Subj : src/sbbs3/zmodem.c zmodem.h
To   : Git commit to main/sbbs/master
From : Rob Swindell (on Debian Linux)
Date : Mon Aug 24 2026 20:32:03

https://gitlab.synchro.net/main/sbbs/-/commit/81e5e3a3a6f008a7e11ff590
Modified Files:
	src/sbbs3/zmodem.c zmodem.h
Log Message:
zmodem: derive the receive fast path from one table of plain bytes

zmodem_rx() decided per byte, through a switch, whether the byte it had just
read needed handling: ZDLE introduces an escape, the four XON/XOFF forms are
dropped as flow control, and when the peer negotiated ZF0_ESCCTL a control
character that arrived unescaped is dropped too.  Everything else is returned
verbatim, which on ordinary data is the overwhelming majority of bytes.

Precompute that predicate into a 256-entry table and test it before the
switch.  The switch is then reached only by bytes that genuinely need it, and
the common case is one indexed load.  A 256 MiB localhost receive goes from
126.9 to 153.0 MB/s and from 2.11 to 1.74 CPU-seconds.

The table is built in zmodem_init() and rebuilt where escape_ctrl_chars is
assigned from the peer's ZRINIT, which is the only place it changes.  Building
it eagerly rather than on demand avoids needing a sentinel to distinguish
"not built yet" from "built for escape_ctrl_chars == FALSE"; zmodem_init()
memsets the struct, so any such sentinel would have to be assigned anyway and
is easy to forget.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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