alhudz opened a new pull request, #1713:
URL: https://github.com/apache/commons-lang/pull/1713
`Repro:` `getCommonPrefix("𐐀", "𐐁")` returns the lone high surrogate
`\uD801`, and `difference("𐐀", "𐐁")` returns the lone low surrogate `\uDC01`.
Both are malformed UTF-16.
`Cause:` both `indexOfDifference` overloads walk the inputs one `char` at a
time, so when two strings share the high surrogate of a supplementary code
point but differ in the low half, the reported index lands inside the pair
(index `1` rather than `0`). `getCommonPrefix` then slices the pair in half and
`difference` starts its result on the orphaned low half.
`Fix:` when the difference falls on a low surrogate whose preceding (common)
`char` is a high surrogate, report the start of the pair. `getCommonPrefix` and
`difference` follow for free. BMP inputs never hit the new branch, so existing
behaviour is unchanged.
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