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Indicator constraint support #709

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merged 35 commits into from
Apr 30, 2019
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matbesancon
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First draft for the addition of indicator constraints to MOI. There is evidence for multiple solvers handling them:
Gurobi
CPLEX
SCIP

One thing we would want to add (suggestion from @leethargo) is a bridge for solvers supporting SOS1 but not indicator constraints natively. From SCIP documentation:

This constraint is equivalent to a linear constraint ax−s≤b and an SOS1 constraint on z and s (at most one should be nonzero). In the indicator context we can, however, separate more inequalities.

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codecov-io commented Apr 11, 2019

Codecov Report

Merging #709 into master will increase coverage by 0.09%.
The diff coverage is 99.04%.

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@@            Coverage Diff             @@
##           master     #709      +/-   ##
==========================================
+ Coverage   93.73%   93.83%   +0.09%     
==========================================
  Files          54       54              
  Lines        5572     5677     +105     
==========================================
+ Hits         5223     5327     +104     
- Misses        349      350       +1
Impacted Files Coverage Δ
src/Test/intlinear.jl 100% <100%> (ø) ⬆️
src/sets.jl 93.75% <66.66%> (-1.81%) ⬇️

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@@ -449,7 +466,7 @@ function Base.copy(set::Union{Reals, Zeros, Nonnegatives, Nonpositives,
PositiveSemidefiniteConeSquare,
LogDetConeTriangle, LogDetConeSquare,
RootDetConeTriangle, RootDetConeSquare,
Integer, ZeroOne, Semicontinuous, Semiinteger})
Integer, ZeroOne, Semicontinuous, Semiinteger, IndicatorSet})
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It is not an isbits type since it contains a vector. The copy should probably copy this vector too

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You're right, I'll re-implement copy. Should we make the vector type generic?

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What about having a set parameter for whether y should be 0 or 1 to activate the constraint?

Then the disjunction ax ≤ b v cx ≤ d could be expressed by the two constraints

(y, x) in IndicatorSet(1, a, LessThan(b))
(y, x) in IndicatorSet(0, c, LessThan(d))

reusing the y variable. Otherwise, another z would have to be introduced with y + z == 1?

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Should we make the indicator set generic on vector type?
IndicatorSet{T,S <: ScalarSet,V <: AbstractVector{<:Real}}

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mlubin commented Apr 12, 2019

What are the implications of having non isbits sets? How does this interact with MOF?

One way to avoid storing the a vector in the set would be to store it in the function, e.g.,

Set of (y, x) that satisfy the indicator constraint:
    y ∈ 𝔹, y = 1 ==> x ∈ S

Then use VAF to write down the constraints, e.g.,

(1.0 * y, 2*x + 3*z) in IndicatorSet(LessThan(b))

It's an error if the first component of the VAF isn't 1.0 times some variable.

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odow commented Apr 12, 2019

The SOS sets already have weight vectors, so this isn't breaking new ground. No problem on the MOF front

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see last version for generic vector

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odow commented Apr 12, 2019

Oh, wait. I misread miles' concern. Not sure how abstractvector would play with MOF. I'd prefer the vectoraffinefunction in set approach.

This reverts commit 6e57763.
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problem with VectorAffine in Indicator is that it requires Set to hold variables. Also keeping a vector of a is consistent with SOS

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mlubin commented Apr 12, 2019

problem with VectorAffine in Indicator is that it requires Set to hold variable

What do you mean by this?

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Either VectorAffine has a weird special first term (1.0, y) or the y needs to be specified in the set

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mlubin commented Apr 12, 2019

The weird special term is fine with me. That's what we do for cones.

We have machinery already to translate JuMP AffExprs into VAF, so today you could write:

@constraint(model, [y, 2x + 3z] in IndicatorSet(LessThan(b)))

OTOH,

@constraint(model, [y, x, z] in IndicatorSet([2.0, 3.0], LessThan(b)))

seems like a regression in modeling because you have to manually create a list of coefficients.

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Ok I see. [y, 2x + 3z] would be specially parsed at the JuMP level? (Meaning, different from [y, 2x, 3z] or [y + 2x + 3z]

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mlubin commented Apr 12, 2019

[y, 2x + 3z] would be specially parsed at the JuMP level? (Meaning, different from [y, 2x, 3z] or [y + 2x + 3z]

Yes, those are all parsed differently.

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one thing is that the set cannot determine the dimension of the expected function

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mlubin commented Apr 12, 2019

The dimension of the set I'm proposing (under the MOI definition) would always be 2.

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ok I hadn't thought of counting like this, so the whole affine term a^T x count as dim 1 right?

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odow commented Apr 12, 2019

What about

@constraint(model, 2x1 + x2 in MOI.IndicatorSet(y=>1, MOI.GreaterThan(1.0)))

So

struct IndicatorSet{S}
    variable::Pair{MOI.VariableIndex, Bool}
    set::S
end

Then if S is a scalar set, it works for any scalar function, and if S is a vector set, it works for any vector function.

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Ok, one design consideration I tried to keep is to represent the constraint as closely as possible to what solvers take as input

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Fixed most of the comments. @blegat I didn't find anything for testing new sets with MockOptimizer.
SOS constraints are not tested using it (just basic unit tests in /test/sets.jl

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tests passing cc @blegat @mlubin

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@blegat blegat self-requested a review April 24, 2019 20:43
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Almost there. Missing a (tested) copy implementation that copies the internal set

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Thanks, added in last commit

s1_copy = copy(s1)
s2_copy = copy(s2)
@test s1_copy isa MOI.IndicatorSet{MOI.ACTIVATE_ON_ONE}
@test s1 == s1_copy
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Check that modifying the set in s1 does not modify the set in s1_copy. You will need a scalar set which is mutable

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do we even have one? I understand the importance of testing it does not happen, but we are calling copy on the subtest, so if error there is, it's because copy is not properly implemented on it

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@matbesancon LessThan{BigInt} is mutable because BigInt is mutable. You call also create a dummy mutable coefficient type or a dummy mutable set just for the test.

we are calling copy on the subtest

That's what we want to test. If you remove this copy on the subset, the tests still pass while they shouldn't

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@blegat I played a bit with this idea. Fundamentally, this is breaking an assumption about numbers that will be everywhere in MOI, this PR doesn't change that. BigInt being mutable is just a constraint for their construction as far as I know, not supposed to be used.

The implementation from src/sets:

# isbits types, nothing to copy
function Base.copy(set::Union{Reals, Zeros, Nonnegatives, Nonpositives,
                              GreaterThan, LessThan, EqualTo, Interval,
                              SecondOrderCone, RotatedSecondOrderCone,
                              GeometricMeanCone, ExponentialCone,
                              DualExponentialCone, PowerCone, DualPowerCone,
                              PositiveSemidefiniteConeTriangle,
                              PositiveSemidefiniteConeSquare,
                              LogDetConeTriangle, LogDetConeSquare,
                              RootDetConeTriangle, RootDetConeSquare,
                              Integer, ZeroOne, Semicontinuous, Semiinteger})
    return set
end

Is then false because it should check isbits on the set

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Good point. We could defer the question of whether we should copy the field of EqualTo... to a separate issue.
Still you might be able to create a mutable abstract set with Float64 to test the fact we ce copy the set for IndicatorSet

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@blegat see issue #721, and tests added here for IndicatorSet copy

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Looking good. Just a few minor comments.

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@blegat blegat merged commit c1d8f6a into jump-dev:master Apr 30, 2019
@matbesancon matbesancon deleted the indicator-cons branch April 30, 2019 11:20
@blegat blegat mentioned this pull request Oct 4, 2019
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6 participants