[stdlib] String: Fix forward implementation of grapheme breaking rule 11 #63043
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Azoy
approved these changes
Jan 15, 2023
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Rule GB11 in Unicode Annex 29 is: GB11: Extended_Pictographic Extend* ZWJ × Extended_Pictographic However, our forward grapheme breaking state machine implements it as: GB11: Extended_Pictographic Extend* ZWJ+ × Extended_Pictographic We implement the correct rules when going backward, which can cause String values to have different counts whether we’re going forward or back. The rule as implemented would be fine (Unicode doesn’t care much about the placement of grapheme breaks in invalid sequences), but the directional inconsistency messes with String’s Collection conformance. rdar://104279671
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Rule GB11 in Unicode Annex 29 is:
However, our forward grapheme breaking state machine implements it as:
We implement the correct rules when going backward, which can cause String values to have different counts whether we’re going forward or back.
The rule as implemented would be fine (Unicode doesn’t care much about the placement of grapheme breaks in invalid sequences), but the directional inconsistency messes with String’s Collection conformance.
rdar://104279671