261322b48e
Port the first retail precipice-slide slice from named retail/ACE: terrain and BSP walkable hits now preserve polygon vertices, failed step-down edges back-probe to rediscover the walkable polygon, and edge-slide can run precipice/cliff slide instead of only hard-stopping. Adds pseudocode anchors plus regression coverage for terrain polygon context and loaded-terrain boundary edge-slide. Co-authored-by: Codex <codex@openai.com>
110 lines
3.5 KiB
Markdown
110 lines
3.5 KiB
Markdown
# Precipice Slide Pseudocode
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Date: 2026-04-30
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Phase: L.2c - Movement & Collision Conformance
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## Retail Anchors
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- Named retail: `CTransition::edge_slide`, `acclient_2013_pseudo_c.txt:273001`
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- Named retail: `CTransition::cliff_slide`, `acclient_2013_pseudo_c.txt:272397`
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- Named retail: `SPHEREPATH::precipice_slide`, `acclient_2013_pseudo_c.txt:274316`
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- ACE cross-check: `Transition.EdgeSlide`, `Transition.CliffSlide`,
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`SpherePath.PrecipiceSlide`
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- ACE cross-check: `Polygon.find_crossed_edge`
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## Edge-Slide Flow
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When a grounded mover has contact state but the next candidate position has no
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walkable surface within step-down reach, retail does not immediately accept the
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fall or hard-stop. It enters `CTransition::edge_slide`.
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```text
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edge_slide(transitionState, stepDownHeight, walkableZ):
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if object is not OnWalkable or EdgeSlide is disabled:
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clear walkable
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restore candidate check position
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clear current contact plane
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mark cell array valid
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transitionState = OK
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return handled
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if current collision has a contact plane below walkableZ:
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transitionState = cliff_slide(contact plane)
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clear walkable and restore candidate check position
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clear current contact plane
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return not-final
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if sphere_path.walkable exists:
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transitionState = precipice_slide()
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clear current contact plane and restore candidate check position
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return transitionState == Collided
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if current collision has any contact plane:
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clear walkable
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restore candidate check position
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clear current contact plane
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transitionState = OK
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return handled
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move CheckPos back from failed candidate to the current sphere center
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step_down(stepDownHeight, walkableZ) to rediscover the walkable polygon
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clear current contact plane
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restore the failed candidate check position
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if a walkable polygon was discovered:
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set walkable_check_pos from the candidate sphere in walkable space
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transitionState = precipice_slide()
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return transitionState == Collided
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clear walkable
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mark cell array valid
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transitionState = Collided
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return handled
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```
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## Precipice Slide
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`SPHEREPATH::precipice_slide` is the edge-normal half of edge-slide. The crucial
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input is the walkable polygon that the mover just left; without that polygon,
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there is no crossed edge to slide along.
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```text
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precipice_slide():
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normal = zero
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found = walkable.find_crossed_edge(walkable_check_pos, walkable_up, normal)
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if not found:
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clear walkable
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return Collided
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clear walkable
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step_up = false
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normal = walkable_pos.frame.LocalToGlobalVec(normal)
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blockOffset = LandDefs.GetBlockOffset(curr cell, check cell)
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movementOffset = global_sphere.center - global_curr_center.center + blockOffset
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if dot(normal, movementOffset) > 0:
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normal = -normal
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return global_sphere.slide_sphere(transition, normal, global_curr_center.center)
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```
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## Porting Notes
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acdream already had the `Polygon.find_crossed_edge` math inside `BSPQuery`, but
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the live diagnostic showed `walkableValid=False` at the failed step-down edge
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branch. The port must therefore preserve or rediscover the walkable polygon,
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not just pass the `EdgeSlide` flag.
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For the first L.2c slice:
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- terrain supplies the exact current triangle vertices alongside its plane;
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- BSP step-down/find-walkable records world-space polygon vertices when the
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caller supplies the object's world origin;
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- the failed step-down edge branch performs the retail back-probe to current
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position before calling precipice slide;
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- `CELLARRAY`, full `cell_bsp` ownership, and cross-cell building portals remain
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L.2e work.
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