnv *env,
   CATCH_STD(env, 0);
 }
 
+JNIEXPORT jlong JNICALL Java_ai_rapids_cudf_Scalar_repeatString(JNIEnv *env, jclass, jlong handle,
+                                                                jint repeat_times) {
+  JNI_NULL_CHECK(env, handle, "scalar handle is null", 0)
+  try {
+    cudf::jni::auto_set_device(env);
+    auto const str = *reinterpret_cast<cudf::string_scalar *>(handle);
+    return reinterpret_cast<jlong>(cudf::strings::repeat_strings(str, repeat_times).release());
+  }
+  CATCH_STD(env, 0);
+}
+
 } // extern "C"

java/src/test/java/ai/rapids/cudf/ColumnVectorTest.java

@@ -2621,6 +2621,42 @@ void testStringConcatWsSingleListColEmptyArrayReturnEmpty() {
     }
   }
 
+  @Test
+  void testRepeatStrings() {
+     // Empty strings column.
+    try (ColumnVector sv = ColumnVector.fromStrings("", "", "");
+         ColumnVector result = sv.repeatStrings(1)) {
+      assertColumnsAreEqual(sv, result);
+    }
+
+    // Zero repeatTimes.
+    try (ColumnVector sv = ColumnVector.fromStrings("abc", "xyz", "123");
+         ColumnVector result = sv.repeatStrings(0);
+         ColumnVector expected = ColumnVector.fromStrings("", "", "")) {
+      assertColumnsAreEqual(expected, result);
+    }
+
+    // Negative repeatTimes.
+    try (ColumnVector sv = ColumnVector.fromStrings("abc", "xyz", "123");
+         ColumnVector result = sv.repeatStrings(-1);
+         ColumnVector expected = ColumnVector.fromStrings("", "", "")) {
+      assertColumnsAreEqual(expected, result);
+    }
+
+    // Strings column containing both null and empty, output is copied exactly from input.
+    try (ColumnVector sv = ColumnVector.fromStrings("abc", "", null, "123", null);
+         ColumnVector result = sv.repeatStrings(1)) {
+      assertColumnsAreEqual(sv, result);
+    }
+
+    // Strings column containing both null and empty.
+    try (ColumnVector sv = ColumnVector.fromStrings("abc", "", null, "123", null);
+         ColumnVector result = sv.repeatStrings( 2);
+         ColumnVector expected = ColumnVector.fromStrings("abcabc", "", null, "123123", null)) {
+      assertColumnsAreEqual(expected, result);
+    }
+  }
+
   @Test
   void testListConcatByRow() {
     try (ColumnVector cv = ColumnVector.fromLists(new HostColumnVector.ListType(true,

java/src/test/java/ai/rapids/cudf/ScalarTest.java

@@ -18,18 +18,18 @@
 
 package ai.rapids.cudf;
 
-import static ai.rapids.cudf.TableTest.assertColumnsAreEqual;
-import static org.junit.jupiter.api.Assertions.*;
-
 import ai.rapids.cudf.HostColumnVector.BasicType;
-import ai.rapids.cudf.HostColumnVector.DataType;
 import ai.rapids.cudf.HostColumnVector.ListType;
-import ai.rapids.cudf.HostColumnVector.StructData;
 import ai.rapids.cudf.HostColumnVector.StructType;
+
+import org.junit.jupiter.api.Test;
+
 import java.math.BigDecimal;
 import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
-import org.junit.jupiter.api.Test;
+
+import static ai.rapids.cudf.TableTest.assertColumnsAreEqual;
+import static org.junit.jupiter.api.Assertions.*;
 
 public class ScalarTest extends CudfTestBase {
   @Test
@@ -405,4 +405,48 @@ public void testStruct() {
       }
     }
   }
+
+  @Test
+  public void testRepeatString() {
+    // Invalid scalar.
+    try (Scalar nullString = Scalar.fromString(null)) {
+      Scalar result = nullString.repeatString(5);
+      assertFalse(result.isValid());
+    }
+
+    // Empty string.
+    try (Scalar emptyString = Scalar.fromString("")) {
+      Scalar result = emptyString.repeatString(5);
+      assertTrue(result.isValid());
+      assertEquals("", result.getJavaString());
+    }
+
+    // Negative repeatTimes.
+    try (Scalar s = Scalar.fromString("Hello World");
+         Scalar result = s.repeatString(-100)) {
+      assertTrue(result.isValid());
+      assertEquals("", result.getJavaString());
+    }
+
+    // Zero repeatTimes.
+    try (Scalar s = Scalar.fromString("Hello World");
+         Scalar result = s.repeatString(0)) {
+      assertTrue(result.isValid());
+      assertEquals("", result.getJavaString());
+    }
+
+    // Trivial input, output is copied exactly from input.
+    try (Scalar s = Scalar.fromString("Hello World");
+         Scalar result = s.repeatString(1)) {
+      assertTrue(result.isValid());
+      assertEquals(s.getJavaString(), result.getJavaString());
+    }
+
+    // Trivial input.
+    try (Scalar s = Scalar.fromString("abcxyz-");
+         Scalar result = s.repeatString(3)) {
+      assertTrue(result.isValid());
+      assertEquals("abcxyz-abcxyz-abcxyz-", result.getJavaString());
+    }
+  }
 }