re: a nested record's +0x08 is its loop length -- the plate's period is 120, not 105

Answers the question the port agent asked: does the `PRESS A` plate's pulse
group loop from its start, or hold at alpha 0 between cycles? It holds.

A nested record is itself a RATC bundle with its own header, and that header's
`+0x08` is the loop length -- the same field ui_header_time_disc already tests
as an animation length at the top level. Its keyframes need not fill it, and
the slack is a hold at the final pose. `ptbtn00f` is 105 units of ramp inside
a 120-unit cycle, so the glow rests dark for 15 units between pulses. The five
main-menu focus records fill their 120 exactly, which is what shows the slack
belongs to this record rather than to the format.

Disc-wide over 1 781 timed nested records: 92.3% declare exactly their last
keyframe time, 7.7% declare more, and 0 declare less. That last row is the
falsifier -- a cycle cannot restart before its own last pose -- and it never
fires; the 7.7% is what keeps the reading from being an unfalsifiable
relabelling of the keyframes.

Falsification against the running game, using a pacing factor measured
INDEPENDENTLY on the main menu's focus ring (declared 120 units, measured
2.177 s, factor 1.0885): to reach the corpus's four measurements of the plate
pulse (2.12/2.19/2.34/2.31 s), a 105-unit period needs a factor of 1.211-1.337,
which EXCLUDES the ring's; a 120-unit period needs 1.060-1.170, which CONTAINS
it. Predicted 2.177 s against a measured 2.12-2.34. The two elements are in
different bundles and were measured in separate runs; the only thing tying
them together is that both declare 120.

So the port should stop shipping 105. Its 123-vs-129 ambiguity straddled the
right answer without containing it, and 129 only fitted because it was
105 + the exit_ramp_units constant it has since correctly deleted.

Reach is stated: this says where a cycle ends, not which records cycle, and
the TOP-level +0x08 is a different field left untouched -- every GP_TITLE
entry declares 300 while its elements end at 244-269.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QsEPXWVaEpyfudtR6re1Pd

crates/sylpheed-formats/examples/_all2.rs

@@ -0,0 +1,38 @@
+use sylpheed_formats::{pak, ui_layout};
+fn main(){
+    let mut a=std::env::args().skip(1);
+    let pk=a.next().unwrap(); let i:usize=a.next().unwrap().parse().unwrap();
+    let ar=pak::PakArchive::open(pk).unwrap();
+    let by=ar.read(&ar.entries()[i]).unwrap();
+    let b=ui_layout::parse_build(&by).unwrap();
+    println!("entry {i}: {} elements, {} records, {} sprites",
+             b.elements.len(), b.records.len(), b.sprites.len());
+    let mut rk:Vec<&String>=b.records.keys().collect(); rk.sort();
+    println!("  records: {:?}", rk);
+    for (rn,&(o,sz)) in &b.records {
+        if o+sz>by.len() { continue }
+        let Some(lb)=ui_layout::parse_build(&by[o..o+sz]) else { continue };
+        println!("  RECORD {rn}: {} elements", lb.elements.len());
+        for le in &lb.elements {
+            let lts:Vec<String>=le.keyframes.iter()
+                .map(|k|format!("t{}a{}",k.time.map(|v|v as i64).unwrap_or(-1),k.fade>>24)).collect();
+            println!("      [{}] {:<22} kind=0x{:<6x} {}", le.index, le.name, le.kind, lts.join(" "));
+        }
+    }
+    for e in &b.elements {
+        let ts:Vec<String>=e.keyframes.iter()
+            .map(|k|format!("t{}a{}",k.time.map(|v|v as i64).unwrap_or(-1),k.fade>>24)).collect();
+        println!("  [{}] {:<24} kind=0x{:<6x} {}", e.index, e.name, e.kind, ts.join(" "));
+        if let Some(&(o,s))=b.records.get(&e.name) {
+            if o+s<=by.len() {
+                if let Some(lb)=ui_layout::parse_build(&by[o..o+s]) {
+                    for le in &lb.elements {
+                        let lts:Vec<String>=le.keyframes.iter()
+                            .map(|k|format!("t{}a{}",k.time.map(|v|v as i64).unwrap_or(-1),k.fade>>24)).collect();
+                        println!("        leaf {:<20} kind=0x{:<6x} {}", le.name, le.kind, lts.join(" "));
+                    }
+                }
+            }
+        }
+    }
+}

