Niels Lohmann 6c32ac94cf Decode \u escapes with a table in the lexer ...

get_codepoint() read the four hex digits of a \u escape with four calls
to get(), each classified by a chain of range comparisons. For contiguous
input, get_codepoint_bulk() now decodes them with one lookup per byte
(hex_codepoint() in string_scan.hpp, after yyjson's read_hex_u16): a
256-entry table maps a byte to its value, or 0xFF for anything else, and
an invalid digit shows in the OR of the four values. It then skips the
four bytes and updates the position counters as four get() calls would.
If a digit is invalid or fewer than four bytes are left, it changes
nothing and the existing loop runs, so errors are reported with the same
message and position as before.

json::parse, best of 5 runs in separate processes (M1 Max): the escaped
twitter.json (every non-ASCII character as \u) -13.6%, all other files
within 0.3%.

Tests compare the contiguous and the streaming path (value or exception
message) for valid escapes, surrogate pairs, truncated and invalid digits
at every position, and 3,000 seeded random escapes.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

2026-09-30 14:56:28 +02:00

..

nlohmann

Decode \u escapes with a table in the lexer

2026-09-30 14:56:28 +02:00