acdream/docs/research/2026-08-08-347-fix-contract.md

# #347 fix contract — retail's within-tick slide continuation (full-rate glide)

**Date:** 2026-08-08. **Implementer: Fable directly (user direction), dual
Opus review, then the user's slope feel gate.** Predecessor: #345 landed at
`ab89ebdf` (validate_walkable return scoping); this contract closes AD-70.

## The defect (measured, not hypothesized)

Post-#345, the glide alternates in a strict two-tick cycle (scratch trace,
2026-08-08, 45-degree diagonal fixture):
- arming tick: ZERO XY yield, sliding normal becomes (0.707,-0.707,0),
  transient gains Sliding;
- moving tick: +0.115/+0.115 (the along-crease component), sliding normal
  CLEARS;
- repeat. 14 of 30 ticks stuck; lateral rate is HALF the input's lateral
  component.

Retail delivers motion EVERY tick: cdb counters (`345-retail-glide.cdb.log`)
show edge_slide/cliff_slide 594 each over one ~15 s glide (~per 30 Hz tick),
set_sliding_normal 538, step_up 0.

## Retail's mechanism (pinned from pc this session)

`CTransition::edge_slide` @0x0050b3d0 (pc:273001+), steep-contact branch
(contact valid AND N.z < allowance @0050b3f7-0050b441):
1. `sphere_path.walkable = null; restore_check_pos()` — back to the SAVED
   (advanced, pre-step-down) candidate;
2. `*outState = cliff_slide(&contact_plane)` — see below;
3. clears contact plane validity/water;
4. **returns 0 (FALSE = do not stop)** — `transitional_insert`'s attempt
   loop CONTINUES and retries at the position cliff_slide just produced.

`CTransition::cliff_slide` @0x0050a6d0 (pc:272397+):
1. crease = cross(steepN, **last_known_contact_plane.N**) — NOT the current
   contact; horizontal-projected (Z forced 0 via *0f arithmetic in BN's
   rendering) and 90-degree-rotated: vector (-crossY, crossX, 0) = the
   horizontal PERPENDICULAR to the crease;
2. `normalize_check_small` degenerate -> return 1 (OK_TS);
3. proj = dot(check-minus-curr displacement + LandDefs::get_block_offset
   (curr cell vs check cell — the AD-69 seam family), perpVector);
4. sign-dependent arm: adds `perpVector * (+-proj)` to the CHECK POSITION
   (add_offset_to_check_pos @0050a804/0050a857) — REMOVING the into-face
   perpendicular component so the check pos keeps only the along-crease
   part — and sets the collision normal (one arm negates the normal:
   0050a813-0050a827);
5. **returns 3 (ADJUSTED)**.

Net: the SAME transitional_insert call retries at the slid position, the
insert validates clean, step-down lands on the flat side, OK — motion
delivered within the tick, every tick. The sliding normal is set as well,
so next tick's AdjustOffset pre-projection ALSO applies (that projection
producing an already-clean request is why retail still fires edge_slide
per tick: the request keeps pressing into the face).

## Our port today (`TransitionTypes.cs`)

- `TransitionalInsert` (:2306-2335) already continues the loop when
  `EdgeSlideAfterStepDownFailed` returns false — the LOOP shape is ported.
- `EdgeSlideAfterStepDownFailed` branch2 (:2534-2544) matches retail's
  steep-contact branch: restore, CliffSlide, clear, return false.
- `CliffSlide` (:2630-2672) computes the same perpVector and even calls
  `sp.AddOffsetToCheckPos` with a sign-dependent arm and returns Adjusted.

**Yet the arming tick yields zero.** So the divergence is INSIDE this
chain, not its shape. Candidates, in test order:

1. **Sign-arm inversion (BN flag-test ambiguity).** Ours adds
   `collideNormal*angle` when angle<=0 and `collideNormal*(-angle)` when
   angle>0 — BOTH are non-positive multiples. Removing the perpendicular
   component requires the CANCELLING sign in both arms: for angle>0
   subtract (ours does), for angle<0 ADD the positive multiple (ours
   subtracts more — DOUBLING the into-face component instead of
   cancelling). Retail's two arms at 0050a7d6/0050a849 vs 0050a7f6 are
   sign-mushed in BN — byte-decode 0050a7a0-0050a870 (fchs placement)
   before concluding. If ours doubles the into-face component on one arm,
   the slid retry re-collides HARDER -> retries exhaust -> zero yield ->
   exactly the alternation (next tick's pre-projection is what moves).
2. **Retry-after-CliffSlide dies.** The Adjusted continue (:2323-2327)
   re-enters InsertIntoCell; if the slid check pos still validates against
   the steep triangle (e.g. because the offset was wrong per candidate 1,
   or because the retry re-runs the PRIMARY phase from the pre-advance
   position rather than the slid candidate), attempts exhaust. The
   transit-fail probe's buffered [transit-fail-insert] lines (19 per stuck
   tick in the scratch run) name each attempt's phase outcomes — READ THEM
   FIRST; they may settle candidates 1 and 2 in one look.
3. **restore_check_pos ordering.** Retail restores BEFORE cliff_slide and
   cliff_slide then adds its offset to the RESTORED (advanced) candidate.
   Ours: branch2 restores then CliffSlide adds to... verify which position
   RestoreCheckPos leaves in GlobalSphere (SaveCheckPos at :2265 saved the
   ADVANCED candidate, so restore should equal retail). Confirm with the
   probe's per-attempt positions.

