The bulk-copy escape loop still iterated Characters and compared each to '&',
'<', '>' (a grapheme comparison per character), the dominant remaining serialize
cost for content that needs escaping. Every escaping trigger is an ASCII marker,
or in ASCII-only mode a non-ASCII lead byte, so find the next one with
value.utf8.firstIndex(where:) and decode no grapheme for the run between markers.
The marker byte always lands on a grapheme boundary: an ASCII byte is its own
scalar and forces a cluster break (so '&', '<', '>', the quote, and whitespace
are never mid-cluster), and after a grapheme-aligned runStart the first byte at
or above 0x80 is a lead byte. So value[marker] and value[runStart ..< marker]
stay valid, and a multi-scalar cluster escapes as a unit (Swift rounds a
mid-grapheme index to its boundary, so the base letter is neither dropped nor
duplicated). The output is byte-identical.
Measured serialize ~23% faster on the 20k-item generated corpus via a same-state
A/B (~2.8x to ~2.2x slower than libxml2). Verified by EscapingBulkCopyTests
(the oracle differential extended with combining-mark, ZWJ, and flag clusters),
the Apache Xalan gold-output and C14N suites, the full suite, the WASM build,
and an adversarial grapheme-alignment review.
Mihaela Mihaljevic
0bda990f80