engineering/mech/part.py is the model the whole track draws from: a revolved
stack of segments along +X, or a closed prismatic outline plus a thickness, each
carrying a list of typed features. Pure arithmetic - no backend, no async, no
DXF - which is what lets the view engine be tested without a drawing.
Every refusal fires before anything is drawn and names the offending path the
way pid_from_spec does: a non-finite or non-positive number as
segments[2].d_outer, a bore that is not smaller than its outside (a bore that
changes diameter is two segments), an outline with fewer than three distinct
vertices, an outline that crosses itself (engineering.measure.is_self_intersecting
- the shoelace cancels crossed lobes rather than failing, so a bowtie would
otherwise measure 0.0), an unknown material, an unknown segment key, a duplicate
feature id.
Two gates are the reason this is a model rather than a dict: a feature whose
placement is off the part is refused by its own name and by the range it missed,
and two features that would remove the same material are refused with both names
and the overlapping box. Neither is clamped.
part_to_dict round-trips through build_part, which is what lets mech_part_draw
write the model onto the drawing as ACADMCP_MECH XDATA and mech_view_add read it
back months later without the caller re-describing the part.
Two fixes to the plan's own code and test list, both recorded in deviations:
_features_from imported engineering.mech.features unconditionally, so a
featureless part - every fixture in this module, and both round-trips - died on
ModuleNotFoundError until Task 3 lands; the import now happens only when there
is a feature dict to build. And the plan's test file omitted
test_a_duplicate_feature_id_is_refused while its own gate deselects it by name,
so the duplicate-id refusal the commit message claims went unpinned; the test is
written here and Task 3 turns it green.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd
engineering/mech/ opens with the four modules every track-B/G branch imports,
so the four worktrees can be cut from this commit and none of them has to
invent a name the others will conflict with.
- primitives.py: the typed descriptions the pure modules emit (Line, Arc,
Circle, Poly, HatchArea, Text, DimIntent) with bbox / translate / rotate /
scale, and ROLE_LAYER - the one place an ISO 128 line role becomes a layer
name, so no module hardcodes a layer. The arc bbox sweeps its quadrants: a
0-180 arc reports y=+r, not its endpoints, which is what keeps the view
layout from stacking two views on top of each other.
- xdata.py: the ACADMCP_MECH codec in Track A's shape - a 1001 application
marker, 1000 chunks of at most 255 ASCII characters, the 16 KB per-entity
ceiling enforced before anything is written, and a foreign application, a
corrupt chunk stream, a wrong version or an unknown kind refused with
ValueError instead of decoded into something plausible.
- standards/__init__.py: the table registry. A size outside the coverage is
refused against the coverage; a size inside it that is not a listed row is
refused against the rows. Nothing is interpolated, an ambiguous variant is
never resolved by taking the first row, and a registration whose SOURCE does
not name the table it was transcribed from is refused.
- standards/materials.py: the section-hatch map for nine materials. ISO
128-50 standardises only the general hatching (narrow continuous lines at 45
degrees) - that is acadiso.pat ANSI31 and what engineering/gear.py already
draws for steel. The distinguishing symbols are ASME Y14.2M's section lining
as shipped in AutoCAD 2026's acadiso.pat, read off the installed file; cast
iron's departure from ASME and the absence of any mechanical symbol for
concrete and wood are stated in the rows themselves. Angles are 0.0 because
the patterns already carry 45 degrees, and the two architectural patterns'
scales are derived from the ANSI31 line spacing and recomputed by the test
rather than chosen.
No tool is registered here.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd
