CelerisLab/docs/development/src_coherence.md

src coherence stage (from main 914906e)

Owner direction 2026-10-28: unified style over historical compatibility, inside src. No numerical change is intended; kernel text was moved verbatim (PTX/bit parity is pending a GPU run).

Conventions (defined once, in lbm.hierarchy.Hierarchy)

• Level l = 0 .. depth-1 (0 = root). Pair hierarchy.pairs[l-1] builds level l.
• Array shapes are (ny, nx); rectangles, origins and centers are (x, y).
• FixedPatchStepper bodies/Jets use the root EDGE frame (root cell i = [i, i+1)); uniform Simulation objects use the integer-NODE frame. Nothing converts between them.
• Hierarchy.edge_frame(l) -> ((ox, oy), h): cell (i, j) of level l's allocated array (ghost band and holes included) covers [o + h*i, o + h*(i+1)). A scalar FixedPatch2D window carries the half-coarse-cell ghost shift bounds - HALO/2 in fine_origin; a set's level-wide grid has origin 0. Level 1 boundaries are built from the root config (the wall-aware fields live there); deeper ones from the parent frame.
• FixedPatch2D.fine_level_offset (was fine_level_origin) is an array INDEX offset (y, x), not an (x, y) origin.

FixedPatchStepper migration map

removed / changed	replacement
patch, child_patch	hierarchy.pairs[0], hierarchy.pairs[1]
base_layout, fine_layout, child_layout (stepper properties)	layouts[0], layouts[1], layouts[2] (FixedPatchSet2D.fine_layout(block) is unchanged)
fine_boundary, child_boundary	boundaries[1], boundaries[2] (host geometry; None = no bodies/Jets)
_jets	jets[1]
_boundaries[l] (dict from L1, level keys)	_level_boundaries[l] (list from L0, None at L0 unless single-level static circles)
_fine_tables, _actuated, _jet_cache	_level_boundaries[1].tables / .actuated / .jet_cache
_fine_leaf, _child_leaf	leaf[1], leaf[2] (copies)
_middle_kind	hierarchy.kinds[1]
_boundary_hosts (private)	_boundaries (private list behind boundaries)
remap_child(rects, child_objects=...)	remap_finest(rects, objects=...)
set_jet_amplitude(i, a) (implicit level 1)	set_jet_amplitude(i, a, level=l) (required)
read_raw_exchange(b) / read_exchange_window(b) (implicit level 1)	read_raw_exchange(b, level=l) / read_exchange_window(b, level=l) (required)
_level_name(l) "base/fine/child" messages	"level l"
refinement.MIN_PATCH_GAP	deleted (no gap policy exists)
params.cuh HaloPlan, collide_trt_no_force	deleted (zero references)

Kept: step_count, constructor keywords (refinements, level_blocks, level_objects, level_jets, coarse_boundary, explosion), Simulation.set_jet_amplitude(i, a) (one level), RemapTopology.from_config(..., child_objects=, child_jets=) (internal remap helper; "child" = the replaced level relative to its parent).

Responsibilities

• Construction: _check_levels (inputs), _kernel_configs (per-level kernel configs), Hierarchy.edge_frame (geometry frames), _build_level / _build_pair / _upload_boundary (device). Observation reads share _observed_boundary + _summarize.
• Blocked kernels: operators/block_collision.cuh block_collide is the one collision extension point of BlockStep and BlockChannelStep. MRT shares one path; SRT keeps its two explicit arithmetic variants (level: unclamped, no-force, shifted direct-momentum feq; channel: uniform OneStep arithmetic with omega clamp), selected by BLOCK_ROOT_BC.
• explosion="projected_linear" stays an optional capability; "constant" stays default.

Not done / open

• No single-Simulation facade over FixedPatchStepper: the two own different state models (uniform node frame, body sync, forcing, checkpoint vs. immutable blocked hierarchy, edge frame, remap-as-new-instance). A facade would only forward calls; no concrete need found.
• Tests, examples and validation callers are migrated separately (pinned runners stay frozen).