# A nested record's `+0x08` is its loop length — and the plate holds dark for 15 units

**Classification: decoded.** The field is read from the disc and checked
disc-wide (1 781 records, 0 violations). The consequence for the `PRESS Ⓐ` plate
is then tested against the corpus's existing measurements of the running game,
which it passes and the previous reading fails.

## The question

The port asked it directly: the plate's pulse group runs t=0 → t=105 with alpha 0
at both ends. **Does it loop from its start, or hold at alpha 0 between cycles?**

It matters because a 105-unit period is **1.750 s**, and the corpus timed the real
pulse four times at **2.12 / 2.19 / 2.34 / 2.31 s** — about 17 % longer. The port
shipped 105 anyway, saying the disc's number was wrong, because the alternative
(129) was built on an `exit_ramp_units` constant that a decode had just deleted.

## The answer: it holds, and the period is 120

A nested record is **itself a RATC bundle, with its own header**, and that
header's `+0x08` is a frame count — the same field
[`ui-header-time-disc`](../../crates/sylpheed-formats/tests/ui_header_time_disc.rs)
already tests as an animation length at the top level. Read it on the record and
the answer is immediate:

| record | `+0x08` | largest keyframe | slack |
|---|---|---|---|
| `ptbtn00f.rat` — the plate glow | **120** | 105 | **15** |
| `ptbtn01f` … `ptbtn05f` — main menu focus | 120 | 120 | 0 |
| `ptloop01.rat` | 600 | 600 | 0 |
| `ptloop02.rat` | 720 | 720 | 0 |

**The glow ramps 0 → 80 → 0 over 105 units inside a 120-unit cycle, so it rests
dark for 15 units between pulses.** The five main-menu focus records fill their
cycle exactly, which is what shows the slack is a property of *this record* rather
than of the format.

## Disc-wide

[`record-loop-length-census.txt`](../data/record-loop-length-census.txt) — every
nested record on the disc that has a timed keyframe:

| | count | share |
|---|---|---|
| nested records with timed keyframes | 1 781 | — |
| `+0x08 == ` largest keyframe time | 1 643 | 92.3 % |
| `+0x08 > ` largest keyframe time (a hold) | 138 | 7.7 % |
| **`+0x08 < ` largest keyframe time** | **0** | **0.00 %** |

The last row is the falsifier and it never fires: **no record declares a cycle
that would restart before its own last pose.** The 7.7 % is what keeps the reading
from being an unfalsifiable relabelling of the keyframes — if every record
declared exactly its own last keyframe time, the field would carry nothing.

## The falsification test against the running game

Both hypotheses are periods in declared units, and both have to be converted by
the same emulator pacing factor. That factor is measured **independently**, on the
main menu's focus ring: declared 120 units, measured **2.177 s**
([`focus-ring-spin-measured.md`](../focus-ring-spin-measured.md)), so the factor
is **1.0885** against a nominal 60 units/s.

| plate period | nominal | factor it would need to reach 2.12–2.34 s | verdict |
|---|---|---|---|
| 105 units | 1.750 s | **1.211 … 1.337** | 🔴 **excludes** the ring's 1.0885 |
| **120 units** | 2.000 s | **1.060 … 1.170** | ✅ **contains** the ring's 1.0885 |

At 120 units and the ring's own factor the plate should pulse every **2.177 s**,
against a measured 2.12–2.34 s. **105 cannot reach the measured range under any
pacing factor that the ring also satisfies.**

This is a genuine test rather than a fit: the ring and the plate are different
elements in different bundles, measured in separate runs, and the only thing tying
them together is that both declare a 120-unit cycle.

## What this replaces

* 🔴 **105 is wrong** and the port should stop shipping it. The number is **120**,
  and it comes from the disc — not from `exit_ramp_units`, the deleted constant
  whose 129 happened to fit.
* ✅ **The port's ambiguity is genuinely resolved**, just not the way it read: the
  old 123-vs-129 pair straddled the right answer without containing it.
* ⚠️ **The 2.24 s mean is still ~3 % above the 2.177 s prediction.** That is inside
  the spread of the four measurements (2.12–2.34) and is not evidence of a further
  hold; it is what a four-sample wall-clock measurement of a ~2 s period in this
  emulator looks like.

## Reach

⚠️ **This says where a cycle ends, not that every record cycles.** 92.3 % of
records declare no slack at all, and a record whose element is not in a repeating
state (a button's base record, a one-shot build-in) has a length that is simply
its own duration. Nothing here establishes *which* records the game restarts —
only that when one does, `+0x08` is where.

❔ **The top-level `+0x08` is not the same thing.** Every `GP_TITLE` entry declares
300 there while its elements end at 244–269, and no screen visibly repeats every
5 s. Whether the top-level field is a loop length, a budget or something else is
untouched by this.

## Reproducing

```bash
cargo run -p sylpheed-formats --example record_loop_length
SYLPHEED_DISC=/disc cargo test -p sylpheed-formats --test ui_record_loop_length_disc
```