crates/sylpheed-formats/examples/_rechdr.rs

@@ -0,0 +1,27 @@
+use sylpheed_formats::{pak, ui_layout};
+fn main(){
+    let mut a=std::env::args().skip(1);
+    let pk=a.next().unwrap();
+    let ar=pak::PakArchive::open(pk).unwrap();
+    for t in a {
+        let i:usize=t.parse().unwrap();
+        let by=ar.read(&ar.entries()[i]).unwrap();
+        let Some(b)=ui_layout::parse_build(&by) else { continue };
+        let top=u32::from_be_bytes(by[8..12].try_into().unwrap());
+        println!("entry {i:2}  TOP +08 = {top}");
+        let mut ks:Vec<&String>=b.records.keys().collect(); ks.sort();
+        for rn in ks {
+            let &(o,s)=b.records.get(rn).unwrap();
+            if o+16>by.len() { continue }
+            let magic=&by[o..o+4];
+            let h4=u32::from_be_bytes(by[o+4..o+8].try_into().unwrap());
+            let h8=u32::from_be_bytes(by[o+8..o+12].try_into().unwrap());
+            let maxt=ui_layout::parse_build(&by[o..o+s])
+                .map(|lb|lb.elements.iter().flat_map(|e|e.keyframes.iter()
+                     .filter_map(|k|k.time)).max().unwrap_or(0)).unwrap_or(0);
+            println!("   {rn:<24} magic={:?} +04={:08x}({:.1}) +08={h8:<6} max keyframe t={maxt}   ratio={:.4}",
+                String::from_utf8_lossy(magic), h4, h4 as f64/65536.0,
+                if maxt>0 {h8 as f64/maxt as f64} else {0.0});
+        }
+    }
+}

crates/sylpheed-formats/examples/record_loop_length.rs

@@ -0,0 +1,69 @@
+//! Is a nested record's header `+0x08` its LOOP LENGTH — and do its keyframes
+//! have to fill it?
+//!
+//! A `*f` focus record animates forever while its button is focused, so
+//! something must say where the cycle restarts. The keyframes cannot: the plate's
+//! `ptbtn00f` runs 0→80→0 over 105 units, and looping at 105 gives a period 15 %
+//! short of every measurement of the real thing.
+//!
+//! Each record is itself a RATC bundle with its own header, and `+0x08` is a
+//! frame count. If it is the loop length then it must never be LESS than the
+//! record's largest keyframe time — an animation cannot restart before its own
+//! last pose — and it may be more, which is a hold at the final pose.
+//!
+//! Two controls, both of which a wrong reading fails:
+//!   * `+0x08 < max keyframe time` must never happen. That is the falsifier.
+//!   * The distribution must not be trivial: if every record had exactly
+//!     `+0x08 == max t`, the field would carry nothing and "loop length" would be
+//!     an unfalsifiable relabelling of the keyframes.
+use sylpheed_formats::{pak, ratc, ui_layout};
+use std::collections::BTreeMap;
+
+fn main() {
+    let root = std::env::var("SYLPHEED_DISC").unwrap_or_else(|_| "/disc".into());
+    let mut paks: Vec<_> = std::fs::read_dir(format!("{root}/dat")).expect("dat/")
+        .flatten().map(|e| e.path())
+        .filter(|p| p.extension().and_then(|s| s.to_str()) == Some("pak")).collect();
+    paks.sort();
+    let (mut total, mut exact, mut holds, mut violations) = (0usize, 0usize, 0usize, 0usize);
+    let mut hold_hist: BTreeMap<i64, usize> = BTreeMap::new();
+    let mut worst: Vec<(i64, String)> = Vec::new();
+    for p in &paks {
+        let Ok(ar) = pak::PakArchive::open(p) else { continue };
+        for e in ar.entries() {
+            let Ok(by) = ar.read(e) else { continue };
+            if !ratc::is_ratc(&by) { continue }
+            let Some(b) = ui_layout::parse_build(&by) else { continue };
+            for (rn, &(o, s)) in &b.records {
+                if o + 12 > by.len() || o + s > by.len() { continue }
+                if &by[o..o + 4] != b"RATC" { continue }
+                let len = u32::from_be_bytes(by[o + 8..o + 12].try_into().unwrap()) as i64;
+                let Some(lb) = ui_layout::parse_build(&by[o..o + s]) else { continue };
+                let maxt = lb.elements.iter()
+                    .flat_map(|el| el.keyframes.iter().filter_map(|k| k.time))
+                    .max().unwrap_or(0) as i64;
+                if maxt == 0 { continue }          // a static record declares no cycle
+                total += 1;
+                let slack = len - maxt;
+                *hold_hist.entry(slack).or_default() += 1;
+                if slack == 0 { exact += 1 } else if slack > 0 { holds += 1 } else {
+                    violations += 1;
+                    if worst.len() < 12 {
+                        worst.push((slack, format!("{}:{rn} len={len} maxt={maxt}",
+                            p.file_name().unwrap().to_string_lossy())));
+                    }
+                }
+            }
+        }
+    }
+    println!("nested records with timed keyframes : {total}");
+    println!("  +08 == max keyframe time (exact)  : {exact}  ({:.1}%)", 100.0*exact as f64/total as f64);
+    println!("  +08 >  max keyframe time (a hold) : {holds}  ({:.1}%)", 100.0*holds as f64/total as f64);
+    println!("  +08 <  max keyframe time  🔴      : {violations}  ({:.2}%)  <- the falsifier",
+             100.0*violations as f64/total as f64);
+    println!("\nslack (+08 - max t) distribution, most common first:");
+    let mut h: Vec<_> = hold_hist.iter().collect();
+    h.sort_by_key(|&(_, n)| std::cmp::Reverse(*n));
+    for (k, n) in h.iter().take(14) { println!("   slack {k:>6} : {n}"); }
+    if !worst.is_empty() { println!("\nviolations:"); for (s, w) in &worst { println!("   {s:>6}  {w}"); } }
+}

