# core_data region bounds How a level's `core_data` lump divides up, and how each region is bounded. ## The rule A region's end is the **next region's start**, never `core_data.len()`. The last region in a layout is the exception, and when its successor is unknown the end is `None` rather than a guess. This exists because region bounds were being re-derived per format during probing, usually by running from one known offset to the next with the last span running to the end of the lump. On level 0 that made the final shrub class a 925,008-byte span absorbing most of the lump's tail. Every aggregate computed over it -- record counts, value distributions, "this data looks random" -- was silently wrong, and nothing errored. `gc_formats::regions::core_layout` computes these once. Callers use it rather than inventing their own. ## Layout | region | start | end | |------------|----------------|----------------------| | `tfrags` | 0 | `occlusion` | | `occlusion`| `occlusion` | `sky` | | `sky` | `sky` | `collision` | | `collision`| `collision` | first moby class | | `moby` | first moby | first tie class | | `tie` | first tie | first shrub class | | `shrub` | first shrub | lump length − 0x38A10 | Class regions start at the lowest non-zero geometry offset in their class table. A zero entry means *absent*, not position zero -- the same trap the occlusion and collision headers each fell into separately. Not every level has every region: level 10 has no occlusion and no shrub. ## The constant trailer Comparing all 27 levels byte-for-byte from the end: the last **231,952 bytes (0x38A10)** of every level's `core_data` are identical. Level-specific content therefore ends at `len - 0x38A10`, and the final region is bounded there rather than at `core_data.len()`. This is what closes shrub. Across the 24 levels that have a shrub region it gives **24,442,928 bytes** total, and it is the same bound the disc test asserts on every level. What the trailer *is* is not established. It may be fixed data, or padding from a fixed-size decompression buffer. Either way it is constant, which is all that is needed to bound the tail. ## Open: what was in the region before it The offset ending the shrub region is not in the core index. Texture texels do **not** bound it. They sit before moby, not after shrub: on level 0 the texel range is 3539328..5326208 while the last shrub class starts at 16044288, leaving ~925 KB of real, non-zero data after the last shrub class that nothing yet accounts for. `core_layout` reports `end: None` for shrub on the 24 levels that have one, and the disc test asserts it, so the gap stays visible rather than being papered over by clamping to the end of the lump. ## Correction: it is mostly texture data, not an unnamed region Adding the region table to `gc-extract probe` made the gap visible immediately, which is the point of printing it. On level 0: ``` collision 2615232 .. 8832192 6216960 bytes ``` but collision's own header declares a payload of 903,248 bytes. So the range between the collision header and the first moby class is **6.2 MB**, and almost none of it is collision. The first conclusion drawn from this -- that ~6.2 MB of unidentified data sat between collision and moby, possibly `moby_anim` -- **was wrong**, and it was wrong because of the same failure that produced the other two bad numbers this session: quoting a figure before bounding it. Checking the bytes rather than just the sizes: the range is dominated by 8-bit indexed texture data. Byte-value concentration (a texture block uses a small palette; code and collision data does not) puts 18 of 21 sampled 4 KB blocks under 200 distinct byte values, against 249 for tfrags and 209 for collision. So the real layout is: | region | start | end | |------------|----------|------------------| | collision | 2615232 | 3518544 | | textures | 3518848 | ~8832192 | | moby | 8832192 | first tie class | The texture table's `data_offset` values only reach 5326208, which is why the gap looked real: mip chains extend well past the last entry's offset. A region sized from a table of first-level offsets understates the data it points at. The "3 of 21 blocks not texture-like" figure in the previous revision was an artifact of sampling every 256 KB. Re-scanned at 4 KB granularity the whole range is texture data: ~100 blocks stand out, and looking at them they are smooth gradient ramps -- sky and fog texture, exactly what gradient texels look like. A noisy gradient has a high distinct byte count without being code. So the range between collision and moby is **entirely texture data**, and there is no separate `moby_anim` region there. Combined with the correction above, two claims made about this range have now been wrong: the "6.2 MB of unknown data" and the "3 unexplained blocks". Where `moby_anim` actually lives is therefore still open, but it is not here. The remaining candidate is the unresolved tail after the last shrub class, which on level 0 is ~925 KB of real, non-zero data that nothing yet accounts for. ## Where shrub data ends, and the region after it **Range measured:** for each level, the last shrub class's `data_offset` field (`class + 0x2c`) through to the start of the constant trailer. Nothing outside that range is read. **Method:** the last shrub class's body is an array of 8-byte records whose final two bytes are `00 00`. Scanning forward in 8-byte steps for the first record whose trailing pair differs locates the body's end. This is a search for a specific stated pattern, not a statistic, so it either finds the boundary or reports that it did not. **Result:** on **24 of 24** levels the boundary lands exactly on an 8-byte record boundary. Body lengths are 192, 208, 224 or 240 bytes -- the same sizes seen when shrub classes were grouped and diffed earlier. On level 0: | what | range | bytes | |---|---|---| | last shrub class data | 16046800 .. 16047040 | 240 | | **next region** | 16047040 .. 16737344 | **690,304** | | constant trailer | 16737344 .. 