When ensure_ascii is false, dump_escaped previously ran every byte of
every string and object key through the UTF-8 DFA decoder, even for the
common case of ordinary text with nothing to escape. This mirrors the
per-byte cost the parser had before the contiguous fast paths.
At a character boundary, bulk-copy the longest run of bytes that need no
escaping using string_bulk_run() - the same SWAR scanner and UTF-8 bulk
validator the lexer's contiguous path uses - and only fall back to the
byte-at-a-time DFA loop for the first byte that needs individual handling
(a quote, backslash, control character, or ill-formed/truncated UTF-8).
Because every "hard" or invalid byte is still processed by the unchanged
byte path, escaping output and error handling (including strict-mode
error 316 position and message) are byte-identical to before.
The ensure_ascii=true path is unchanged: it must escape non-ASCII and
0x7F, which string_bulk_run does not stop on, so a separate predicate
would be needed for it.
Verified byte-for-byte identical dump output against the pre-change
implementation across ~20k randomized byte strings plus curated edge
cases (all escapes, control chars, valid multibyte, surrogates,
overlong, truncated sequences) for both ensure_ascii settings and all
three error handlers, in C++11/17/20 at -O2/-O3.
Throughput (g++ -O3, ensure_ascii=false, vs pre-change):
long ASCII strings 4.2x
twitter-like objects 2.3x
dense CJK 1.4x (further headroom with JSON_USE_SIMDUTF)
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XAYM1qhSA2FDaDcGfPW3fG
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
