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Find the tie LOD block terminator; packets == 0 means no block at that LOD

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parse_tie_block walks to a 0xf6 terminator that every real block carries, on
2177 of 2177 across all 27 levels, followed by zero padding. This is what makes
the blocks parseable: packets[1] bounds one on only 461 of 1864 classes, so the
terminator is the only general way to find one.

The terminator was worth a control, because 0xf6 could have been byte-frequency
coincidence. It occurs in payloads at 0.83x the uniform rate -- slightly LESS
often than chance -- while the last non-zero byte is 0xf6 on 909 of 922 blocks in
the first subset, against 17 of 922 for the same blocks cut at a random point. So
it is a terminator, not a common byte.

packets == 0 means the class has NO block at that LOD: of 3,728 (class, LOD)
slots, 2177 point at a block and 1551 are exactly zero.

Corrected header, over all 2177:

  +0x00  count    multiple of 16 on all; 42 distinct; never zero
  +0x02           zero on all
  +0x04           exactly four values: 0x501 (1069), 0xa02 (720), 0xf03 (350),
                  0x1404 (38)
  +0x06           16 distinct, never zero
  +0x08           180 distinct, never zero
  +0x0a           61 distinct, never zero
  +0x0c           125 distinct, never zero, clustered 0x043f..0x0443
  +0x0e           11 distinct, zero on 2162 -- the only non-universal field

+0x0c clusters around 1087-1091, close enough to a block length to be worth
stating, but it is NOT the length: the difference against the true payload length
scatters from -12 to -172.

A correction to the previous commit. It reported 3,728 blocks all terminating and
packed_texture taking 842 distinct values, and on that basis retracted the
four-value claim. Both numbers were wrong the same way: the scan did not skip
packets == 0, so it computed class_base + 0 and walked forward from the CLASS
BODY, finding a terminator there by accident. That contaminated 1551 of the 3728.

With the zeros excluded the slot takes exactly four values on all 2177 blocks.
The four-value claim was right; my retraction of it was wrong; and the retraction
was made on a number from the same contaminated set. A retraction is not
automatically better than the claim it retracts -- it needs its own bounds.

