Niels LohmannandClaude Opus 4.8 9ea2d28ef9 Reserve output capacity up front for binary serialization ...

The vector-returning to_cbor/to_msgpack/to_ubjson/to_bjdata/to_bson grew
the output buffer purely by geometric reallocation. Reserving an estimate
up front avoids the early reallocations, which is the dominant per-byte
cost for array/object-heavy output.

The estimate (binary_reserve_hint) is deliberately conservative and safe
against untrusted input: it consults only the top-level element count
(O(1), no walk of the DOM), guards the multiplication against overflow,
and clamps the result to a fixed 1 MiB ceiling, so a large or hostile DOM
can never force an oversized allocation here. The buffer still grows
geometrically past the hint, so an underestimate only costs a few later
reallocations; scalars/strings/binary are written in one shot and get no
hint. Reserving capacity does not change the bytes produced.

Throughput (g++/clang -O3, vs the previous commit):
  cbor int array     +10% / +13%
  cbor object array  +20% / +38%

Output is byte-for-byte identical to develop across the binary
differential corpus.

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>

2026-07-22 11:01:03 +00:00

..

nlohmann

Reserve output capacity up front for binary serialization

2026-07-22 11:01:03 +00:00