maartenbreddels commented on a change in pull request #7449:

File path: cpp/src/arrow/compute/kernels/
@@ -81,5 +147,40 @@ TYPED_TEST(TestStringKernels, 
StrptimeDoesNotProvideDefaultOptions) {
   ASSERT_RAISES(Invalid, CallFunction("strptime", {input}));
+TEST(TestStringKernels, UnicodeLibraryAssumptions) {
+  uint8_t output[4];
+  for (utf8proc_int32_t codepoint = 0x100; codepoint < 0x110000; codepoint++) {
+    utf8proc_ssize_t encoded_nbytes = utf8proc_encode_char(codepoint, output);
+    utf8proc_int32_t codepoint_upper = utf8proc_toupper(codepoint);
+    utf8proc_ssize_t encoded_nbytes_upper = 
utf8proc_encode_char(codepoint_upper, output);
+    if (encoded_nbytes == 2) {
+      EXPECT_LE(encoded_nbytes_upper, 3)
+          << "Expected the upper case codepoint for a 2 byte encoded codepoint 
to be "
+             "encoded in maximum 3 bytes, not "
+          << encoded_nbytes_upper;
+    }
+    if (encoded_nbytes == 3) {

Review comment:
       This code tests the assumption that the byte length growth for case 
mapping can only be at max 3/2, which is what we use for pre-allocation of the 
buffers. Unicode codepoints encoded in 4 bytes can only be 4 bytes or less when 
upper cased or lower cased, so we don't need to check them here. We can also 
skip the test that tests that case for `encoded_nbytes=3`, since it will not 
matter. I'll add a comment why we do the test.

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