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

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

namespace Connected.Expressions.Translation.Rewriters;

public sealed class ComparisonRewriter : DatabaseVisitor
{
    private ComparisonRewriter()
    {
    }

    public static Expression Rewrite(Expression expression)
    {
        return new ComparisonRewriter().Visit(expression);
    }

    protected override Expression VisitBinary(BinaryExpression b)
    {
        switch (b.NodeType)
        {
            case ExpressionType.Equal:
            case ExpressionType.NotEqual:
                var result = Compare(b);

                if (result == b)
                    goto default;

                return Visit(result);
            default:
                return base.VisitBinary(b);
        }
    }

    private static Expression SkipConvert(Expression expression)
    {
        while (expression.NodeType == ExpressionType.Convert)
            expression = ((UnaryExpression)expression).Operand;

        return expression;
    }

    private Expression Compare(BinaryExpression bop)
    {
        var e1 = SkipConvert(bop.Left);
        var e2 = SkipConvert(bop.Right);
        var oj1 = e1 as OuterJoinedExpression;
        var oj2 = e2 as OuterJoinedExpression;
        var entity1 = oj1 is not null ? oj1.Expression as EntityExpression : e1 as EntityExpression;
        var entity2 = oj2 is not null ? oj2.Expression as EntityExpression : e2 as EntityExpression;
        var negate = bop.NodeType == ExpressionType.NotEqual;

        if (oj1 is not null && e2.NodeType == ExpressionType.Constant && ((ConstantExpression)e2).Value is null)
            return MakeIsNull(oj1.Test, negate);
        else if (oj2 is not null && e1.NodeType == ExpressionType.Constant && ((ConstantExpression)e1).Value is null)
            return MakeIsNull(oj2.Test, negate);

        if (entity1 is not null)
            return MakePredicate(e1, e2, MappingsCache.Get(entity1.EntityType).Members.Where(f => f.IsPrimaryKey).Select(f => f.MemberInfo), negate);
        else if (entity2 is not null)
            return MakePredicate(e1, e2, MappingsCache.Get(entity2.EntityType).Members.Where(f => f.IsPrimaryKey).Select(f => f.MemberInfo), negate);

        var dm1 = GetDefinedMembers(e1);
        var dm2 = GetDefinedMembers(e2);

        if (dm1 is null && dm2 is null)
            return bop;

        if (dm1 is not null && dm2 is not null)
        {
            var names1 = new HashSet<string>(dm1.Select(m => m.Name));
            var names2 = new HashSet<string>(dm2.Select(m => m.Name));

            if (names1.IsSubsetOf(names2) && names2.IsSubsetOf(names1))
                return MakePredicate(e1, e2, dm1, negate);
        }
        else if (dm1 is not null)
            return MakePredicate(e1, e2, dm1, negate);
        else if (dm2 is not null)
            return MakePredicate(e1, e2, dm2, negate);

        throw new InvalidOperationException("Cannot compare two constructed types with different sets of members assigned.");
    }

    private static Expression MakeIsNull(Expression expression, bool negate)
    {
        var isnull = new IsNullExpression(expression);

        return negate ? Expression.Not(isnull) : isnull;
    }

    private static Expression? MakePredicate(Expression e1, Expression e2, IEnumerable<MemberInfo> members, bool negate)
    {
        var pred = members.Select(m => Binder.Bind(e1, m).Equal(Binder.Bind(e2, m))).Join(ExpressionType.And);

        if (negate)
            pred = Expression.Not(pred);

        return pred;
    }

    private static IEnumerable<MemberInfo> GetDefinedMembers(Expression expr)
    {
        var mini = expr as MemberInitExpression;

        if (mini is not null)
        {
            var members = mini.Bindings.Select(b => FixMember(b.Member));

            if (mini.NewExpression.Members is not null)
                members = members.Concat(mini.NewExpression.Members.Select(FixMember));

            return members;
        }
        else
        {
            var nex = expr as NewExpression;

            if (nex is not null && nex.Members is not null)
                return nex.Members.Select(FixMember);
        }

        return null;
    }

    private static MemberInfo FixMember(MemberInfo member)
    {
        if (member is MethodInfo && member.Name.StartsWith("get_"))
            return member.DeclaringType.GetTypeInfo().GetDeclaredProperty(member.Name[4..]);

        return member;
    }
}