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fix: fix duplicate variable handling in unhack_system #78

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Merged
AayushSabharwal merged 1 commit into from
May 5, 2026
Merged

fix: fix duplicate variable handling in unhack_system #78

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20 changes: 7 additions & 13 deletions lib/ModelingToolkitTearing/src/ModelingToolkitTearing.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -107,7 +107,7 @@ function MTKBase.unhack_system(sys::System)
resize!(obs_mask, length(obseqs))
fill!(obs_mask, true)
additional_eqs = Equation[]
additional_vars = SymbolicT[]
additional_vars = Set{SymbolicT}()
additional_subs = Dict{SymbolicT, SymbolicT}()

# Also need to update schedule
Expand Down Expand Up @@ -142,21 +142,15 @@ function MTKBase.unhack_system(sys::System)

resid = A * x - b
for res in resid
SU._iszero(res) && continue
# If a linear SCC contains both `D(w)` and `w_t`, it'll contain the equation `D(w) ~ w_t`.
# When unhacking it, `D(w)` will be totermed into `w_t`. Avoid adding the `0 ~ 0` equations.
# The duplicate variables are automatically removed by the `Set`.
# See https://github.com/SciML/ModelingToolkit.jl/issues/4196 for further details.
push!(additional_eqs, Symbolics.COMMON_ZERO ~ res)
end
end
@assert length(additional_eqs) == length(additional_vars)
# If a linear SCC contains both `D(w)` and `w_t`, it'll contain the equation `D(w) ~ w_t`.
# When unhacking it, `D(w)` will be totermed into `w_t`. This, `additional_vars` contains
# two `w_t` and an equation that is `0 ~ 0`. Find the `0 ~ 0` equations, and remove them
# along with the duplicate variables.
# See https://github.com/SciML/ModelingToolkit.jl/issues/4196 for further details.
additional_eqs_mask = trues(length(additional_eqs))
for (i, eq) in enumerate(additional_eqs)
additional_eqs_mask[i] = !SU._iszero(eq.rhs)
end
additional_eqs = additional_eqs[additional_eqs_mask]
additional_vars = additional_vars[additional_eqs_mask]
subst = SU.Substituter{false}(additional_subs, SU.default_substitute_filter)
obseqs = obseqs[obs_mask]
map!(subst, obseqs, obseqs)
Expand All @@ -167,7 +161,7 @@ function MTKBase.unhack_system(sys::System)
map!(subst, values(sched.dummy_sub))
end

dvs = [unknowns(sys); additional_vars]
dvs = [unknowns(sys); collect(additional_vars)]

newsys = @set sys.observed = obseqs
@set! newsys.eqs = eqs
Expand Down
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