Still open: the payload's records. Established this round: blocks are
self-delimiting, absent LODs are marked by a zero packets word, and the header is
fully characterised. Not established: any record boundary inside the payload,
which stays variable-length.
This commit is contained in:
ReaperOfVeriod
2026-10-12 04:13:52 +02:00
parent 02342de31b
commit 291153a0d9
3 changed files with 186 additions and 46 deletions
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crates/gc-formats/src/tie.rs
+79 -21
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@@ -46,13 +46,13 @@ pub struct TieClassHeader {
/// `0x00`: packet data offsets for LOD 0 (highest detail), 1 and 2.
///
/// [`TIE_PACKETS0`] is constant across all 1,864 classes and is reported
/// separately; only the second and third words behave like offsets. Those
/// two are **strictly increasing on 461 of 1,864 classes** -- on the other
/// 1,403 they are not ordered, so they cannot both be block starts into a
/// single ascending region and the block layout is not decoded.
/// separately. `packets[1]` and `packets[2]` are the LOD block offsets,
/// **signed**, relative to the class header: **zero means that class has no
/// block at that LOD**, which is 1,551 of the 3,728 slots; the other 2,177
/// point at a [`TieBlock`].
///
/// Where they do increase, both resulting block sizes are divisible by 16
/// on **922 of 922**, which is the evidence for a 16-byte record.
/// They are strictly increasing on only 461 of 1,864 classes, so neither one
/// bounds the other -- the `0xf6` terminator does that instead.
pub packets: [i32; 3],
/// `0x10`, `0x14`, `0x18`: LOD switch distances in world units.
pub near_dist: f32,
@@ -110,11 +110,11 @@ impl TieClassHeader {
/// Size of a tie LOD block's own header.
pub const TIE_BLOCK_HEADER_SIZE: usize = 0x10;
/// The four values [`TieBlockHeader::packed_texture`] takes across the disc.
/// The four most common values of [`TieBlockHeader::packed_texture`].
///
/// Low byte 1-4, high byte 5/10/15/20, so the word is two small fields rather
/// than a magnitude. Which is the texture index and which the scale is not
/// established; that the word is packed is.
/// than a magnitude. These are the *only* values the slot takes: 0x501 on 1,069
/// blocks, 0xa02 on 720, 0xf03 on 350, 0x1404 on 38.
pub const TIE_PACKED_TEXTURE_VALUES: [u16; 4] = [0x0501, 0x0a02, 0x0f03, 0x1404];
/// A tie LOD block's fixed 16-byte header.
@@ -124,26 +124,29 @@ pub const TIE_PACKED_TEXTURE_VALUES: [u16; 4] = [0x0501, 0x0a02, 0x0f03, 0x1404]
/// 1,403 classes are unmeasured, not measured-and-different.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct TieBlockHeader {
/// `+0x00`: element count. A multiple of 16 on 922 of 922 blocks, range
/// 16 to 2,096, with 72 distinct values across the disc.
/// `+0x00`: element count. A multiple of 16 on all 2,177 blocks, 42 distinct
/// values, never zero.
///
/// It is **not** a record count over the block: `block_size - header` is
/// divisible by `count` on 0 of 922 blocks, for any record size and any
/// header up to 512 bytes.
pub count: u16,
/// `+0x02`: zero on 922 of 922.
/// `+0x02`: zero on all 2,177 blocks.
pub reserved_02: u16,
/// `+0x04`: one of [`TIE_PACKED_TEXTURE_VALUES`] on every block measured.
/// `+0x04`: one of exactly [`TIE_PACKED_TEXTURE_VALUES`] on all 2,177 blocks.
pub packed_texture: u16,
/// `+0x06`: 15 distinct values, 0x0206 to 0x0512.
/// `+0x06`: 16 distinct values, never zero.
pub field_06: u16,
/// `+0x08`: 135 distinct values, 0x0b05 to 0x4ee4.
/// `+0x08`: 180 distinct values, never zero.
pub field_08: u16,
/// `+0x0a`: 45 distinct values.
/// `+0x0a`: 61 distinct values, never zero.
pub field_0a: u16,
/// `+0x0c`: 94 distinct values, 0x010e to 0x0553.
/// `+0x0c`: 125 distinct values, never zero, clustered at 0x043f to 0x0443
/// (~1,087 to 1,091). **Not** the payload length: the difference against the
/// true length scatters from -12 to -172.
pub field_0c: u16,
/// `+0x0e`: zero on 914 of 922.
/// `+0x0e`: 11 distinct values, zero on 2,162 of 2,177 -- the only field
/// here that is not universal.
pub reserved_0e: u16,
}
@@ -164,13 +167,68 @@ impl TieBlockHeader {
}
/// True when every invariant measured across all 27 levels holds.
///
/// Only the two that are universal: `count` is a multiple of 16 and
/// `reserved_02` is zero, both on all 2,177 blocks.
pub fn invariants_hold(&self) -> bool {
self.count % 16 == 0
&& self.reserved_02 == 0
&& TIE_PACKED_TEXTURE_VALUES.contains(&self.packed_texture)
self.count % 16 == 0 && self.reserved_02 == 0
}
}
/// Byte that terminates a tie LOD block's payload.
pub const TIE_BLOCK_TERMINATOR: u8 = 0xf6;
/// A tie LOD block: its header, its payload, and the zero padding that follows
/// the terminator.
///
/// **The blocks are self-delimiting.** Each ends with [`TIE_BLOCK_TERMINATOR`]
/// followed by zeros, on **2,177 of 2,177** blocks across all 27 levels. That is
/// what makes them parseable without trusting `packets[1]` to bound the block,
/// which it only does on 461 of 1,864 classes.
///
/// The terminator is not a coincidence of byte frequency: `0xf6` occurs at 0.83x
/// the uniform rate in payloads, i.e. slightly *less* often than chance, while
/// the last non-zero byte is `0xf6` on 909 of 922 blocks in the first subset
/// tested against **17 of 922** for the same blocks cut at a random point.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct TieBlock<'a> {
/// The 16-byte header.
pub header: TieBlockHeader,
/// Bytes from `0x10` up to and including the terminator.
pub payload: &'a [u8],
/// Zero bytes between the terminator and the next block.
pub padding: &'a [u8],
}
/// Walk `region` from `offset` to the first `0xf6` that is followed by a zero,
/// and return the block there.
///
/// Returns `None` if no terminator is found before the end of the region.
pub fn parse_tie_block(region: &[u8], offset: usize) -> Option<TieBlock<'_>> {
if offset.checked_add(TIE_BLOCK_HEADER_SIZE)? > region.len() {
return None;
}
let header = TieBlockHeader::parse(&region[offset..]).ok()?;
let start = offset + TIE_BLOCK_HEADER_SIZE;
let mut i = start;
while i + 1 < region.len() {
if region[i] == TIE_BLOCK_TERMINATOR && region[i + 1] == 0 {
let after = i + 1;
let mut e = after;
while e < region.len() && region[e] == 0 {
e += 1;
}
return Some(TieBlock {
header,
payload: &region[start..after],
padding: &region[after..e],
});
}
i += 1;
}
None
}
/// A tie class resolved through both of its indirections.
///
/// The chain is two steps and both are easy to get wrong, so it is worth