Discretization RFC — Implementation Status

SUPERSEDED (v0.8.0 clean break). The bespoke discretization / spatial-grid machinery this document concerns was removed from the schema in v0.8.0; grid geometry and discretization are now expressed as aggregate Functional Aggregate Query nodes (see semiring-faq-unified-ir.md). Retained for historical context only.

Audit status for discretization.md sections, as they land in esm-schema.json / esm-spec.md / bindings. Scope is the on-wire schema surface — binding-side runtime work is tracked in the per-binding beads.

§6 Grids — landed (gt-5kq3, v2.1)

Top-level grids map with cartesian / unstructured families is in esm-schema.json. Each grid declares dimensions, locations, metric_arrays and connectivity (unstructured). Loader-backed geometry works by reference:

• GridMetricGenerator.kind = "loader" + {loader, field} (schema line ~1947–1986)
• GridConnectivity.{loader, field} (schema line ~2016–2040)
• GridMetricGenerator.kind = "builtin" with a closed name set (currently empty); unknown names reject with E_UNKNOWN_BUILTIN.

The tests/grids/unstructured_mpas.esm fixture rounds-trips in all five bindings and exercises both loader-backed connectivity and loader-backed metric arrays.

§8 data_loaders — landed (gt-xboy audit → gt-pgls amendment)

What’s landed

Grid → loader wiring is end-to-end: a grid’s metric_arrays.<m>.generator.{loader, field} and connectivity.<c>.{loader, field} resolve by name against a top-level data_loaders entry and pick a field that the loader declares under variables.<name>. Pipeline Step 3 (§11) treats loader / builtin generators as deferred handles — the rewrite engine does not materialize bulk data.

RFC §8.A (new kind: "mesh", mesh subobject with topology, connectivity_fields, metric_fields, dimension_sizes, plus a determinism block) landed under gt-pgls:

The MPAS fixture now uses kind: "mesh" with the mesh + determinism blocks. Pre-gt-pgls files that used kind: "grid" as a workaround still load (the grid enum value is unchanged); new authors should prefer kind: "mesh" for MPAS-style meshes.

Adjacent gap — Parameter.value: "from_loader" (§6.6)

The Grid.parameters description (schema line 2092) claims grid-level parameters “with value='from_loader' are resolved at load time from a referenced data_loaders entry (§6.6)”. The Parameter $def (schema line 658–671), however, does not declare a value field at all — only default: number. The MPAS fixture works around this by providing numeric defaults ("nCells": 2562) and a prose note. This is flagged here for awareness; it is a §6 gap, not a §8 gap, and is out of scope for gt-xboy.

What §8.A adds (now active)

1. Structured, schema-validated declaration of which loader fields are integer-typed connectivity vs float-typed metric (mesh.connectivity_fields / mesh.metric_fields).
2. A determinism / reproducibility contract that bindings can reject against at load (§14 item 4) — determinism.{endian, float_format, integer_width}.
3. A dimension-sizes map that feeds from_loader parameter resolution without forcing authors to pick a numeric default (mesh.dimension_sizes.<dim>: int | "from_file").

Load-bearing once bindings start enforcing the determinism contract (tracked separately) and once Parameter.value: "from_loader" lands (see §6 gap above).

§11 Pipeline — landed (gt-gbs2, gt-l3dg, gt-q7sh)

discretize(esm) end-to-end runner, DAE binding contract, and the Julia reference pipeline are in. Not in audit scope for gt-xboy.