[WIP][SQL] Return zero for instr with zero start and empty substring

common/unsafe/src/main/java/org/apache/spark/sql/catalyst/util/CollationAwareUTF8String.java

@@ -825,8 +825,8 @@ private static int lowercaseIndexOfSlow(final UTF8String target, final UTF8Strin
   public static int lowercaseIndexOf(final UTF8String target, final UTF8String pattern,
       final int start, final int occurrence) {
     assert occurrence > 0;
-    if (pattern.numBytes() == 0) return target.indexOfEmpty(start);
     if (start == 0) return MATCH_NOT_FOUND;
+    if (pattern.numBytes() == 0) return target.indexOfEmpty(start);
     if (target.isFullAscii() && pattern.isFullAscii()) {
       return target.toLowerCase().indexOf(pattern.toLowerCase(), start, occurrence);
     }
@@ -928,8 +928,9 @@ public static int indexOf(final UTF8String target, final UTF8String pattern,
   public static int indexOf(final UTF8String target, final UTF8String pattern,
       final int start, final int occurrence, final int collationId) {
     assert occurrence > 0;
+    if (start == 0) return MATCH_NOT_FOUND;
     if (pattern.numBytes() == 0) return target.indexOfEmpty(start);
-    if (target.numBytes() == 0 || start == 0) return MATCH_NOT_FOUND;
+    if (target.numBytes() == 0) return MATCH_NOT_FOUND;
 
     String targetStr = target.toValidString();
     String patternStr = pattern.toValidString();

common/unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java

@@ -1319,12 +1319,12 @@ public int indexOf(UTF8String v, int start) {
    */
   public int indexOf(UTF8String pattern, int start, int occurrence) {
     assert occurrence > 0;
-    if (pattern.numBytes() == 0) {
-      return indexOfEmpty(start);
-    }
     if (start == 0) {
       return -1;
     }
+    if (pattern.numBytes() == 0) {
+      return indexOfEmpty(start);
+    }
 
     int charCount = 0;
     int byteIdx;

common/unsafe/src/test/java/org/apache/spark/unsafe/types/CollationSupportSuite.java

@@ -4047,6 +4047,19 @@ private void assertStringInstrWithOccurrence(String string, String substring, in
     assertEquals(expected, res);
   }
 
+  @Test
+  public void testStringInstrWithOccurrenceZeroStart() throws SparkException {
+    for (String collationName : testSupportedCollations) {
+      for (String string : new String[]{"", "abc", "\u4F60\u597D"}) {
+        for (String substring : new String[]{"", "a"}) {
+          for (int occurrence : new int[]{1, 2}) {
+            assertStringInstrWithOccurrence(string, substring, 0, occurrence, collationName, 0);
+          }
+        }
+      }
+    }
+  }
+
   @Test
   public void testStringInstrWithOccurrence() throws SparkException {
     // Test start = 1 and occurrence = 1 (equivalent to StringInstr)

common/unsafe/src/test/java/org/apache/spark/unsafe/types/UTF8StringSuite.java

@@ -388,6 +388,10 @@ public void indexOf() {
     assertEquals(0, fromString("hello").indexOf(EMPTY_UTF8, 1, 1));
     assertEquals(0, fromString("hello").indexOf(EMPTY_UTF8, 5, 1));
     assertEquals(0, fromString("hello").indexOf(EMPTY_UTF8, -1, 1));
+    assertEquals(-1, fromString("hello").indexOf(EMPTY_UTF8, 0, 1));
+    assertEquals(-1, fromString("hello").indexOf(EMPTY_UTF8, 0, 2));
+    assertEquals(-1, EMPTY_UTF8.indexOf(EMPTY_UTF8, 0, 1));
+    assertEquals(-1, EMPTY_UTF8.indexOf(EMPTY_UTF8, 0, 2));
 
     // Boundary cases
     assertEquals(0, fromString("x").indexOf(fromString("x"), 1, 1));

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/StringExpressionsSuite.scala

@@ -1348,6 +1348,24 @@ class StringExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
     // scalastyle:on
   }
 
+  test("INSTR with start = 0 returns 0 for empty substrings") {
+    for (collation <- Seq("UTF8_BINARY", "UTF8_LCASE", "UNICODE", "UNICODE_CI")) {
+      val stringType = StringType(collation)
+      val expression = StringInstrWithOccurrence(
+        BoundReference(0, stringType, nullable = false),
+        BoundReference(1, stringType, nullable = false),
+        BoundReference(2, IntegerType, nullable = false),
+        BoundReference(3, IntegerType, nullable = false))
+      for {
+        string <- Seq("", "abc", "\u4F60\u597D")
+        substring <- Seq("", "a")
+        occurrence <- Seq(1, 2)
+      } {
+        checkEvaluation(expression, 0, create_row(string, substring, 0, occurrence))
+      }
+    }
+  }
+
   test("StringInstrExpressionBuilder") {
     val seq1 = Seq(Literal("abcabc"), Literal("a"), Literal(-1), Literal(2))
     val seq2 = Seq(Literal("abcabc"), Literal("a"), Literal(-1))