crates/sylpheed-formats/tests/ui_record_loop_length_disc.rs

@@ -0,0 +1,121 @@
+//! A nested record's header `+0x08` is its LOOP LENGTH, and its keyframes need
+//! not fill it.
+//!
+//! A `*f` focus record animates for as long as its button is focused, so
+//! something has to say where the cycle restarts. The keyframes cannot: the
+//! `PRESS Ⓐ` plate's `ptbtn00f` ramps 0→80→0 over **105** units, and a 105-unit
+//! period is 15 % short of every measurement of the real thing.
+//!
+//! Each record is itself a RATC bundle with its own header. `+0x08` is a frame
+//! count, and if it is the loop length it must never be **less** than the
+//! record's largest keyframe time — an animation cannot restart before its own
+//! last pose. Disc-wide that holds 1 781 times out of 1 781, and 7.7 % of records
+//! declare *more*, which is a hold at the final pose before the cycle repeats.
+//!
+//! `ptbtn00f` is one of those: 105 units of ramp inside a **120**-unit cycle, so
+//! it rests dark for 15 units between pulses.
+//!
+//! Argument, the falsification test against the measured period, and the census:
+//! `docs/re/structures/ui-record-loop-length.md`.
+
+use std::path::PathBuf;
+
+use sylpheed_formats::{pak::PakArchive, ratc, ui_layout};
+
+fn disc_root() -> Option<PathBuf> {
+    let p = PathBuf::from(std::env::var("SYLPHEED_DISC").ok()?);
+    p.join("dat").is_dir().then_some(p)
+}
+
+/// Read a nested record's declared length and its largest keyframe time.
+fn record_len_and_maxt(bundle: &[u8], off: usize, size: usize) -> Option<(i64, i64)> {
+    if off + 12 > bundle.len() || off + size > bundle.len() || &bundle[off..off + 4] != b"RATC" {
+        return None;
+    }
+    let len = u32::from_be_bytes(bundle[off + 8..off + 12].try_into().ok()?) as i64;
+    let lb = ui_layout::parse_build(&bundle[off..off + size])?;
+    let maxt = lb
+        .elements
+        .iter()
+        .flat_map(|el| el.keyframes.iter().filter_map(|k| k.time))
+        .max()? as i64;
+    (maxt > 0).then_some((len, maxt))
+}
+
+/// The falsifier: a loop cannot restart before its own last keyframe.
+#[test]
+fn a_records_declared_length_is_never_shorter_than_its_keyframes() {
+    let Some(root) = disc_root() else {
+        eprintln!("SYLPHEED_DISC unset — skipping");
+        return;
+    };
+    let mut paks: Vec<PathBuf> = std::fs::read_dir(root.join("dat"))
+        .expect("dat/")
+        .flatten()
+        .map(|e| e.path())
+        .filter(|p| p.extension().and_then(|s| s.to_str()) == Some("pak"))
+        .collect();
+    paks.sort();
+
+    let (mut total, mut exact, mut holds) = (0usize, 0usize, 0usize);
+    for p in &paks {
+        let Ok(ar) = PakArchive::open(p) else { continue };
+        for e in ar.entries() {
+            let Ok(by) = ar.read(e) else { continue };
+            if !ratc::is_ratc(&by) {
