milesgranger commented on code in PR #14395:
URL: https://github.com/apache/arrow/pull/14395#discussion_r1016288306


##########
cpp/src/arrow/compute/kernels/scalar_nested_test.cc:
##########
@@ -116,6 +117,125 @@ TEST(TestScalarNested, ListElementInvalid) {
               Raises(StatusCode::Invalid));
 }
 
+TEST(TestScalarNested, ListSliceVariableOutput) {
+  const auto value_types = {float32(), int32()};
+  for (auto value_type : value_types) {
+    /* Variable list size output required variable size list input. */
+    auto inputs = {ArrayFromJSON(list(value_type), "[[1, 2, 3], [4, 5], [6], 
null]")};
+    for (auto input : inputs) {
+      ListSliceOptions args(/*start=*/0, /*stop=*/2, /*step=*/1,
+                            /*return_fixed_size_list=*/false);
+      auto expected = ArrayFromJSON(list(value_type), "[[1, 2], [4, 5], [6], 
null]");
+      CheckScalarUnary("list_slice", input, expected, &args);
+
+      args.start = 1;
+      expected = ArrayFromJSON(list(value_type), "[[2], [5], [], null]");
+      CheckScalarUnary("list_slice", input, expected, &args);
+
+      args.start = 2;
+      args.stop = 4;
+      expected = ArrayFromJSON(list(value_type), "[[3], [], [], null]");
+      CheckScalarUnary("list_slice", input, expected, &args);
+    }
+  }
+
+  // Verify passing `return_fixed_size_list=false` with fixed size input
+  // returns variable size even if stop is beyond list_size
+  ListSliceOptions args(/*start=*/0, /*stop=*/2, /*step=*/1,
+                        /*return_fixed_size_list=*/false);
+  auto input = ArrayFromJSON(fixed_size_list(int32(), 1), "[[1]]");
+  auto expected = ArrayFromJSON(list(int32()), "[[1]]");
+  CheckScalarUnary("list_slice", input, expected, &args);
+}
+
+TEST(TestScalarNested, ListSliceFixedOutput) {
+  const auto value_types = {float32(), int32()};
+  for (auto value_type : value_types) {
+    auto inputs = {ArrayFromJSON(list(value_type), "[[1, 2, 3], [4, 5], [6], 
null]"),
+                   ArrayFromJSON(fixed_size_list(value_type, 3),
+                                 "[[1, 2, 3], [4, 5, null], [6, null, null], 
null]")};
+    for (auto input : inputs) {
+      ListSliceOptions args(/*start=*/0, /*stop=*/2, /*step=*/1,
+                            /*return_fixed_size_list=*/true);
+      auto expected = ArrayFromJSON(fixed_size_list(value_type, 2),
+                                    "[[1, 2], [4, 5], [6, null], null]");
+      CheckScalarUnary("list_slice", input, expected, &args);
+
+      args.start = 1;
+      expected =
+          ArrayFromJSON(fixed_size_list(value_type, 1), "[[2], [5], [null], 
null]");
+      CheckScalarUnary("list_slice", input, expected, &args);
+
+      args.start = 2;
+      args.stop = 4;
+      expected = ArrayFromJSON(fixed_size_list(value_type, 2),
+                               "[[3, null], [null, null], [null, null], 
null]");
+      CheckScalarUnary("list_slice", input, expected, &args);
+    }
+  }
+}
+
+TEST(TestScalarNested, ListSliceOutputEqualsInputType) {
+  // Default is to return same type as the one passed in.
+  auto inputs = {
+      ArrayFromJSON(list(int8()), "[[1, 2, 3], [4, 5], [6, null], null]"),
+      ArrayFromJSON(large_list(int8()), "[[1, 2, 3], [4, 5], [6, null], 
null]"),
+      ArrayFromJSON(fixed_size_list(int8(), 2), "[[1, 2], [4, 5], [6, null], 
null]")};
+  for (auto input : inputs) {
+    ListSliceOptions args(/*start=*/0, /*stop=*/2, /*step=*/1);
+    auto expected = ArrayFromJSON(input->type(), "[[1, 2], [4, 5], [6, null], 
null]");
+    CheckScalarUnary("list_slice", input, expected, &args);
+  }
+}
+
+TEST(TestScalarNested, ListSliceBadParameters) {
+  auto input = ArrayFromJSON(list(int32()), "[[1]]");
+
+  // negative start
+  ListSliceOptions args(/*start=*/-1, /*stop=*/1, /*step=*/1,
+                        /*return_fixed_size_list=*/true);
+  EXPECT_RAISES_WITH_MESSAGE_THAT(
+      Invalid,
+      ::testing::HasSubstr(
+          "`start`(-1) should be greater than 0 and smaller than `stop`(1)"),
+      CallFunction("list_slice", {input}, &args));
+  // start greater than stop
+  args.start = 1;
+  args.stop = 0;
+  EXPECT_RAISES_WITH_MESSAGE_THAT(
+      Invalid,
+      ::testing::HasSubstr(
+          "`start`(1) should be greater than 0 and smaller than `stop`(0)"),
+      CallFunction("list_slice", {input}, &args));
+  // start same as stop
+  args.stop = args.start;
+  EXPECT_RAISES_WITH_MESSAGE_THAT(
+      Invalid,
+      ::testing::HasSubstr(
+          "`start`(1) should be greater than 0 and smaller than `stop`(1)"),
+      CallFunction("list_slice", {input}, &args));
+  // stop not set and FixedSizeList requested
+  args.stop = std::nullopt;
+  EXPECT_RAISES_WITH_MESSAGE_THAT(
+      NotImplemented,
+      ::testing::HasSubstr("NotImplemented: Unable to produce 
FixedSizeListArray without "
+                           "`stop` being set."),
+      CallFunction("list_slice", {input}, &args));
+  // stop not set and ListArray requested
+  args.stop = std::nullopt;
+  args.return_fixed_size_list = false;
+  EXPECT_RAISES_WITH_MESSAGE_THAT(
+      NotImplemented, ::testing::HasSubstr("Slicing to end not yet 
implemented"),
+      CallFunction("list_slice", {input}, &args));
+  // step other than `1` not implmented
+  args.stop = 2;
+  args.step = 2;
+  EXPECT_RAISES_WITH_MESSAGE_THAT(
+      NotImplemented,
+      ::testing::HasSubstr("Setting `step` to anything other than 1 is not 
supported"),
+      CallFunction("list_slice", {input}, &args));
+}
+

Review Comment:
   :man_facepalming: but of course, that makes more sense. Thank you for that. 
Updated in 
https://github.com/apache/arrow/pull/14395/commits/dcff4d1e8e098d3fe47f7de6ecf1f1eae65b210b



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