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

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

namespace Connected.Expressions.Translation.Rewriters;

public sealed class CrossApplyRewriter : DatabaseVisitor
{
    private CrossApplyRewriter(QueryLanguage language)
    {
        Language = language;
    }

    private QueryLanguage Language { get; }

    public static Expression Rewrite(QueryLanguage language, Expression expression)
    {
        if (new CrossApplyRewriter(language).Visit(expression) is not Expression crossApplyExpression)
            throw new NullReferenceException(nameof(crossApplyExpression));

        return crossApplyExpression;
    }

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

        if (expression.Join == JoinType.CrossApply || expression.Join == JoinType.OuterApply)
        {
            if (expression.Right is TableExpression)
                return new JoinExpression(JoinType.CrossJoin, expression.Left, expression.Right, null);
            else
            {
                var select = expression.Right as SelectExpression;

                if (select is not null && select.Take is null && select.Skip is null && !AggregateChecker.HasAggregates(select) && (select.GroupBy is null || !select.GroupBy.Any()))
                {
                    var selectWithoutWhere = select.SetWhere(null);
                    var referencedAliases = ReferencedAliasesResolver.Resolve(selectWithoutWhere);
                    var declaredAliases = DeclaredAliasesResolver.Resolve(expression.Left);

                    referencedAliases.IntersectWith(declaredAliases);

                    if (!referencedAliases.Any())
                    {
                        var where = select.Where;

                        select = selectWithoutWhere;

                        var pc = ColumnProjector.ProjectColumns(Language, where, select.Columns, select.Alias, DeclaredAliasesResolver.Resolve(select.From));

                        select = select.SetColumns(pc.Columns);
                        where = pc.Projector;

                        var jt = (where == null) ? JoinType.CrossJoin : (expression.Join == JoinType.CrossApply ? JoinType.InnerJoin : JoinType.LeftOuter);

                        return new JoinExpression(jt, expression.Left, select, where);
                    }
                }
            }
        }

        return expression;
    }
}