+                continue;
+            }
+            let Some(b) = ui_layout::parse_build(&by) else { continue };
+            for (rn, &(o, s)) in &b.records {
+                let Some((len, maxt)) = record_len_and_maxt(&by, o, s) else { continue };
+                total += 1;
+                assert!(
+                    len >= maxt,
+                    "{}:{rn} declares a {len}-unit cycle but has a keyframe at t={maxt} — \
+a loop cannot restart before its own last pose",
+                    p.file_name().unwrap().to_string_lossy()
+                );
+                if len == maxt { exact += 1 } else { holds += 1 }
+            }
+        }
+    }
+    assert!(total > 1500, "expected >1500 timed nested records, got {total}");
+    // The field must carry information. If every record declared exactly its own
+    // last keyframe time, "loop length" would be an unfalsifiable relabelling.
+    assert!(
+        holds > 50,
+        "only {holds} of {total} records declare a hold — the field would be carrying nothing"
+    );
+    eprintln!("{total} records: {exact} exact, {holds} with a hold before the cycle repeats");
+}
+
+/// The case that motivated it, pinned by name.
+#[test]
+fn the_press_a_plate_glow_holds_dark_for_fifteen_units() {
+    let Some(root) = disc_root() else {
+        eprintln!("SYLPHEED_DISC unset — skipping");
+        return;
+    };
+    let ar = PakArchive::open(root.join("dat/GP_TITLE.pak")).expect("GP_TITLE.pak");
+    let by = ar.read(&ar.entries()[2]).expect("entry 2 — the PRESS A plate");
+    let b = ui_layout::parse_build(&by).expect("parse");
+    let &(o, s) = b.records.get("ptbtn00f.rat").expect("the focus record");
+    let (len, maxt) = record_len_and_maxt(&by, o, s).expect("a timed record");
+    assert_eq!(maxt, 105, "the glow's ramp ends at t=105");
+    assert_eq!(len, 120, "but the cycle is 120 units");
+    assert_eq!(len - maxt, 15, "so it holds dark for 15 units between pulses");
+
+    // The five main-menu focus records fill their cycle exactly — the contrast
+    // that shows the slack is a property of this record, not of the format.
+    let menu = ar.read(&ar.entries()[5]).expect("entry 5 — the main menu");
+    let mb = ui_layout::parse_build(&menu).expect("parse");
+    let mut checked = 0;
+    for n in 1..=5 {
+        let name = format!("ptbtn0{n}f.rat");
+        let Some(&(mo, ms)) = mb.records.get(&name) else { continue };
+        let (l, m) = record_len_and_maxt(&menu, mo, ms).expect("timed");
+        assert_eq!((l, m), (120, 120), "{name} should fill its 120-unit cycle exactly");
+        checked += 1;
+    }
+    assert_eq!(checked, 5, "expected five main-menu focus records");
+}

docs/port/HANDOFF.md

@@ -1870,3 +1870,60 @@ the other six.
 remain refuted. This finding does not answer it — it makes it cheap to get wrong.
 
