Connected.Framework/Connected.Expressions/Translation/Resolvers/RedundandSubqueriesResolver.cs

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

namespace Connected.Expressions.Translation.Resolvers;

internal class RedundandSubqueriesResolver : DatabaseVisitor
{
   private RedundandSubqueriesResolver()
   {
   }

   private List<SelectExpression>? Redundant { get; set; }

   internal static List<SelectExpression> Resolve(Expression expression)
   {
      var retriever = new RedundandSubqueriesResolver();

      retriever.Visit(expression);

      return retriever.Redundant;
   }

   private static bool IsRedudantSubquery(SelectExpression expression)
   {
      return (IsSimpleProjection(expression) || RedundantSubqueries.IsNameMapProjection(expression))
          && !expression.IsDistinct
          && !expression.IsReverse
          && expression.Take is null
          && expression.Skip is null
          && expression.Where is null
          && (expression.OrderBy is null || !expression.OrderBy.Any())
          && (expression.GroupBy is null || !expression.GroupBy.Any());
   }

   internal static bool IsSimpleProjection(SelectExpression select)
   {
      foreach (var decl in select.Columns)
      {
         if (decl.Expression is not ColumnExpression col || decl.Name != col.Name)
            return false;
      }

      return true;
   }

   protected override Expression VisitSelect(SelectExpression expression)
   {
      if (IsRedudantSubquery(expression))
      {
         Redundant ??= new List<SelectExpression>();

         Redundant.Add(expression);
      }

      return expression;
   }

   protected override Expression VisitSubquery(SubqueryExpression expression)
   {
      return expression;
   }
}