docs(physics): #347 fix contract — retail's within-tick slide continuation, pinned from pc with candidate mechanisms ordered
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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docs/research/2026-08-08-347-fix-contract.md
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# #347 fix contract — retail's within-tick slide continuation (full-rate glide)
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**Date:** 2026-08-08. **Implementer: Fable directly (user direction), dual
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Opus review, then the user's slope feel gate.** Predecessor: #345 landed at
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`ab89ebdf` (validate_walkable return scoping); this contract closes AD-70.
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## The defect (measured, not hypothesized)
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Post-#345, the glide alternates in a strict two-tick cycle (scratch trace,
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2026-08-08, 45-degree diagonal fixture):
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- arming tick: ZERO XY yield, sliding normal becomes (0.707,-0.707,0),
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transient gains Sliding;
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- moving tick: +0.115/+0.115 (the along-crease component), sliding normal
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CLEARS;
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- repeat. 14 of 30 ticks stuck; lateral rate is HALF the input's lateral
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component.
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Retail delivers motion EVERY tick: cdb counters (`345-retail-glide.cdb.log`)
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show edge_slide/cliff_slide 594 each over one ~15 s glide (~per 30 Hz tick),
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set_sliding_normal 538, step_up 0.
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## Retail's mechanism (pinned from pc this session)
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`CTransition::edge_slide` @0x0050b3d0 (pc:273001+), steep-contact branch
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(contact valid AND N.z < allowance @0050b3f7-0050b441):
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1. `sphere_path.walkable = null; restore_check_pos()` — back to the SAVED
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(advanced, pre-step-down) candidate;
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2. `*outState = cliff_slide(&contact_plane)` — see below;
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3. clears contact plane validity/water;
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4. **returns 0 (FALSE = do not stop)** — `transitional_insert`'s attempt
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loop CONTINUES and retries at the position cliff_slide just produced.
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`CTransition::cliff_slide` @0x0050a6d0 (pc:272397+):
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1. crease = cross(steepN, **last_known_contact_plane.N**) — NOT the current
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contact; horizontal-projected (Z forced 0 via *0f arithmetic in BN's
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rendering) and 90-degree-rotated: vector (-crossY, crossX, 0) = the
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horizontal PERPENDICULAR to the crease;
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2. `normalize_check_small` degenerate -> return 1 (OK_TS);
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3. proj = dot(check-minus-curr displacement + LandDefs::get_block_offset
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(curr cell vs check cell — the AD-69 seam family), perpVector);
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4. sign-dependent arm: adds `perpVector * (+-proj)` to the CHECK POSITION
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(add_offset_to_check_pos @0050a804/0050a857) — REMOVING the into-face
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perpendicular component so the check pos keeps only the along-crease
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part — and sets the collision normal (one arm negates the normal:
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0050a813-0050a827);
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5. **returns 3 (ADJUSTED)**.
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Net: the SAME transitional_insert call retries at the slid position, the
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insert validates clean, step-down lands on the flat side, OK — motion
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delivered within the tick, every tick. The sliding normal is set as well,
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so next tick's AdjustOffset pre-projection ALSO applies (that projection
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producing an already-clean request is why retail still fires edge_slide
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per tick: the request keeps pressing into the face).
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## Our port today (`TransitionTypes.cs`)
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- `TransitionalInsert` (:2306-2335) already continues the loop when
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`EdgeSlideAfterStepDownFailed` returns false — the LOOP shape is ported.
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- `EdgeSlideAfterStepDownFailed` branch2 (:2534-2544) matches retail's
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steep-contact branch: restore, CliffSlide, clear, return false.
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- `CliffSlide` (:2630-2672) computes the same perpVector and even calls
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`sp.AddOffsetToCheckPos` with a sign-dependent arm and returns Adjusted.
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**Yet the arming tick yields zero.** So the divergence is INSIDE this
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chain, not its shape. Candidates, in test order:
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1. **Sign-arm inversion (BN flag-test ambiguity).** Ours adds
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`collideNormal*angle` when angle<=0 and `collideNormal*(-angle)` when
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angle>0 — BOTH are non-positive multiples. Removing the perpendicular
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component requires the CANCELLING sign in both arms: for angle>0
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subtract (ours does), for angle<0 ADD the positive multiple (ours
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subtracts more — DOUBLING the into-face component instead of
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cancelling). Retail's two arms at 0050a7d6/0050a849 vs 0050a7f6 are
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sign-mushed in BN — byte-decode 0050a7a0-0050a870 (fchs placement)
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before concluding. If ours doubles the into-face component on one arm,
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the slid retry re-collides HARDER -> retries exhaust -> zero yield ->
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exactly the alternation (next tick's pre-projection is what moves).
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2. **Retry-after-CliffSlide dies.** The Adjusted continue (:2323-2327)
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re-enters InsertIntoCell; if the slid check pos still validates against
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the steep triangle (e.g. because the offset was wrong per candidate 1,
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or because the retry re-runs the PRIMARY phase from the pre-advance
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position rather than the slid candidate), attempts exhaust. The
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transit-fail probe's buffered [transit-fail-insert] lines (19 per stuck
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tick in the scratch run) name each attempt's phase outcomes — READ THEM
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FIRST; they may settle candidates 1 and 2 in one look.
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3. **restore_check_pos ordering.** Retail restores BEFORE cliff_slide and
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cliff_slide then adds its offset to the RESTORED (advanced) candidate.
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Ours: branch2 restores then CliffSlide adds to... verify which position
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RestoreCheckPos leaves in GlobalSphere (SaveCheckPos at :2265 saved the
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ADVANCED candidate, so restore should equal retail). Confirm with the
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probe's per-attempt positions.
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## D1 — the minimal port
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Fix ONLY what the instrumentation names (expected: the CliffSlide sign
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arm(s), possibly one retry-entry position). Do NOT touch: the loop shape,
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branch1/3/4 responses, PrecipiceSlide, the #331 sliding-normal
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persistence/clearing, AdjustOffset (AD-66!), ValidateWalkable (#345 just
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landed), DoStepDown internals.
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## D2 — tests
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1. Tighten `Issue345SteepSlopeGlideTests.Angled45Approach_GlidesAlongTheDiagonal`:
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full-rate — stuckTicks small (crossing transient only, e.g. <= 3) and
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lateral advance ~= the input's lateral component times the post-crossing
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tick count (assert >= 85% of it, ordering-safe), replacing the
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alternation-tolerant `Ticks/2 + 2` bound and its comment (and drop the
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#347-residual comment).
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2. The 30/60-degree ordering pin and perpendicular stop stay green
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unchanged.
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3. AD-65/AD-66 conformance, the #331 absorb pin, RetailEdgeResponseOrdering,
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Issue265, TransitFailProbe, EdgeSlideBackProbePrecipiceSlideTests, and
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the #271 staircase guard ALL stay green untouched.
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4. Sabotage: revert the sign/continuation fix -> the tightened full-rate
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assertion reds with the alternation numbers; restore.
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## Acceptance
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Clean-room complete suite; dual Opus review (conformance byte-check of
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cliff_slide's fchs arms + blast radius over every CliffSlide caller);
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retire AD-70 in the same commit; user slope feel gate (~2 min: glide speed
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now matches retail side-by-side; downhill/uphill/hover unchanged).
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