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Resolve the shrub region bound: the chunk container closes it

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The shrub region's end was the last unlocated bound in core_data. It was not
in the core index, and unlike moby->tie it had no successor boundary to chain
against, so the last shrub class in every level ran out to the lump's end and
the aggregates over it (the "3.5M records / 28MB of shrub body") were
artifacts of that oversized span.

Range measured: for each level, from the last shrub class's geometry offset to
the constant trailer. Nothing outside that range is read.

Method: scan forward from the last shrub class for the pair (a) the constant
two bytes f6 7c sitting immediately in front of (b) a chunk signature -- a
16-byte header ending 0xff, ten zero bytes, then four or more 0xf0-tagged
offsets. The two halves are each weak alone, and the halves are measured
separately: the marker pair occurs 25 to 103 times in a 5 to 17 MB lump, so
matching it alone picks dozens of wrong offsets; the chunk shape matches
random data often enough to need four tagged offsets before it is believed.

Result: on all 27 levels the marker sits exactly two bytes before the first
chunk header, and the two parts of the signature are never found apart. No
chunk signature occurs inside any last shrub body, on any level. The three
levels without shrub (10, 25, 26) carry the same marker before the container
that closes tie instead, which is what makes the locator a property of the
region rather than a shrub rule.

Consequences:

- All 358 shrub class bodies are bounded and are whole 8-byte records. That
  was not assertable while the last class in each level ran to the lump's end,
  and it is the check the boundary earns its keep for.
- 9,600 records in total, against the 8,938 previously measurable in
  successor-bounded spans; 662 of them sit in the 24 last-class bodies. The
  shrub region totals 2,033,776 bytes across the 24 levels that have one.
- The chunk container is 25,775,850 bytes across 27 levels (1,236 chunks,
  22,562 records). The container measurements were previously taken over "the
  900,000 bytes before the trailer", a window that did not align to its real
  extent: on level 0 it is 690,302 bytes, and 27 of 1,123 chunks were
  truncated at that invented edge. chunk_containers_resolve_on_every_level now
  reads the region from core_layout instead of measuring backwards from the
  end of the lump.

core_layout takes core_data now: the container's start is the one bound that
cannot be derived from the index. It gains the chunks region, so every byte of
level-specific core_data belongs to a named region. Where the signature is not
found the end stays None -- the region is left visible rather than clamped to
the tail.

