Also return determined range constraints. (#3068)

Sometimes we want to run an optimization that depends on range
constraints. Using the solver is the best way to determine range
constraints, so it should also return them.

constraint-solver/src/solver.rs

@@ -29,7 +29,7 @@ pub fn solve_system<T, V>(
 ) -> Result<SolveResult<T, V>, Error>
 where
     T: RuntimeConstant + VarTransformable<V, Variable<V>> + Display,
-    T::Transformed: RuntimeConstant
+    T::Transformed: RuntimeConstant<FieldType = T::FieldType>
         + VarTransformable<Variable<V>, V, Transformed = T>
         + ReferencedSymbols<Variable<V>>
         + Substitutable<Variable<V>>
@@ -43,7 +43,7 @@ where
     let result = Solver::new(constraint_system)
         .with_bus_interaction_handler(bus_interaction_handler)
         .solve()?;
-    Ok(bus_interaction_variable_wrapper.finalize(result.assignments))
+    Ok(bus_interaction_variable_wrapper.finalize(result))
 }
 
 /// The result of the solving process.
@@ -52,6 +52,9 @@ pub struct SolveResult<T: RuntimeConstant, V> {
     /// Values might contain variables that are replaced as well,
     /// and because of that, assignments should be applied in order.
     pub assignments: Vec<VariableAssignment<T, V>>,
+    /// The range constraints the solver was able to determine
+    /// for the variables.
+    pub range_constraints: RangeConstraints<T::FieldType, V>,
     /// Maps a (bus interaction index, field index) to a concrete value.
     pub bus_field_assignments: BTreeMap<(usize, usize), T::FieldType>,
 }
@@ -73,7 +76,7 @@ pub enum Error {
 pub type VariableAssignment<T, V> = (V, GroupedExpression<T, V>);
 
 /// Given a list of constraints, tries to derive as many variable assignments as possible.
-struct Solver<T: RuntimeConstant, V: Clone + Eq, BusInterHandler> {
+pub struct Solver<T: RuntimeConstant, V: Clone + Eq, BusInterHandler> {
     /// The constraint system to solve. During the solving process, any expressions will
     /// be simplified as much as possible.
     constraint_system: IndexedConstraintSystem<T, V>,
@@ -127,11 +130,12 @@ where
     }
 
     /// Solves the constraints as far as possible, returning concrete variable
-    /// assignments and a simplified version of the algebraic constraints.
+    /// assignments and determined range constraints.
     pub fn solve(mut self) -> Result<SolveResult<T, V>, Error> {
         self.loop_until_no_progress()?;
         Ok(SolveResult {
             assignments: self.assignments,
+            range_constraints: self.range_constraints,
             bus_field_assignments: Default::default(),
         })
     }
@@ -280,8 +284,8 @@ impl<T: RuntimeConstant, V: Clone + Hash + Eq, B> RangeConstraintProvider<T::Fie
 }
 
 /// The currently known range constraints for the variables.
-struct RangeConstraints<T: FieldElement, V> {
-    range_constraints: HashMap<V, RangeConstraint<T>>,
+pub struct RangeConstraints<T: FieldElement, V> {
+    pub range_constraints: HashMap<V, RangeConstraint<T>>,
 }
 
 // Manual implementation so that we don't have to require `V: Default`.

constraint-solver/src/solver/bus_interaction_variable_wrapper.rs

@@ -7,7 +7,7 @@ use crate::{
     constraint_system::{BusInteraction, ConstraintSystem},
     grouped_expression::GroupedExpression,
     runtime_constant::{RuntimeConstant, Substitutable, VarTransformable},
-    solver::{SolveResult, VariableAssignment},
+    solver::{RangeConstraints, SolveResult},
 };
 
 /// A wrapped variable: Either a regular variable or a bus interaction field.
@@ -35,7 +35,7 @@ pub struct BusInteractionVariableWrapper<T, V> {
 impl<T, V: Ord + Clone + Hash + Eq + Display> BusInteractionVariableWrapper<T, V>
 where
     T: RuntimeConstant + VarTransformable<V, Variable<V>> + Display,
-    T::Transformed: RuntimeConstant
+    T::Transformed: RuntimeConstant<FieldType = T::FieldType>
         + VarTransformable<Variable<V>, V, Transformed = T>
         + Substitutable<Variable<V>>,
 {
@@ -82,9 +82,9 @@ where
 
     pub fn finalize(
         mut self,
-        assignments: Vec<VariableAssignment<T::Transformed, Variable<V>>>,
+        solve_result: SolveResult<T::Transformed, Variable<V>>,
     ) -> SolveResult<T, V> {
-        for (variable, expr) in &assignments {
+        for (variable, expr) in &solve_result.assignments {
             // Apply any assignments to the bus interaction field definitions.
             if let Variable::BusInteractionField(..) = variable {
                 // Non-concrete assignments are only generated by the `quadratic_equivalences` module,
@@ -105,7 +105,8 @@ where
         }
 
         // Unwrap assignments. Note that this uses the updated bus interaction field definitions.
-        let assignments = assignments
+        let assignments = solve_result
+            .assignments
             .into_iter()
             .filter_map(|(v, expr)| match v {
                 Variable::Variable(v) => {
@@ -127,8 +128,23 @@ where
             })
             .collect();
 
+        let range_constraints = solve_result
+            .range_constraints
+            .range_constraints
+            .into_iter()
+            .fold(
+                RangeConstraints::<T::FieldType, V>::default(),
+                |mut range_constraints, (v, rc)| {
+                    if let Variable::Variable(v) = v {
+                        range_constraints.update(&v, &rc);
+                    }
+                    range_constraints
+                },
+            );
+
         SolveResult {
             assignments,
+            range_constraints,
             bus_field_assignments,
         }
     }