# AD-55 byte-decode — independent confirmation of a fix that already shipped (VERDICT CORRECTED)

**CORRECTION, same night:** this note's original verdict — "our port is wrong"
— was STALE. The production code has carried the byte-confirmed `0.98480775f`
since `252e8068` (2026-07-30), whose commit message contains this exact
instruction listing. The register row was retired in that commit and then
**resurrected by `a8a7d64b`**, a revert of the unrelated TS-4 commit whose
register hunk restored the older row text. This note's derivation was
performed against that zombie row; it stands as an INDEPENDENT confirmation
of the 2026-07-30 result (identical bytes, identical constant, identical
conclusion) and as the measured cost of a register row surviving its own
retirement. No code change is needed; no feel gate is owed — the user has
been playing on the fixed constant for a week.

*Original note below, its evidence valid, its verdict superseded.*

**Date:** 2026-08-07 (overnight). **Method:** `reference_pe_byte_decode` — raw
bytes from the PDB-paired v11.4186 binary (`check_exe_pdb.py` → MATCH,
CodeView GUID `9e847e2f-777c-4bd9-886c-22256bb87f32`), not the decomp text.

## The question AD-55 filed

`PhysicsBody.calc_friction`'s Sledding near-flat branch compares
`GroundNormal.Z > 0.99999536f` (≈0.175° from flat). The raw decomp of
`CPhysicsObj::calc_friction` @0x0050ee70 instead shows
`__fcos(0.17453292519943295)` — cos(10°) ≈ 0.984808 — compared against
`contact_plane.N.z`. One of the two had to be a decode artifact.

## The bytes @0x0050ef53 (verbatim from the binary)

```
d9 86 38 01 00 00    fld   dword [esi+0x138]      ; contact_plane.N.z
dd 05 28 6b 7c 00    fld   qword [0x007c6b28]     ; the constant
d9 ff                fcos
de d9                fcompp                        ; cos(const) vs N.z
df e0                fnstsw ax
f6 c4 41             test  ah, 0x41
7a 0a                jp    +0x0a                   ; skip the friction load
8b 86 bc 00 00 00    mov   eax, [esi+0xbc]        ; this->friction
89 44 24 04          mov   [esp+4], eax
d9 44 24 04          fld   dword [esp+4]
```

`qword [0x007c6b28]` = **0.17453292519943295 = π/18 exactly** (verified by
direct read at the mapped file offset). The binary genuinely executes `FCOS`
at runtime — the compiler did not fold it — so the threshold retail compares
against `N.z` is **cos(π/18 rad) = cos(10°) = 0.984807753...**.

## The verdict

- **The decomp was RIGHT. Our port is wrong.** `0.99999536f` is
  `cos(0.17453292519943295°)` — the *radian* literal read as *degrees* and
  run through a degree→radian cosine. A one-character-class unit slip that
  survived because nothing gates slope feel numerically.
- **Felt consequence:** the branch means "on ground flatter than the
  threshold, use the object's own friction; on steeper ground (while slow —
  the `arg3 < 6.25` speed² gate at 0x0050ef46 guards this), keep the 0.2
  sliding friction." With our constant, "flat" requires < 0.175° — real
  terrain triangles essentially never qualify, so the object-friction arm of
  Sledding is unreachable in practice and slow movers keep sliding friction
  on gentle slopes retail treats as flat. Ice-feel in exactly the S4/S5
  slope-feel family.
- **Fix shape (S5):** replace the constant with retail's semantics. Either
  the folded `0.98480775f` with a comment carrying this evidence, or the
  exact `MathF.Cos(MathF.PI / 18f)` computed once — prefer the folded
  constant + comment, matching how AP-7's 0.25f landed. Polarity must be
  ported from the `test ah,0x41; jp` idiom above, not assumed: the friction
  load is SKIPPED when the jump is taken (cos(10°) > N.z, i.e. steeper than
  10°, or unordered), and taken when N.z ≥ cos(10°). Verify our branch's
  existing polarity against this before changing only the constant.
- The speed² gates in the same function — 1.5625 (= 1.25²) at 0x0050ef24 and
  6.25 (= 2.5²) at 0x0050ef46 — matched our port already at AP-7 and are
  untouched.

## Bookkeeping owed at the fix

Retire AD-55 (the row's open question is now answered against our constant);
conformance test pinning `0.98480775f` + the ported polarity; S4/S5's
slope-feel session covers the felt change. Until the fix lands, the register
row stands corrected by this note.