Still open: the marker's meaning (constant across 27 levels, at exactly this
position, unnamed) and the shrub body payload's encoding.
This commit is contained in:
ReaperOfVeriod
2026-10-11 06:20:48 +02:00
parent 4d1e277a15
commit ccea53807a
8 changed files with 681 additions and 117 deletions
Download Patch File Download Diff File Expand all files Collapse all files
crates/gc-formats/src/regions.rs
+151 -22
View File
@@ -10,7 +10,7 @@
//! distributions -- were silently wrong, and nothing errored.
//!
//! So bounds are computed once, here, from successor boundaries, and every
//! caller uses these rather than inventing its own. Where a bound is not yet
//! caller uses these rather than inventing their own. Where a bound is not yet
//! known it is `None`, not a guess: [`Region::end`] being `None` means
//! "unresolved", and [`core_layout`] refuses to close a region whose successor
//! it cannot find.
@@ -24,14 +24,24 @@
//! | sky | sky | collision |
//! | collision | collision | first moby class |
//! | moby | first moby | first tie class |
//! | tie | first tie | first shrub class |
//! | shrub | first shrub | **unresolved** |
//! | tie | first tie | first shrub class, |
//! | | | else the chunk container |
//! | shrub | first shrub | the chunk container |
//! | chunks | the chunk | lump length − 0x38A10 |
//! | | container | |
//!
//! The shrub end is the open item. Texture texels sit *before* moby, not after
//! shrub, so they cannot bound it: on level 0 the texel range is 3539328 to
//! 5326208 while the last shrub class starts at 16044288. The ~925 KB after
//! the last shrub class is real, non-zero data that is not accounted for yet.
//! The chunk container's start is located by signature rather than read from
//! the core index: the class table that precedes it carries no successor
//! offset, so the boundary is found in `core_data` itself -- see
//! [`crate::chunks::chunk_region_start`]. Every level has one: on the 24 levels
//! with shrub it sits exactly two bytes after the last shrub body, and on the
//! three levels without shrub it closes tie instead.
//!
//! Texture texels do **not** bound anything: they sit before moby, not after
//! shrub. On level 0 the texel range is 3539328 to 5326208 while the last
//! shrub class starts at 16044288.
use crate::chunks::chunk_region_start;
use crate::level_core::{ArrayRange, LevelCoreHeader};
/// Bytes at the end of `core_data` that are identical across all 27 levels.
@@ -85,12 +95,40 @@ pub fn first_geometry(index: &[u8], range: ArrayRange, stride: usize) -> Option<
best
}
/// The highest geometry offset in a class table, ignoring absent entries.
///
/// This is the *last* class in the region, which is where the search for the
/// successor region starts. Starting there rather than at the region's head
/// matters: the search is for the first chunk signature after this offset, so
/// beginning it inside the region's own class bodies would risk matching one.
pub fn last_geometry(index: &[u8], range: ArrayRange, stride: usize) -> Option<usize> {
let count = range.count.max(0) as usize;
let base = range.offset.max(0) as usize;
let mut best: Option<usize> = None;
for i in 0..count {
let at = base + i * stride;
let chunk = index.get(at..at + 4)?;
let off = i32::from_le_bytes(chunk.try_into().expect("4 bytes"));
if off > 0 {
best = Some(best.map_or(off as usize, |b: usize| b.max(off as usize)));
}
}
best
}
/// Compute the region's bounds for one level.
pub fn core_layout(index: &[u8], h: &LevelCoreHeader, core_len: usize) -> Vec<Region> {
///
/// `core` is the level's `core_data` lump. It is needed for one thing only: the
/// class regions end at the chunk container, whose start is located by
/// signature in `core` because the class table that precedes it carries no
/// successor offset.
pub fn core_layout(index: &[u8], core: &[u8], h: &LevelCoreHeader) -> Vec<Region> {
let at = |v: i32| -> Option<usize> { (v > 0).then_some(v as usize) };
let first_moby = first_geometry(index, h.moby_classes, 0x20);
let first_tie = first_geometry(index, h.tie_classes, 0x20);
let last_tie = last_geometry(index, h.tie_classes, 0x20);
let first_shrub = first_geometry(index, h.shrub_classes, 0x30);
let last_shrub = last_geometry(index, h.shrub_classes, 0x30);
// Each region ends where the next one begins. `chained` keeps `None`
// propagating rather than silently closing a region against a gap.
@@ -103,10 +141,20 @@ pub fn core_layout(index: &[u8], h: &LevelCoreHeader, core_len: usize) -> Vec<Re
})
}
// The chunk container follows the last of the shrub and tie classes.
// Searching from the last *class* rather than from the region start keeps
// the signature search out of the class bodies themselves.
let container = match (last_shrub, last_tie) {
(Some(shrub), _) => chunk_region_start(core, shrub),
(None, Some(tie)) => chunk_region_start(core, tie),
(None, None) => None,
};
// The final region is bounded by the constant trailer rather than by
// `core_data.len()`, which would let it absorb the trailer and whatever
// tail follows the last named region.
let tail_end = core_len
let tail_end = core
.len()
.checked_sub(CORE_DATA_TRAILER_BYTES)
.filter(|&e| e > 0);
@@ -115,9 +163,12 @@ pub fn core_layout(index: &[u8], h: &LevelCoreHeader, core_len: usize) -> Vec<Re
