Connected.Framework/Connected.Expressions/Translation/Rewriters/CrossJoinRewriter.cs

using Connected.Expressions.Expressions;
using Connected.Expressions.Translation.Resolvers;
using Connected.Expressions.Visitors;
using System.Linq.Expressions;

namespace Connected.Expressions.Translation.Rewriters;

public sealed class CrossJoinRewriter : DatabaseVisitor
{
    public static Expression Rewrite(Expression expression)
    {
        if (new CrossJoinRewriter().Visit(expression) is not Expression crossJoinExpression)
            throw new NullReferenceException(nameof(crossJoinExpression));

        return crossJoinExpression;
    }

    private Expression? CurrentWhere { get; set; }

    protected override Expression VisitSelect(SelectExpression select)
    {
        var saveWhere = CurrentWhere;

        try
        {
            CurrentWhere = select.Where;

            var result = (SelectExpression)base.VisitSelect(select);

            if (CurrentWhere != result.Where)
                return result.SetWhere(CurrentWhere);

            return result;
        }
        finally
        {
            CurrentWhere = saveWhere;
        }
    }

    protected override Expression VisitJoin(JoinExpression expression)
    {
        expression = (JoinExpression)base.VisitJoin(expression);

        if (expression.Join == JoinType.CrossJoin && CurrentWhere is not null)
        {
            var declaredLeft = DeclaredAliasesResolver.Resolve(expression.Left);
            var declaredRight = DeclaredAliasesResolver.Resolve(expression.Right);
            var declared = new HashSet<Alias>(declaredLeft.Union(declaredRight));
            var exprs = CurrentWhere.Split(ExpressionType.And, ExpressionType.AndAlso);
            var good = exprs.Where(e => CanBeJoinCondition(e, declaredLeft, declaredRight, declared)).ToList();

            if (good.Any())
            {
                if (good.Join(ExpressionType.And) is not Expression conditionExpression)
                    throw new NullReferenceException(nameof(conditionExpression));

                expression = UpdateJoin(expression, JoinType.InnerJoin, expression.Left, expression.Right, conditionExpression);

                var newWhere = exprs.Where(e => !good.Contains(e)).Join(ExpressionType.And);

                CurrentWhere = newWhere;
            }
        }

        return expression;
    }

    private static bool CanBeJoinCondition(Expression expression, HashSet<Alias> left, HashSet<Alias> right, HashSet<Alias> all)
    {
        var referenced = ReferencedAliasesResolver.Resolve(expression);
        var leftOkay = referenced.Intersect(left).Any();
        var rightOkay = referenced.Intersect(right).Any();
        var subset = referenced.IsSubsetOf(all);

        return leftOkay && rightOkay && subset;
    }
}