Enhancement-880: a resumed transient keeps the breakpoint spacing it ran with — a resume, in memory or from a checkpoint, took a minbreak 5e6 times larger and stepped over edges the run had landed on

Enhancement-880: a resumed transient keeps the breakpoint spacing it ran with — a resume, in memory or from a checkpoint, took a minbreak 5e6 times larger and stepped over edges the run had landed on

Scope: F7 of the
transient stress campaign of 2026-10-12.

ngspice: spicelib/analysis/dctran.c: tran_minbreak, the run's rule for a minbreak that is not
given, used at the start, on a resume and on a checkpoint continuation. Stock ngspice-46
behaviour for the in-memory resume; the checkpoint branch of
Enhancement-131 copied it.

examples/resumebreak_examples/ (new). ngspice only.

Suites:

• resumebreak_examples: 5 of 5 per solver. On the E-875
  binaries [1] to [3] fail; [4] and [5] are controls.
• checkpoint, ckorder and stagedslots pass.
• The campaign's Family D, rerun: no resumed run and no checkpoint continuation is more than 10×
  less accurate than the uninterrupted run, except one under ordfix=6. Before, there were 5
  and 4. Every one of those nine now matches the uninterrupted run's error.
• The full sweep: 584 of 584.

What was wrong

A checkpoint continuation did not land where the uninterrupted run landed. On a network of the
campaign, driven by a pulse with an 82 fs rise, it was 0.19 off against the exact solution where
the uninterrupted run was 0.011. On an RC driven by a pulse with 10 fs edges, the uninterrupted run
landed on 17 of the 18 corners after the stop, and the continuation on 9. Every rise's end was
stepped over.

minbreak is the smallest spacing kept between breakpoints; a source skips a corner closer than
that. With no .option minbreak, a run starts with ten times delmin in a build with XSPICE, as
ours is, and with tmax/20 000 in a build without it. CKTdoJob puts the option's 0 back for every
job, a resume included. The resume branch of DCtran then re-derived it with tmax/20 000 in
either build, and the checkpoint branch was written the same way. On these decks that is 1e-12 s
against 2e-18 s, 5e6 times larger. The 10 fs edges fell under it, and the continuation skipped
the corners the run had landed on.

The checkpoint continuation is the in-memory resume, point for point, on both the E-875 and the
current binaries. The defect was the resume's, and F7's checkpoint cases were two of its faces.
Enhancement-877 fixed three of the five Family D resumes it explained (a
current source's chain). The other two, under ordfix and trbdf2, were this.

The change

tran_minbreak returns the run's rule: ten times delmin with XSPICE, tmax/20 000 without. A run's
start, a resume and a checkpoint continuation all take it when minbreak is not given. A given
minbreak is used as given, as before.

The RC continuation now lands on the same 17 corners as the run, and the campaign network's
continuation follows the uninterrupted run to 1e-6 of the range; it was 0.16.

A continuation is still not the uninterrupted run point for point. stop when time > t sets a
breakpoint at t, so the stopped run lands there and restarts its step. The continuation equals a
run with the same stop.

The checks

resumebreak_examples, every deck under .option sparse (checkpoints need it):

• [1] An RC driven by a pulse with 10 fs edges: after the stop, a checkpoint continuation
  lands on every corner the uninterrupted run lands on.
• [2] The in-memory resume: the same.
• [3] The campaign network: the continuation follows the uninterrupted run to 1e-4 of the
  range.
• [4] (control) The checkpoint continuation and the in-memory resume are the same run, point
  for point.
• [5] (control) With .option minbreak=1p, the 10 fs edges' ends are skipped by the run and by
  the continuation alike.

Write-up: enhancements_doc/Enhancement-880.md