16969296 | 231,952 | So the 690,304 bytes between the last shrub class and the trailer are a distinct region, not shrub data. What they contain is not established. Two things are worth noting about how this was found, given the rule in `AGENTS.md`: the earlier NaN-filler search found nothing at the start of this range and was about to conclude the body had zero length, which would have been wrong. The last shrub class is a real class with real data, and the NaN filler only appears in degenerate ones. Checking the actual byte pattern rather than concluding from a failed heuristic is what caught it. ### First structure in the region after shrub **Range measured:** level 0, `16047040 .. 16737344` (690,304 bytes), starting at the first byte after the last shrub class's 240-byte body. Nothing outside that range is read. **Method:** reading the bytes at `+0x20` as `u32` shows ten consecutive entries whose low halves increase by a constant **0x94 (148)**, followed by a zero. Read as u32 the high half is `0xf000` for the first nine and `0x0a40` for the tenth, so the table is ten entries long and ends at `+0x48` (absolute `16047112`). **Result, stated plainly:** a ten-entry table on a 148-byte stride. What the entries mean, what 148 refers to, and why there are ten, are all unestablished. Ten does not match any count already known for this level -- 238 moby classes, 74 tie, 7 shrub, 93 tfrag textures -- so this is not yet tied to any structure elsewhere in the lump. ### Follow-up: the 148 stride is a coincidence Treating the ten entries' low halves as offsets from the region start lands on blocks exactly 148 bytes apart, the first nine of which begin `08 00` followed by a 16-bit id stepping by 0x40. That is a convincing-looking structure and it does not survive being extended: walking the same 148-byte stride across the rest of the region, **only 14 of 4657 blocks** start with `08 00`. The first nine agree and then the pattern breaks. So the ten entries are not a pointer table. The constant stride held for eight consecutive values, which is far more than chance should produce and far less than a structure needs. Recorded because the lead was committed as a lead and this is where it goes. Nothing else about the region has been identified: it is real, non-zero data of unknown type, bounded, and unopened beyond these ~150 bytes. ## The region after shrub has a repeating structure at stride 368 **Range measured:** level 0, `16047040 .. 16737344`, sampled as the first 65,536 bytes. Cross-checked on every level that has the region, each sampled the same way from its own region start. **Method:** autocorrelation. For each candidate stride, count how often a 4-byte sequence at offset *i* repeats at *i + stride*, as a fraction of comparable positions. A random control of the same size is measured identically and reads flat at 0.0000 for every stride, so the statistic discriminates. **Result:** | stride | region match rate | |---|---| | 368 | 0.1110 | | 340 | 0.0935 | | 337–339 | 0.026–0.029 (background) | | random control | 0.0000 | Stride **368** is the strongest peak. On level 0, 340 is a secondary peak well clear of the background; the two differ by 28, which is unexplained. **Cross-check:** the peak stride was recomputed independently for every level that has the region, and **all 24 peak at exactly 368**. That is the part that makes this worth recording -- a single level's peak could be a property of that level's data, but 24 of 24 agreeing is a format invariant. What a 368-byte record *is* remains unestablished. This is the same position the shrub body was in before differential analysis identified its records, and the same method is the obvious next step. ### Differential analysis at stride 368: no constant substructure **Range measured:** level 0, `16047040 .. 16737344`. 1,875 whole 368-byte records from the region start; variation checked across the first 60. **Method:** the same differential analysis that identified the shrub body -- compare records byte by byte and record which positions differ. **Result:** **all 368 positions vary.** There is no constant field, no padding and no header inside a record. Unlike the shrub body, where differential analysis immediately separated a varying payload from a constant two-byte tail, this format has nothing constant to separate. That does not contradict the autocorrelation peak. Checking which byte positions carry the 368-stride matches over 20,000 positions: 168 exact 4-byte repeats, distributed evenly at 0.84% across all four byte offsets. So whole 4-byte sequences recur at exactly 368 while no individual position is constant -- consistent with records containing entries that repeat at a fixed relative index, rather than with a fixed-layout record. **So the 368 stride stands and the records remain unidentified.** The shrub method failed here not because it was the wrong tool but because it needs something constant to latch onto, and there is nothing. ## Sky opens with a 256-byte-row image **Range measured:** level 0, `2192576 .. 2615232` (the sky region, 422,656 bytes). Periodicity start and end located by sliding an 8 KB window in 256-byte steps through the region and measuring the 256-stride match rate. **Method:** autocorrelation as above, then a periodicity scan. A random control of the same size reads flat at 0.0000 for every stride tested. **Result:** - Peak stride in the first 64 KB is **256** at a 0.772 match rate, against a flat random control. Strides 255, 257 and 4 score nearly as high, which is what a true period looks like smeared by sampling. - Across levels the peak is 256 on 21, 512 on 5 and 128 on 1. 512 and 128 are a harmonic and a sub-multiple, so the fundamental period is **256 bytes**. - The periodicity holds from `+0` to about `+60160` and then stops: **236 rows of 256 bytes**, i.e. a 256-wide, 236-tall image at one byte per pixel. - The remaining ~362,240 bytes of the sky region are *not* 256-periodic and remain unidentified. That is 86% of the region, and it is where the offset table seen at `+0x20` earlier points. So the sky region is not one thing. Its first ~60 KB is an image; the rest is whatever the layer offsets address. Both halves still need work, and the image is the tractable one.