Fix slice_strings wide strings logic with multi-byte characters

cpp/src/strings/slice.cu

@@ -122,26 +122,28 @@ CUDF_KERNEL void substring_from_kernel(column_device_view const d_strings,
       break;
     }
     size_type const cc = (itr < end) && is_begin_utf8_char(*itr);
-    size_type const bc = (itr < end);
+    size_type const bc = (itr < end) ? bytes_in_utf8_byte(*itr) : 0;
     char_count += cg::reduce(warp, cc, cg::plus<int>());
     byte_count += cg::reduce(warp, bc, cg::plus<int>());
     itr += cudf::detail::warp_size;
   }
 
+  __syncwarp();
+
   if (warp.thread_rank() == 0) {
     if (start >= char_count) {
       d_output[str_idx] = string_index_pair{"", 0};
       return;
     }
 
-    // we are just below start/stop and must now increment up to it from here
+    // we are just below start/stop and must now increment up to them from here
     auto first_byte = start_counts.second;
     if (start_counts.first < start) {
       auto const sub_str = string_view(d_str.data() + first_byte, d_str.size_bytes() - first_byte);
       first_byte += std::get<0>(bytes_to_character_position(sub_str, start - start_counts.first));
     }
 
-    stop           = max(stop, char_count);
+    stop           = min(stop, char_count);
     auto last_byte = stop_counts.second;
     if (stop_counts.first < stop) {
       auto const sub_str = string_view(d_str.data() + last_byte, d_str.size_bytes() - last_byte);

cpp/tests/strings/slice_tests.cpp

@@ -268,6 +268,25 @@ TEST_F(StringsSliceTest, MaxPositions)
   CUDF_TEST_EXPECT_COLUMNS_EQUAL(*results, expected);
 }
 
+TEST_F(StringsSliceTest, MultiByteChars)
+{
+  auto input = cudf::test::strings_column_wrapper({
+    // clang-format off
+    "quick brown fox jumped over the lazy brown dog; the fat cats jump in place without moving "
+    "the following code snippet demonstrates how to use search for values in an ordered range  "
+            // this placement tests proper multi-byte chars handling  ------vvvvv
+    "it returns the last position where value could be inserted without the ééééé ordering ",
+    "algorithms execution is parallelized as determined by an execution policy; this is a 12345"
+    "continuation of previous row to make sure string boundaries are honored 012345678901234567"
+           //   v--- this one also
+    "01234567890é34567890012345678901234567890"
+    // clang-format on
+  });
+
+  auto results = cudf::strings::slice_strings(cudf::strings_column_view(input), 0);
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(*results, input);
+}
+
 TEST_F(StringsSliceTest, Error)
 {
   cudf::test::strings_column_wrapper strings{"this string intentionally left blank"};