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fix: fix simplification of underdetermined systems #80

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Merged
AayushSabharwal merged 1 commit into from
May 14, 2026
Merged

fix: fix simplification of underdetermined systems #80

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11 changes: 10 additions & 1 deletion lib/ModelingToolkitTearing/src/reassemble.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -503,7 +503,7 @@ function generate_system_equations!(state::TearingState, neweqs::Vector{Equation
offset = 1
findnextfn = let diff_vars_set = diff_vars_set, solved_vars_set = solved_vars_set,
diff_to_var = diff_to_var, ispresent = ispresent
j -> !(j in diff_vars_set || j in solved_vars_set) && diff_to_var[j] === nothing &&
j -> !(j in diff_vars_set || j in solved_vars_set || j in extra_vars) && diff_to_var[j] === nothing &&
ispresent(j)
end
for (i, v) in enumerate(var_ordering)
Expand Down Expand Up @@ -1028,6 +1028,15 @@ function update_simplified_system!(
unknown_idxs = filter(filterer, eachindex(state.fullvars))
unknowns = state.fullvars[unknown_idxs]
unknowns = [unknowns; extra_unknowns]
# `generate_system_equations!` will include the extra unknowns in `var_ordering` due to
# the `setdiff` at the end. Excluding them leads to issues when running `singularity_check`
# on the simplified `state` in `InitializationProblem`. However, `ispresent` can filter
# some of them out of `unknown_idxs` if they don't occur in the selected equations. As
# a result, we still add them here. `unique!` serves to remove duplicates. Otherwise,
# `tearing_hacks` may count an array variable occuring `length + 1` times in
# unknowns+observables, and will thus not add the `x ~ [x[1], x[2]...]` array hack
# observed equation.
unique!(unknowns)
if StateSelection.is_only_discrete(structure)
# Algebraic variables are shifted forward by one, so we backshift them.
_unknowns = SymbolicT[]
Expand Down
9 changes: 9 additions & 0 deletions lib/ModelingToolkitTearing/test/runtests.jl
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Expand Up @@ -220,3 +220,12 @@ end
@test length(equations(ts)) == 1
@test length(ts.fullvars) == 2
end

@testset "Unknowns in underdetermined systems aren't counted twice" begin
@variables x[1:3]
@mtkcompile sys = System([x[1] ~ 2x[2]+3x[3], x[2]^3 + x[3]^3 - 9 ~ 0], [x[1], x[2], x[3]], []) fully_determined=false
# Without the fix in https://github.com/JuliaComputing/StateSelection.jl/pull/80
# `unknowns` prior to running `tearing_hacks` will contain `x[3]` twice, so
# the observed equation `x ~ [x[1], x[2], x[3]]` won't be added.
@test isequal(observables(sys)[2], x)
end
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