## D1 — the minimal port

Fix ONLY what the instrumentation names (expected: the CliffSlide sign
arm(s), possibly one retry-entry position). Do NOT touch: the loop shape,
branch1/3/4 responses, PrecipiceSlide, the #331 sliding-normal
persistence/clearing, AdjustOffset (AD-66!), ValidateWalkable (#345 just
landed), DoStepDown internals.

## D2 — tests

1. Tighten `Issue345SteepSlopeGlideTests.Angled45Approach_GlidesAlongTheDiagonal`:
   full-rate — stuckTicks small (crossing transient only, e.g. <= 3) and
   lateral advance ~= the input's lateral component times the post-crossing
   tick count (assert >= 85% of it, ordering-safe), replacing the
   alternation-tolerant `Ticks/2 + 2` bound and its comment (and drop the
   #347-residual comment).
2. The 30/60-degree ordering pin and perpendicular stop stay green
   unchanged.
3. AD-65/AD-66 conformance, the #331 absorb pin, RetailEdgeResponseOrdering,
   Issue265, TransitFailProbe, EdgeSlideBackProbePrecipiceSlideTests, and
   the #271 staircase guard ALL stay green untouched.
4. Sabotage: revert the sign/continuation fix -> the tightened full-rate
   assertion reds with the alternation numbers; restore.

## Acceptance

Clean-room complete suite; dual Opus review (conformance byte-check of
cliff_slide's fchs arms + blast radius over every CliffSlide caller);
retire AD-70 in the same commit; user slope feel gate (~2 min: glide speed
now matches retail side-by-side; downhill/uphill/hover unchanged).

## PREMISE REVISION (2026-08-08, same session — before any code change)

Three findings force the "retail = full-rate within-tick" premise back to
OPEN:

1. **cliff_slide's arms are conformant in all three sources.** ACE
   (`Transition.cs:242-266`), our port, and the byte decode agree on the
   crease vector, the sign arms, and the compare-vs-0.0 (double at
   0x794610, verified zero). For the conformance fixture's geometry those
   arms move the check position INTO the face (the arming tick's probe
   trace shows primary dist -0.056 -> -0.200 -> -0.487 across the three
   attempts) — the "dig" is what the code as written does, in retail's
   bytes too.
2. **The round-1 counters refute a per-tick retry storm in retail.**
   slidn:edge = 538:594 ~ 1:1.1. Our alternation shape produces THREE edge
   entries per sliding-normal set (the dig-retries); retail produces ~ONE.
   The glide window in the counter progression (edge 0 -> 594 across six
   1280-vwalk blocks, then frozen) also shows edge growth stopping the
   moment the hold ended.
3. **The user's side-by-side observation** ("I cant detect any speed
   change from retail", #345 gate, 2026-08-08) is consistent with retail
   ALTERNATING exactly as we do — arm one tick, move the next — and
   inconsistent with retail moving at double our rate.

Competing hypotheses now:
- **H-A (retail alternates too):** retail's arming tick runs ONE edge
  entry (no retry after the cliff-slide Adjusted) and yields nothing; the
  next tick's pre-projection moves. Then our ONLY divergence is the two
  extra futile dig-retries per arming tick (invisible — the tick's output
  is discarded), #347 closes as measured-identical, and AD-70 is RETIRED
  as a wrong inference, not fixed.
- **H-B (retail yields within the tick):** the single edge entry precedes
  a successful same-tick commit. Requires the retried/continued insert to
  land clean — mechanism unknown given the dig direction — and would keep
  AD-70 open as written.

**The discriminator is one number in the round-2 capture:**
`CTransition::find_transitional_position` (~1/tick for the player) vs
`edge_slide` during the glide window. H-A predicts ftp:edge ~ 2:1
(edge every other tick); H-B predicts ~ 1:1 (edge every tick). The
round-2 script (`tools/cdb/345-glide-stacks.cdb`) now counts ftp, prints
the periodic progression, samples 6 stacks each for edge/cliff/step_down,
and auto-detaches at 300 edge hits via the fall-through-then-top-level-qd
recipe. NO code changes until this capture runs.

## RESOLUTION (2026-08-08, round-2 capture): H-A confirmed in its strong form — CLOSED, no code change

Round-2 (`345-glide-stacks.cdb.log`, auto-detached at 768 edge hits):
glide-window steady state per 1280-vwalk block: edge +133..158, ftp
+93..105, stepdown +351..376, cliff lockstep with edge, stepup 0. edge/ftp
~ 1.45 with ftp INCLUDING background movers — the player's true ratio is
~1.5, which is precisely the alternation's signature (3 arming-tick
entries, 0 moving-tick entries, averaged). All six edge_slide stack
samples: transitional_insert -> find_transitional_position ->
CPhysicsObj::transition -> UpdateObjectInternal — our exact path. Retail
performs the same dig-retries and the same alternate-tick yield. #347
closed; AD-70 retired; the D2 "tighten to full-rate" plan is CANCELLED —
the existing alternation-tolerant assertion is the correct retail pin.
The H-B mechanism does not exist in the binary's behavior.