Refactor & cleanup Grammar.field_index

[BenWeber42]

The Grammar.field_index has become way too complex. It's a key piece for the location semantics, but its complexity hinders development severely:

dusk/dusk/grammar.py

Lines 509 to 594 in d6bbfaf

	@transform(
	OneOf(
	Tuple(
	elts=FixedList(
	Capture(OneOf(Compare, Name)).to("hindex"),
	Capture(expr).to("vindex"),
	),
	ctx=Load,
	),
	# FIXME: ensure built-ins (like `Edge`) aren't _shadowed_ by variables
	# TODO: hardcoded string
	Capture(OneOf(Compare, name(OneOf("Edge", "Cell", "Vertex")))).to("hindex"),
	Capture(expr).to("vindex"),
	None,
	)
	)
	def field_index(self, field: DuskField, vindex=None, hindex=None):
	voffset, vbase = (
	self.relative_vertical_offset(vindex) if vindex is not None else (0, None)
	)
	include_center, hindex = (
	self.location_chain(hindex) if hindex is not None else (False, None)
	)
	if not self.ctx.location.in_neighbor_iteration:
	if hindex is not None:
	raise DuskSyntaxError(
	f"Invalid horizontal index for field '{field.sir.name}' "
	"outside of neighbor iteration!"
	)
	return make_unstructured_offset(False), voffset, vbase
	neighbor_chain = self.ctx.location.current_neighbor_iteration.chain
	field_dimension = self.ctx.location.get_field_dimension(field.sir)
	if hindex is not None and not self.ctx.location.is_dense(field_dimension):
	raise DuskSyntaxError(
	f"Invalid horizontal offset for field '{field.sir.name}',"
	"sparse fields do not take an offset (only non-offset read makes sense)!"
	)
	# TODO: `vindex` is _non-sensical_ if the field is 2d
	# TODO: we should check that `field_dimension` is valid for
	# the current neighbor iteration(s?)
	if hindex is None:
	if self.ctx.location.is_dense(field_dimension):
	if self.ctx.location.is_ambiguous(neighbor_chain):
	raise DuskSyntaxError(
	f"Field '{field.sir.name}' requires a horizontal index "
	"inside of ambiguous neighbor iteration!"
	)
	return (
	make_unstructured_offset(field_dimension[0] == neighbor_chain[-1]),
	voffset,
	vbase,
	)
	else:
	return make_unstructured_offset(False), voffset, vbase
	# TODO: check if `hindex` is valid for this field's location type
	if (
	self.ctx.location.current_neighbor_iteration.include_center
	!= include_center
	):
	raise DuskSyntaxError(
	f"Invalid horizontal offset for field '{field.sir.name}'! "
	"inconsistent center inclusion"
	)
	if len(hindex) == 1:
	if neighbor_chain[0] != hindex[0]:
	raise DuskSyntaxError(
	f"Invalid horizontal offset for field '{field.sir.name}'!"
	)
	return make_unstructured_offset(False), voffset, vbase
	if hindex != neighbor_chain:
	raise DuskSyntaxError(
	f"Invalid horizontal offset for field '{field.sir.name}'!"
	)
	return make_unstructured_offset(True), voffset, vbase