alhudz opened a new pull request, #756:
URL: https://github.com/apache/commons-text/pull/756
Port of apache/commons-lang#1730 to the Commons Text copy of
`TextStringBuilder`, as requested by @garydgregory.
`TextStringBuilder.reverse()` swaps the buffer one `char` at a time, so
every surrogate pair is left in low-high order and a supplementary code point
becomes malformed UTF-16. `StringBuilder`/`StringBuffer`, which this class is
documented to mimic, reverse the same input correctly.
Repro: `new TextStringBuilder("a😀b").reverse().toString()` (`a`, `U+1F600`,
`b`).
Before: `b\uDE00\uD83Da`, a low surrogate ahead of its high surrogate.
After: `b😀a`, matching `new StringBuilder("a😀b").reverse()`.
Fix: after the char swap, walk the buffer once and swap each adjacent
low-high surrogate pair back to high-low, gated on whether any surrogate was
seen during the swap. BMP text, lone unpaired surrogates and odd-length buffers
are untouched.
Added `TextStringBuilderTest#testReverseSurrogatePairs`, which fails on the
current tree and passes with the fix.
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