Connected.Framework/Connected.Expressions/Evaluation/ExpressionNominator.cs

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

namespace Connected.Expressions.Evaluation;

internal sealed class ExpressionNominator : DatabaseVisitor
{
	private ExpressionNominator(QueryLanguage language, ProjectionAffinity affinity)
	{
		Language = language;
		Affinity = affinity;

		Candidates = new HashSet<Expression>();
	}

	private QueryLanguage Language { get; }
	private ProjectionAffinity Affinity { get; }

	private HashSet<Expression> Candidates { get; set; }
	private bool IsBlocked { get; set; }

	public static HashSet<Expression> Nominate(QueryLanguage language, ProjectionAffinity affinity, Expression expression)
	{
		var nominator = new ExpressionNominator(language, affinity);

		nominator.Visit(expression);

		return nominator.Candidates;
	}

	protected override Expression? Visit(Expression? expression)
	{
		if (expression is not null)
		{
			var saveIsBlocked = IsBlocked;

			IsBlocked = false;

			if (Language.MustBeColumn(expression))
				Candidates.Add(expression);
			else
			{
				base.Visit(expression);

				if (!IsBlocked)
				{
					if (Language.MustBeColumn(expression) || (Affinity == ProjectionAffinity.Server && Language.CanBeColumn(expression)))
						Candidates.Add(expression);
					else
						IsBlocked = true;
				}

				IsBlocked |= saveIsBlocked;
			}
		}

		return expression;
	}

	protected override Expression VisitProjection(ProjectionExpression expression)
	{
		Visit(expression.Projector);

		return expression;
	}
}