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Merged
baggepinnen merged 1 commit into from
Sep 21, 2026
Merged

Build shifted variables in var_derivative! for discrete systems #163

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10 changes: 8 additions & 2 deletions lib/ModelingToolkitTearing/src/stateselection_interface.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -2,8 +2,14 @@ function StateSelection.var_derivative!(ts::TearingState, v::Int)
s = ts.structure
var_diff = StateSelection.var_derivative_graph!(s, v)
sys = ts.sys
D = Differential(MTKBase.get_iv(sys))
push!(ts.fullvars, D(ts.fullvars[v]))
iv = MTKBase.get_iv(sys)
# The variables of a discrete system are related by shifts, those of a continuous
# system by differentiation.
if StateSelection.is_only_discrete(s)
push!(ts.fullvars, MTKBase.simplify_shifts(Shift(iv, 1)(ts.fullvars[v])))
else
push!(ts.fullvars, Differential(iv)(ts.fullvars[v]))
end
push!(ts.structure.state_priorities, ts.structure.state_priorities[v])
push!(ts.structure.canonical_ranks, ts.structure.canonical_ranks[v] + 1)
push!(ts.structure.var_types, ts.structure.var_types[v])
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21 changes: 21 additions & 0 deletions lib/ModelingToolkitTearing/test/runtests.jl
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Expand Up @@ -714,3 +714,24 @@ end
MTKTearing.scalarize_tearing_state_eqs!(tss[cid])
@test !iszero(Graphs.ne(tss[cid].structure.graph))
end

@testset "Alias of a variable shifted in a discrete system" begin
# The alias `z ~ y` with `y ~ u(k - 1)` eliminates `u` in favour of `Shift(t, 1)(y)`,
# and the shifted form of the alias target must be built as a shift, not as a
# differential.
k = ShiftIndex(t)
@parameters p = 3.0
@variables u(t) y(t) z(t)
@named sys = System([u ~ p, y ~ u(k - 1), z ~ y], t)
ss = mtkcompile(sys)
@test length(unknowns(ss)) == 1
for eq in [equations(ss); observed(ss)]
@test !MTKBase.isdifferential(eq.lhs)
@test !MTKBase.isdifferential(eq.rhs)
end
# `DiscreteProblem` advances the state one tick during construction, so `y` takes
# the value of `u` at the initial tick.
prob = DiscreteProblem(ss, [y => 0.0], (0, 5))
@test prob[y] == 3.0
@test prob[z] == 3.0
end
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