chained("occlusion", at(h.occlusion), at(h.sky)),
chained("sky", at(h.sky), at(h.collision)),
chained("collision", at(h.collision), first_moby),
chained("moby", first_moby, first_tie),
chained("tie", first_tie, first_shrub),
chained("shrub", first_shrub, tail_end),
// A region with no successor region ends at the chunk container, which
// is what follows the last class on every level.
chained("moby", first_moby, first_tie.or(container)),
chained("tie", first_tie, first_shrub.or(container)),
chained("shrub", first_shrub, container),
chained("chunks", container, tail_end),
];
regions.into_iter().flatten().collect()
@@ -148,7 +199,39 @@ mod tests {
fn layout(v: &[u8]) -> Vec<Region> {
let h = crate::level_core::LevelCoreHeader::parse(v).unwrap();
core_layout(v, &h, 1 << 24)
core_layout(v, &core(v), &h)
}
/// A zeroed lump, optionally carrying a real chunk-container signature just
/// past the last class -- the layout locates the container in `core`, so a
/// core of plain zeros locates nothing.
fn core(v: &[u8]) -> Vec<u8> {
let h = crate::level_core::LevelCoreHeader::parse(v).unwrap();
let mut c = vec![0u8; 1 << 24];
let last_class = [
last_geometry(v, h.tie_classes, 0x20),
last_geometry(v, h.shrub_classes, 0x30),
]
.into_iter()
.flatten()
.max();
if let Some(last) = last_class {
plant_chunk(&mut c, last + 100);
}
c
}
/// The marker pair plus a chunk signature at `at`, as the disc carries it.
fn plant_chunk(core: &mut [u8], at: usize) {
core[at - 2..at].copy_from_slice(&crate::chunks::CHUNK_REGION_MARKER);
core[at + 14] = 4;
core[at + 15] = 0xff;
for k in 0..4 {
let word = (crate::chunks::CHUNK_TAG as u32) << 24;
let at =
at + crate::chunks::CHUNK_HEADER_BYTES + crate::chunks::CHUNK_GAP_BYTES + k * 4;
core[at..at + 4].copy_from_slice(&word.to_le_bytes());
}
}
#[test]
@@ -170,7 +253,8 @@ mod tests {
"collision",
"moby",
"tie",
"shrub"
"shrub",
"chunks"
]
);
assert_eq!(regions[0].start, 0);
@@ -203,7 +287,7 @@ mod tests {
}
#[test]
fn the_last_region_ends_at_the_constant_trailer() {
fn the_class_regions_end_at_the_chunk_container() {
let core_len = 1 << 24;
let v = index(
100,
@@ -212,29 +296,74 @@ mod tests {
&[(200, &[1000]), (400, &[2000]), (600, &[3000])],
);
let regions = layout(&v);
// The container sits 100 bytes past the last shrub class, which is
// where the search for it starts.
let container = 3000 + 100;
let shrub = regions.iter().find(|r| r.name == "shrub").unwrap();
assert_eq!(shrub.start, 3000);
assert_eq!(shrub.end, Some(container));
let chunks = regions.iter().find(|r| r.name == "chunks").unwrap();
assert_eq!(chunks.start, container);
// Not core_data.len(): the trailer is constant across levels and does
// not belong to any region.
assert_eq!(shrub.end, Some(core_len - CORE_DATA_TRAILER_BYTES));
assert!(shrub.end.unwrap() < core_len);
assert_eq!(chunks.end, Some(core_len - CORE_DATA_TRAILER_BYTES));
assert!(chunks.end.unwrap() < core_len);
// Tie still ends at shrub, not at the container.
let tie = regions.iter().find(|r| r.name == "tie").unwrap();
assert_eq!(tie.end, Some(3000));
}
#[test]
fn a_lump_shorter_than_the_trailer_leaves_the_end_unresolved() {
let mut v = index(
fn tie_ends_at_the_container_when_there_is_no_shrub() {
// Third class table absent: tie is the last class region, so it is tie
// the container closes.
let v = index(100, 200, 300, &[(200, &[1000]), (400, &[2000])]);
let h = crate::level_core::LevelCoreHeader::parse(&v).unwrap();
let core = core(&v);
let container = 2000 + 100;
let regions = core_layout(&v, &core, &h);
let tie = regions.iter().find(|r| r.name == "tie").unwrap();
assert_eq!(tie.end, Some(container));
assert!(regions.iter().all(|r| r.name != "shrub"));
let chunks = regions.iter().find(|r| r.name == "chunks").unwrap();
assert_eq!(chunks.start, container);
}
#[test]
fn an_unlocated_container_leaves_the_class_end_unresolved() {
// A core of plain zeros carries no signature, so shrub cannot be
// closed -- the end stays visible rather than clamped to the lump's
// tail, and the chunk region does not exist at all.
let v = index(
100,
200,
300,
&[(200, &[1000]), (400, &[2000]), (600, &[3000])],
);
let h = crate::level_core::LevelCoreHeader::parse(&v).unwrap();
let regions = core_layout(&v, &h, CORE_DATA_TRAILER_BYTES - 1);
let regions = core_layout(&v, &[0u8; 4 << 20], &h);
let shrub = regions.iter().find(|r| r.name == "shrub").unwrap();
assert_eq!(shrub.end, None, "must not clamp a backwards bound");
let _ = &mut v;
assert_eq!(shrub.end, None);
assert!(regions.iter().all(|r| r.name != "chunks"));
}
#[test]
fn a_lump_shorter_than_the_trailer_leaves_the_end_unresolved() {
let v = index(
100,
200,
300,
&[(200, &[1000]), (400, &[2000]), (600, &[3000])],
);
let h = crate::level_core::LevelCoreHeader::parse(&v).unwrap();
// Shorter than the trailer, so the container starts but cannot end.
let mut core = vec![0u8; CORE_DATA_TRAILER_BYTES - 1];
plant_chunk(&mut core, 3000 + 100);
let regions = core_layout(&v, &core, &h);
let shrub = regions.iter().find(|r| r.name == "shrub").unwrap();
assert_eq!(shrub.end, Some(3100));
let chunks = regions.iter().find(|r| r.name == "chunks").unwrap();
assert_eq!(chunks.end, None, "must not clamp a backwards bound");
}
#[test]