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The UTF-16 decoder was incorrectly accepting low surrogates (#xdc00-#xdfff) as the first word of a surrogate pair. According to the UTF-16 specification, only high surrogates (#xd800-#xdbff) are valid as the first word. This fix changes the validation in both UTF-16 LE and BE decoders to only accept the correct range of high surrogates, properly rejecting invalid sequences where low surrogates appear as the first word. Also added comprehensive test cases to verify that low surrogate pairs are correctly rejected with encoding errors.
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Summary
This PR fixes a bug in the UTF-16 decoder where low surrogates (#xdc00-#xdfff) were incorrectly accepted as the first word of a surrogate pair. According to the UTF-16 specification, only high surrogates (#xd800-#xdbff) are valid as the first word.
Problem
The UTF-16 decoder in
decode.lispwas using the range check(<= #xd800 word #xdfff)to detect surrogate pairs. This range includes both:This allowed malformed UTF-16 data with unpaired low surrogates to be incorrectly decoded as valid surrogate pairs, potentially producing garbage Unicode code points.
Solution
Changed the validation in both UTF-16 LE and BE decoders (lines 366 and 397 in decode.lisp) from:
to:
This ensures only high surrogates are accepted as the first word of a surrogate pair.
Testing
Added 8 comprehensive test cases to
test/test.lispthat verify:All existing tests pass, and the new tests correctly fail before the fix and pass after.
Test Results
Before fix: 7 new tests failed (incorrectly decoded invalid data)
After fix: All tests pass (properly reject invalid sequences)
Files Changed
decode.lisp: Fixed surrogate validation in UTF-16 LE and BE decoders (2 lines)test/test.lisp: Added 8 test cases for low surrogate validation (14 lines)