 Detail, controls and reach: [`docs/re/structures/ui-tie-break-cost-at-settle.md`](../re/structures/ui-tie-break-cost-at-settle.md).
+
+## 2026-08-29 (later still) — the plate's pulse period is 120, not 105
+
+✅ **decoded, and it answers the question you asked.** The `ptbtn00f` group holds
+at alpha 0 between cycles. It does not loop from t=105.
+
+**A nested record is itself a RATC bundle with its own header, and that header's
+`+0x08` is the loop length.** The keyframes do not have to fill it; the slack is a
+hold at the final pose:
+
+| record | `+0x08` | largest keyframe | slack |
+|---|---|---|---|
+| `ptbtn00f.rat` — the plate glow | **120** | 105 | **15** |
+| `ptbtn01f` … `ptbtn05f` — your main-menu focus records | 120 | 120 | 0 |
+| `ptloop01` / `ptloop02` | 600 / 720 | 600 / 720 | 0 |
+
+So the glow ramps 0→80→0 over 105 units inside a **120-unit** cycle and rests dark
+for 15. Your five menu focus records fill their cycle exactly, which is what shows
+the slack belongs to this record rather than to the format.
+
+**Disc-wide, 1 781 timed nested records:** 92.3 % declare exactly their last
+keyframe time, **7.7 % declare more**, and **0 declare less** — a cycle never
+restarts before its own last pose. That last row is the falsifier and it never
+fires; the 7.7 % is what stops the reading being a relabelling of the keyframes.
+
+**And it survives the test your objection implies.** Both candidate periods have to
+be converted by the same emulator pacing factor, and that factor is measured
+*independently* on your focus ring — declared 120 units, measured 2.177 s, so
+**1.0885**:
+
+| plate period | nominal | factor needed to reach the measured 2.12–2.34 s | |
+|---|---|---|---|
+| 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 it** |
+
+At 120 units the predicted period is **2.177 s** against a measured 2.12–2.34 s.
+**105 cannot reach that range under any pacing factor the ring also satisfies.**
+The ring and the plate are different elements in different bundles measured in
+separate runs; the only thing tying them together is that both declare 120.
+
+🔴 **So stop shipping 105.** The number is 120 and it is on the disc — not from
+`exit_ramp_units`, the constant you correctly deleted, whose 129 merely happened to
+fit. Your 123-vs-129 ambiguity straddled the right answer without containing it.
+
+⚠️ The 2.24 s mean is still ~3 % above the 2.177 s prediction. That sits inside the
+spread of four wall-clock samples of a ~2 s period and is not evidence of a further
+hold — I looked for one and the disc does not declare it.
+
+⚠️ **Scope:** this says where a cycle *ends*, not which records cycle. 92.3 % of
+records declare no slack, and a one-shot build-in's length is simply its duration.
+❔ The **top-level** `+0x08` is a different field and is untouched: every `GP_TITLE`
+entry declares 300 while its elements end at 244–269, and no screen visibly repeats
+every 5 s.
+
+Detail, census and the falsification test:
+[`docs/re/structures/ui-record-loop-length.md`](../re/structures/ui-record-loop-length.md).
+

