Presolve div by const

ampl · Jun 3, 2024 · b6c299a · b6c299a
1 parent 7a7b928
commit b6c299a
Show file tree

Hide file tree

Showing 4 changed files with 46 additions and 1 deletion.
diff --git a/include/mp/flat/expr_quadratic.h b/include/mp/flat/expr_quadratic.h
@@ -210,6 +210,12 @@ class QuadAndLinTerms :
     QuadTerms::add(qlt.GetQPTerms());
   }
 
+  /// Multiply by const
+  void operator*=(double n) {
+    GetLinTerms() *= n;
+    GetQPTerms() *= n;
+  }
+
   /// Value at given variable vector
   template <class VarInfo>
   long double ComputeValue(const VarInfo& x) const {

diff --git a/include/mp/flat/problem_flattener.h b/include/mp/flat/problem_flattener.h
@@ -497,6 +497,25 @@ class ProblemFlattener :
     return AssignResult2Args( std::move(fc) );
   }
 
+  /// Specialize for Div:
+  /// For constant divisor, return linear expression.
+  /// Why we need this: AMPL 20240331 does not preprocess this.
+  template <class ExprArray=std::initializer_list<Expr> >
+  EExpr VisitDivExpression(ExprArray ea) {
+    auto dividend = Convert2EExpr(*ea.begin());
+    auto it = ea.begin();
+    auto divisor = Convert2EExpr(*(++it));
+    if (divisor.is_constant()) {
+      if (!divisor.constant_term())
+        MP_RAISE("Division by 0 in the model.");
+      dividend *= (1.0 / divisor.constant_term());
+      return dividend;
+    }
+    DivConstraint fc
+        {{ Convert2Var(std::move(dividend)), Convert2Var(std::move(divisor)) }};
+    return AssignResult2Args( std::move(fc) );
+  }
+
   template <class ExprArray>
   void Exprs2Vars(const ExprArray& ea, std::vector<int>& result) {
     assert(result.empty());
@@ -666,7 +685,7 @@ class ProblemFlattener :
   }
 
   EExpr VisitDiv(BinaryExpr e) {
-    return VisitFunctionalExpression<DivConstraint>(
+    return VisitDivExpression(
           { e.lhs(), e.rhs() });
   }
 

diff --git a/test/end2end/cases/categorized/fast/nonlinear/div_by_const.mod b/test/end2end/cases/categorized/fast/nonlinear/div_by_const.mod
@@ -0,0 +1,15 @@
+#### See if Div/const is well presolved
+#### MP does this because AMPL 20240331 does not
+#### See if abs() is well redefined (or used well natively)
+###################################################
+
+var x {1..3} >=-200, <=200;
+
+minimize Unbalance:
+   sum {i in 1..3} abs(x[i] - sum {i1 in 1..3} x[i1] / 3);
+
+## Some extra constraints
+ s.t. C1: sum {I in 1..3} x[I] == 15;
+
+ s.t. C2: x[1] + 2*x[2] + 4*x[3] == 18;
+
diff --git a/test/end2end/cases/categorized/fast/nonlinear/modellist.json b/test/end2end/cases/categorized/fast/nonlinear/modellist.json
@@ -1,4 +1,9 @@
 [
+  {
+    "name" : "div_by_const",
+    "tags" : ["linear"],
+    "objective": 11.33333333333333
+  },
   {
     "name" : "expA_1",
     "tags" : ["nonlinear"],