using System;
using System.Collections.Generic;
using System.Linq;

namespace jive
{
  static public class SpecificationExtensions
  {
    static public IEnumerable<T> that_satisfy<T>(this IEnumerable<T> items_to_peek_in_to,
        Predicate<T> criteria_to_satisfy)
    {
      foreach (var item in items_to_peek_in_to ?? Enumerable.Empty<T>())
        if (item.satisfies(criteria_to_satisfy)) yield return item;
    }

    static public bool satisfies<T>(this T item_to_interrogate, Predicate<T> criteria_to_satisfy)
    {
      return criteria_to_satisfy(item_to_interrogate);
    }

    static public bool satisfies<T>(this T item_to_validate, Specification<T> criteria_to_satisfy)
    {
      return item_to_validate.satisfies(criteria_to_satisfy.is_satisfied_by);
    }

    static public Specification<T> and<T>(this Specification<T> left, Specification<T> right)
    {
      return new PredicateSpecification<T>(x => left.is_satisfied_by(x) && right.is_satisfied_by(x));
    }

    static public Specification<T> or<T>(this Specification<T> left, Specification<T> right)
    {
      return new PredicateSpecification<T>(x => left.is_satisfied_by(x) || right.is_satisfied_by(x));
    }

    static public Specification<T> not<T>(this Specification<T> original)
    {
      return new PredicateSpecification<T>(x => !original.is_satisfied_by(x));
    }
  }
}