framework/src/main/java/org/checkerframework/framework/util/typeinference/constraint/F2AReducer.java

package org.checkerframework.framework.util.typeinference.constraint;

import java.util.Set;
import org.checkerframework.framework.type.AnnotatedTypeFactory;
import org.checkerframework.framework.type.AnnotatedTypeMirror;

/**
 * F2AReducer takes an F2A constraint that is not irreducible (@see AFConstraint.isIrreducible) and
 * reduces it by one step. The resulting constraint may still be reducible.
 *
 * <p>Generally reductions should map to corresponding rules in
 * https://docs.oracle.com/javase/specs/jls/se11/html/jls-15.html#jls-15.12.2.7
 */
public class F2AReducer implements AFReducer {

  protected final F2AReducingVisitor visitor;

  public F2AReducer(final AnnotatedTypeFactory typeFactory) {
    this.visitor = new F2AReducingVisitor(typeFactory);
  }

  @Override
  public boolean reduce(AFConstraint constraint, Set<AFConstraint> newConstraints) {
    if (constraint instanceof F2A) {
      final F2A f2A = (F2A) constraint;
      visitor.visit(f2A.formalParameter, f2A.argument, newConstraints);
      return true;

    } else {
      return false;
    }
  }

  /**
   * Given an F2A constraint of the form: F2A( typeFromFormalParameter, typeFromMethodArgument )
   *
   * <p>F2AReducingVisitor visits the constraint as follows: visit ( typeFromFormalParameter,
   * typeFromMethodArgument, newConstraints )
   *
   * <p>The visit method will determine if the given constraint should either:
   *
   * <ul>
   *   <li>be discarded -- in this case, the visitor just returns
   *   <li>reduced to a simpler constraint or set of constraints -- in this case, the new constraint
   *       or set of constraints is added to newConstraints
   * </ul>
   *
   * Sprinkled throughout this class are comments of the form:
   *
   * <pre>{@code
   * // If F has the form G<..., Yk-1, ? super U, Yk+1, ...>, where U involves Tj
   * }</pre>
   *
   * These are excerpts from the JLS, if you search for them you will find the corresponding JLS
   * description of the case being covered.
   */
  private static class F2AReducingVisitor extends AFReducingVisitor {

    public F2AReducingVisitor(AnnotatedTypeFactory typeFactory) {
      super(F2A.class, typeFactory);
    }

    @Override
    public AFConstraint makeConstraint(AnnotatedTypeMirror subtype, AnnotatedTypeMirror supertype) {
      return new F2A(subtype, supertype);
    }

    @Override
    public AFConstraint makeInverseConstraint(
        AnnotatedTypeMirror subtype, AnnotatedTypeMirror supertype) {
      return new A2F(subtype, supertype);
    }

    @Override
    public AFConstraint makeEqualityConstraint(
        AnnotatedTypeMirror subtype, AnnotatedTypeMirror supertype) {
      return new FIsA(subtype, supertype);
    }
  }
}