docs/re/INDEX.md

@@ -169,3 +169,4 @@ files, which is how the same ground got covered twice.
 | [`menu-idle-and-b-2026-08-29.md`](menu-idle-and-b-2026-08-29.md) | The main menu does not idle back to the title — and four durations that were a pipeline | ✅ **refuted**: no self-return in **≥ 60 s** untouched; the ~8–10 s idle belongs to the **title**. 🟡 Ⓑ→title ordering measured, latency not. 🔴 `classify_array` at **1503 ms/frame** drained an 8 fps stream at 0.64 fps and manufactured four latencies (24.66 s / 15.58 s / 25.60 s / 20.26 s) — all withdrawn; a backlog preserves ordering and destroys durations |
 | [`structures/ui-settle-time.md`](structures/ui-settle-time.md) | Which instant a "settled screen" composite depicts | ✅ **decoded**: a settled screen is **one instant every element is posed at**, and the disc names it — the midpoint of the **longest keyframe-free interval** in the build (`UiBuild::settle_time` / `settle_window`). 🔴 `rest()` is *not* that: it picks each element's last hold **independently**, so a two-frame flash holds at its **peak** and burns forever. `GP_TITLE` build 4 has five staggered flashes (`ptlogo_back2eff1`…`eff5`, all extinguished by t110) that `rest()` draws simultaneously and permanently, saturating the light arc. Predicted t=198 from `[160,236]` **before scoring**: arc band **33.22 → 11.79**, clipped pixels **8 581 → 1 452** against the console's **1 459** (an unfitted statistic), whole frame 14.07 → 12.06; controls at t=100 and t=358 are far worse, and a hand-picked visibility list reaches the identical 12.06/11.79/1 452. Controls: `at=None` byte-identical (`cmp`), pre- and post-rotation tags both 14.07, 13 paint-order tests green. ⚠️ **Reach**: of 1 758 bundles with ≥2 keyframe times only **30 %** have a window ≥ 30 units and **42 %** under 10 — mostly `loop*` fragments that never settle; check the width. 🔴 Withdraws two claims in [`ui-rotation-implemented.md`](structures/ui-rotation-implemented.md) — its "Flat. No minimum." (`at` posed **leaves only**) and its "Reborn does not draw `ptlogo1`/`ptlogo2`" (both **are** drawn; only kind-`0x4` ghosts are skipped, and hiding the real ones makes the error *worse* by +5.20/+7.47). ❔ its **10.92** baseline is unreproducible — 14.07 at both tags |
 | [`structures/ui-tie-break-cost-at-settle.md`](structures/ui-tie-break-cost-at-settle.md) | What the unknown paint-order tie-break costs, in pixels | ✅ **decoded**, closing the open half of Q3: at the settled instant the tie-break costs **at most 1 px at Δ1**, on the **Japanese title only** (`ptlogo2`×`ptlogo_tm`, 5 px shared ink); **exactly 0 px on all five port screens**. The earlier 24-pair bound was a `rest()` count — and 10 of the title's 11 tied pairs are between `ptlogo_back2eff1`…`eff5`, five transient flashes that are **transparent** on the settled screen ([`ui-settle-time.md`](structures/ui-settle-time.md)). Live pairs at settle: entry 4 → **1**, entry 7 → **2**, the four loading bundles → **0**. ✅ Not a knife-edge — sweeping every keyframe time and midpoint, the count is **flat across the whole settle window**, and the loading bundles' tie is live only at t17–t33. ✅ Controls: an overlapping *different*-key swap moves 25 310 / 268 698 / ~765 000 px on the entries reporting zero; zeros are explained by shared-ink counts (the `ptframe` pairs share **0 px** of ink). ⚠️ Entries 0/1/12/15 have **no live control** — their zeros rest on keyframe data, not a render. 🟡 Refutation attempt on the corpus's "24 pairs": **survives** as a rest-pose bound, 16/16 on entry 7. ❔ *Why* ties order as they do is still unknown — and now worth one pixel |
+| [`structures/ui-record-loop-length.md`](structures/ui-record-loop-length.md) | Where a looping record's cycle restarts — and the `PRESS Ⓐ` plate's real period | ✅ **decoded**: a nested record is itself a RATC bundle and its header **`+0x08` is the loop length**; its keyframes need not fill it, and the slack is a hold at the final pose. Disc-wide over **1 781** timed nested records: 92.3 % declare exactly their last keyframe time, **7.7 % declare more**, and **0 declare less** — the falsifier (a cycle cannot restart before its own last pose) never fires. 🔴 **The plate's `ptbtn00f` is 105 units of ramp inside a 120-unit cycle, so it holds dark for 15 units** — the port was shipping **105**, and the answer is **120**. ✅ Falsification test against the running game, using a pacing factor measured *independently* on the focus ring (declared 120 → **2.177 s**, factor **1.0885**): to reach the corpus's measured 2.12–2.34 s, 105 units needs a factor of **1.211–1.337** (🔴 excludes the ring's) while 120 needs **1.060–1.170** (✅ contains it). Different elements, different bundles, separate runs — tied only by both declaring 120. ⚠️ Says where a cycle *ends*, not which records cycle. ❔ the **top-level** `+0x08` (300 on every `GP_TITLE` entry, elements ending at 244–269) is a different question, untouched |

docs/re/data/record-loop-length-census.txt

@@ -0,0 +1,20 @@
+nested records with timed keyframes : 1781
+  +08 == max keyframe time (exact)  : 1643  (92.3%)
+  +08 >  max keyframe time (a hold) : 138  (7.7%)
+  +08 <  max keyframe time  🔴      : 0  (0.00%)  <- the falsifier
+
+slack (+08 - max t) distribution, most common first:
+   slack      0 : 1643
+   slack     40 : 32
+   slack     10 : 20
+   slack      4 : 13
+   slack     54 : 12
+   slack     30 : 8
+   slack      1 : 6
+   slack      6 : 6
+   slack      9 : 6
+   slack     36 : 6
+   slack    405 : 6
+   slack     16 : 5
+   slack     58 : 5
+   slack     80 : 5

docs/re/structures/ui-record-loop-length.md

@@ -0,0 +1,106 @@
+# 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
+```