detect/detect.gui/Commons/PredicateBuilder.cs

using System;
using System.Linq.Expressions;

namespace detect.gui.Commons;

public static class PredicateBuilder
{
    public static Expression<Func<T, bool>> GetTrue<T>()
    {
        return f => true;
    }

    public static Expression<Func<T, bool>> GetFalse<T>()
    {
        return f => false;
    }

    public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first,
        Expression<Func<T, bool>> second)
    {
        return first.AndAlso(second, Expression.AndAlso);
    }

    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first,
        Expression<Func<T, bool>> second)
    {
        return first.AndAlso(second, Expression.OrElse);
    }

    private static Expression<Func<T, bool>> AndAlso<T>(this Expression<Func<T, bool>> expr1,
        Expression<Func<T, bool>> expr2, Func<Expression, Expression, BinaryExpression> func)
    {
        var parameter = Expression.Parameter(typeof(T));

        var leftVisitor = new ReplaceExpressionVisitor(expr1.Parameters[0], parameter);
        var left = leftVisitor.Visit(expr1.Body);

        var rightVisitor = new ReplaceExpressionVisitor(expr2.Parameters[0], parameter);
        var right = rightVisitor.Visit(expr2.Body);

        if (left == null || right == null)
            return Expression.Lambda<Func<T, bool>>(() => true);
        
        return Expression.Lambda<Func<T, bool>>(func(left, right), parameter);
    }

    private class ReplaceExpressionVisitor : ExpressionVisitor
    {
        private readonly Expression? _oldValue;
        private readonly Expression? _newValue;

        public ReplaceExpressionVisitor(Expression? oldValue, Expression? newValue)
        {
            _oldValue = oldValue;
            _newValue = newValue;
        }

        public override Expression? Visit(Expression? node)
        {
            return node == _oldValue ? _newValue : base.Visit(node);
        }
    }
}