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### genconstr_vb.vb

' Copyright 2021, Gurobi Optimization, LLC

' In this example we show the use of general constraints for modeling
' some common expressions. We use as an example a SAT-problem where we
' want to see if it is possible to satisfy at least four (or all) clauses
' of the logical for
'
' L = (x0 or ~x1 or x2)  and (x1 or ~x2 or x3)  and
'     (x2 or ~x3 or x0)  and (x3 or ~x0 or x1)  and
'     (~x0 or ~x1 or x2) and (~x1 or ~x2 or x3) and
'     (~x2 or ~x3 or x0) and (~x3 or ~x0 or x1)
'
' We do this by introducing two variables for each literal (itself and its
' negated value), a variable for each clause, and then two
' variables for indicating if we can satisfy four, and another to identify
' the minimum of the clauses (so if it one, we can satisfy all clauses)
' and put these two variables in the objective.
' i.e. the Objective function will be
'
' maximize Obj0 + Obj1
'
'  Obj0 = MIN(Clause1, ... , Clause8)
'  Obj1 = 1 -> Clause1 + ... + Clause8 >= 4
'
' thus, the objective value will be two if and only if we can satisfy all
' clauses; one if and only of at least four clauses can be satisfied, and
' zero otherwise.
'

Imports Gurobi

Class genconstr_vb

Public Const n As Integer = 4
Public Const NLITERALS As Integer = 4  'same as n
Public Const NCLAUSES As Integer = 8
Public Const NOBJ As Integer = 2

Shared Sub Main()

Try

' Example data:
' e.g. {0, n+1, 2} means clause (x0 or ~x1 or x2)
Dim Clauses As Integer(,) = New Integer(,) { _
{    0, n + 1, 2}, {    1, n + 2, 3}, _
{    2, n + 3, 0}, {    3, n + 0, 1}, _
{n + 0, n + 1, 2}, {n + 1, n + 2, 3}, _
{n + 2, n + 3, 0}, {n + 3, n + 0, 1}}

Dim i As Integer, status As Integer

' Create environment
Dim env As New GRBEnv("genconstr_vb.log")

' Create initial model
Dim model As New GRBModel(env)
model.ModelName = "genconstr_vb"

' Initialize decision variables and objective
Dim Lit As GRBVar() = New GRBVar(NLITERALS - 1) {}
Dim NotLit As GRBVar() = New GRBVar(NLITERALS - 1) {}
For i = 0 To NLITERALS - 1
Lit(i) = model.AddVar(0.0, 1.0, 0.0, GRB.BINARY, String.Format("X{0}", i))
NotLit(i) = model.AddVar(0.0, 1.0, 0.0, GRB.BINARY, String.Format("notX{0}", i))
Next

Dim Cla As GRBVar() = New GRBVar(NCLAUSES - 1) {}
For i = 0 To NCLAUSES - 1
Cla(i) = model.AddVar(0.0, 1.0, 0.0, GRB.BINARY, String.Format("Clause{0}", i))
Next

Dim Obj As GRBVar() = New GRBVar(NOBJ - 1) {}
For i = 0 To NOBJ - 1
Obj(i) = model.AddVar(0.0, 1.0, 1.0, GRB.BINARY, String.Format("Obj{0}", i))
Next

Dim lhs As GRBLinExpr
For i = 0 To NLITERALS - 1
lhs = New GRBLinExpr()
Next

For i = 0 To NCLAUSES - 1
Dim clause As GRBVar() = New GRBVar(2) {}
For j As Integer = 0 To 2
If Clauses(i, j) >= n Then
clause(j) = NotLit(Clauses(i, j) - n)
Else
clause(j) = Lit(Clauses(i, j))
End If
Next
Next

lhs = New GRBLinExpr()
For i = 0 To NCLAUSES - 1
Next
model.AddGenConstrIndicator(Obj(1), 1, lhs, GRB.GREATER_EQUAL, 4.0, "CNSTR_Obj1")

' Set global objective sense
model.ModelSense = GRB.MAXIMIZE

' Save problem
model.Write("genconstr_vb.mps")
model.Write("genconstr_vb.lp")

' Optimize
model.Optimize()

' Status checking
status = model.Status

If status = GRB.Status.INF_OR_UNBD OrElse _
status = GRB.Status.INFEASIBLE OrElse _
status = GRB.Status.UNBOUNDED Then
Console.WriteLine("The model cannot be solved " & _
"because it is infeasible or unbounded")
Return
End If

If status <> GRB.Status.OPTIMAL Then
Console.WriteLine("Optimization was stopped with status {0}", status)
Return
End If

' Print result
Dim objval As Double = model.ObjVal

If objval > 1.9 Then
Console.WriteLine("Logical expression is satisfiable")
ElseIf objval > 0.9 Then
Console.WriteLine("At least four clauses can be satisfied")
Else
Console.WriteLine("Not even three clauses can be satisfied")
End If

' Dispose of model and environment
model.Dispose()
env.Dispose()

Catch e As GRBException
Console.WriteLine("Error code: {0}. {1}", e.ErrorCode, e.Message)

End Try
End Sub
End Class


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