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Forward-shift whole-array references in shift_discrete_system #162

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Forward-shift whole-array references in shift_discrete_system #162

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19 changes: 18 additions & 1 deletion lib/ModelingToolkitTearing/src/tearingstate.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -1067,6 +1067,23 @@ function lower_order_var(dervar::SymbolicT, t::SymbolicT)
end
end

"""
$(TYPEDSIGNATURES)

Whether `k` is an array variable that is not itself in `fullvars_set` but all of whose
scalar elements are. `fullvars` only ever holds scalarized array elements, so an array
used as a whole (e.g. `f(x(k - 1))` with a registered `f(::AbstractVector)`) must be
recognized through its elements. Otherwise `shift_discrete_system` shifts the elements
forward but leaves the whole-array reference behind, and backshifting the resulting
observed equation later produces a shift one step further back than the model asked for.
"""
function is_scalarized_in(k::SymbolicT, fullvars_set::Set{SymbolicT})
sh = SU.shape(k)
sh isa SU.ShapeVecT || return false
isempty(sh) && return false
return all(i -> k[i] in fullvars_set, SU.stable_eachindex(k))
end

function shift_discrete_system(ts::TearingState)
# NOTE: `original_eqs` is intentionally not shifted here. This behavior is
# necessary for hybrid system handling.
Expand All @@ -1080,8 +1097,8 @@ function shift_discrete_system(ts::TearingState)
iv = MTKBase.get_iv(sys)::SymbolicT
discmap = Dict{SymbolicT, SymbolicT}()
for k in discvars
k in fullvars_set || continue
MTKBase.isoperator(k, Union{Sample, Hold, Pre}) && continue
k in fullvars_set || is_scalarized_in(k, fullvars_set) || continue
discmap[k] = MTKBase.simplify_shifts(Shift(iv, 1)(k))
end

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