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formats: REGN section 3 is the cell index, self-checked on all 11 objects

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The fourth section is one 8-byte (count, offset) record per grid cell, followed
by the 32-byte records it points at. The check: the lowest offset any cell refers
to equals align16(offsets[3] + cells*8) on 11 of 11 objects - and the alignment
term is visible rather than assumed because the three 5x5x5 maps have 125*8 =
1000 bytes of index, which is not 16-aligned, so their payload starts 8 bytes
later than the six 10x10x10 maps'.

Two further invariants from the same sweep: every occupied cell has count exactly
1 (total items == occupied cells on all 11, so it is one record per cell rather
than a bucket list), and counts[4] equals occupied cells + 2 exactly on all 11 -
the +2 unexplained and recorded as such.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PMRJjbxLqZtsb5Vb7KunPE
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Sylpheed RE agent
2026-08-24 10:31:42 +00:00
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docs/re/structures/regn-map-grid.md
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@@ -50,7 +50,34 @@ Two independent fields reproducing the grid is what rules out coincidence.
So every map is a cube partitioned into 125 or 1 000 uniform cells — 10 km cells
in a 100 km cube for the common case, and one pair of maps five times larger.
## ❔ The four sections
## ✅ Section 3 is the cell index — and it self-checks 11/11
The fourth section is a **one entry per cell** table of 8-byte records
`(count, offset)`, immediately followed by the records those offsets point at:
```
index = offsets[3] .. offsets[3] + cells*8
payload = align16(index end) ..
record = 32 bytes (offsets step by 0x20)
```
Checked over all 11 objects: the lowest offset any cell refers to equals
**`align16(offsets[3] + cells × 8)` — 11 of 11**. On the six 10×10×10 maps
`cells × 8` is already 16-aligned and the payload butts straight up against the
index; on the three 5×5×5 maps `125 × 8 = 1000` is not, and the payload starts
8 bytes later, which is what makes the alignment rule visible rather than assumed.
Two more invariants from the same sweep:
* every occupied cell has count **exactly 1** — total items equals occupied cells
on all 11 objects, so this is "one record per cell", not a bucket list;
* **`counts[4] = occupied cells + 2`**, exactly, on all 11 (e.g. 997/995,
880/878, 125/123). The `+2` is unexplained — two sentinels, or two cells
counted differently.
Most cells are occupied: 878–998 of 1 000, 123 of 125.
## ❔ The other three sections
Their offsets scale with the object, and `counts[0..2]` scale with them —
`(318, 1172, 2584)` for the 350 KB map against `(2936, 14967, 31155)` for the