S4.3 — Space resolution: valid segment count

Intent

Resolution belongs to the state space, not to the planners. Today compound, SE2 and SE3 merge their components' longest valid segment lengths by weighted RMS, which under-samples rotation: SE2 on [0,10]² at default settings would inspect a 180° turn every 36° once the ×0.1 factor goes (rotation alone needs 9°). Adopt OMPL's rule (max of per-component segment counts) and OMPL's default fraction. Supersedes part of S2.2 (cancelled).

Decisions (ADR-0003; grilling Q2/Q3/Q13)

• New provided method StateSpace::valid_segment_count(&self, s1, s2) -> usize, default ceil(distance(s1, s2) / longest_valid_segment_length()). Custom spaces keep working unchanged.
• CompoundStateSpace, SE2StateSpace, SE3StateSpace override it with the max over components of each component's own count (OMPL CompoundStateSpace::validSegmentCount).
• Default longest-valid-segment fraction 0.01 (was 0.05) in RealVector, SO2, SO3. Single spaces become 2× coarser than today's effective spacing; compound spaces become finer than today.
• Migration note for users: set the fraction to 0.005 to reproduce the old single-space spacing.
• OMPL's extra longestValidSegmentCountFactor is not adopted (the fraction covers it; can be added later as a provided method).
• The ×0.1 factor itself is removed in S4.4, not here: S4.3 only adds the method and changes defaults, so behaviour shifts in two observable steps.

Acceptance criteria

• valid_segment_count provided method on StateSpace (oxmpl/src/base/space.rs), plus a _dyn counterpart on AnyStateSpace so compound spaces can ask their components
• Max-of-components overrides on compound, SE2, SE3
• Defaults changed to 0.01 in real_vector_state_space.rs (:98), so2_state_space.rs (:72), so3_state_space.rs (:77)
• Numeric test: SE2 on [0,10]², default fractions, pure 180° rotation ⇒ count equals the SO2 component's count (20 at 0.05 / 100 at 0.01 — recompute), not the RMS-derived one
• Numeric test: compound(RV2 + SO2) count equals the max of the component counts
• Unit tests for the default implementation on RealVector
• just test-all green (planners still use their own check_motion until S4.4)

Tasks

• Add the method and the _dyn plumbing (spaces/any_state_space.rs :51–:89 shows the existing _dyn pattern)
• Overrides: compound_state_space.rs (RMS at :137), se2_state_space.rs (:134), se3_state_space.rs (:140)
• Decide whether the RMS get_longest_valid_segment_length stays as is (it is still used by the default valid_segment_count of non-compound spaces); document it
• Change the three defaults; update doc comments that mention 0.05
• Tests as above

Open question (resolve at implementation, flag to owner)

• The fraction setters clamp to [0, 1] (real_vector_state_space.rs:121, so2:82, so3:87), so a fraction of 0 gives a zero length and a division by zero in valid_segment_count. OMPL throws for fractions outside (ε, 1−ε). Options: reject non-positive fractions (breaking: setter becomes fallible) or guard the division (count = usize::MAX/error). Breaking is acceptable in 0.9.0; pick one and record it in the PR.

Source

Agreed in the senior-engineer grilling 2026-10-08 (decisions: repo docs/planning/adr/0003-motion-validation.md; glossary: CONTEXT.md). Regeneration spec: oxmpl - sprint-003. Line references are as of origin/main c558556 (2026-10-08), before E3. E3 renames things (rand Rng→RngExt, PyO3 with_gil→attach, edition 2024), so re-grep before trusting a line number.