edges_runs

Turns the sorted sides of a mesh into the edge table. Two stages share one dispatch shape, chosen by the STAGE specialization:

• STAGE heads flags, per sorted place, where a new edge starts;
• STAGE table, after the flags are compacted into starts, writes each edge's two points (a < b) and the number of sides along it (fcount).

The whole sequence for a mesh with nv vertices: edges writes lo, hi, face, rev; a stable sort by hi and then by lo gives order; STAGE heads flags the heads; compact of the flags gives starts and the live edge count; STAGE table writes the edges. An edge's faces are the primitives face[order[starts[e] + j]] for j below fcount[e], so gather(face, order) is the faces column the core.edges node answers, with starts as its faceStart and fcount its faceCount. An edge is open when fcount is 1 and manifold when it is at most 2. Integers are 32-bit words, exact.

In the plain variant the n param is the number of sorted places (the vertex slots, also the most edges there can be); stage table runs over the edges, and an invocation past the live edges writes values nothing reads.

Over a live count (COUNT=device, see the abi README) domain_count is the number of live sorted places in both stages, and the n param is not read. Stage heads runs over the capacity of places and holds 0 in flag past the live ones. Stage table runs over the capacity of edges: an edge is live where starts holds a place below the live places (the slots past the live edges hold -1 where compact over a live count makes them), a run ends at the live places, and every other slot holds -1 and -1 in a and b and 0 in fcount. The device variant binds the buffers of its own stage only: 5 storage buffers in stage heads and 8 in stage table (8 in the plain variant).