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fix(hypergraph): keep a slider's line through from_mechanism, count pairs in indeterminacy - #61
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…airs in indeterminacy Three loose ends from #56 (PR #60). from_mechanism dropped the slide line of a PrismaticJoint, which has line_point and axis but no joints on the line to hang a hyperedge on, so slider_crank() → from_mechanism → to_mechanism failed with "Could not determine solve order". It now writes the encoding to_mechanism reads: a hyperedge over the slider and two ground nodes on the line. Ground joints already on the line are reused, and missing ones are added as <id>_rail0 / <id>_rail1 at line_point and one axis length along. The round trip reproduces the original simulation and a second pass adds no node. to_mechanism built a prismatic driver node — a LinearActuator — as a rotary crank without a word. Mechanism has no linear driver link, so it now raises NotImplementedError. Mechanism.indeterminacy() counted every joint as a kinematic pair. A joint on k links joins them with k − 1 pairs: a ground joint that only marks a point of the frame, or a coupler point, is no pair, and a PrismaticJoint adds the prismatic pair with the frame its slide line stands for. The round-tripped slider-crank reported −1, a four-bar with a coupler point and a frame mark −3; both are 1 now.
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Follow-up to #60 / #56 — the three gaps noted there.
from_mechanismkeeps aPrismaticJoint's slide line. APrismaticJointhasline_pointandaxisbut no joints on the line, so nothing was emitted for it andslider_crank()→from_mechanism→to_mechanismfailed with "Could not determine solve order". It now writes the encodingto_mechanismreads: a hyperedge over the slider and two ground nodes on the line. Ground joints already on the line are reused (the motor pivot ofslider_crank()is one), and missing ones are added as<id>_rail0/<id>_rail1atline_pointand one axis length along. The round trip reproduces the original to ~1e-15 for horizontal, slanted and vertical rails, and a second pass adds no node.to_mechanismrefuses a prismatic driver node. ALinearActuatorwas silently built as a rotary crank.Mechanismhas no linear driver link, so this is now aNotImplementedErrorwith the reason — adding aLinearDriverLinkis a feature (step, derivatives, builder, serialization, numba path) and out of scope here.Mechanism.indeterminacy()counts pairs, not joints. A joint onklinks joins them withk − 1pairs: a ground joint that only marks a point of the frame, or a coupler point, is no pair, and aPrismaticJointadds the prismatic pair with the frame its slide line stands for. The round-tripped slider-crank reported −1, a four-bar with a coupler point and a frame mark −3; both report 1 now.fourbar()andslider_crank()are unchanged at 1.