我可以缓存部分执行的LINQ查询吗？

Question

我有以下代码:

IEnumerable<KeyValuePair<T, double>> items =
    sequence.Select(item => new KeyValuePair<T, double>(item, weight(item)));
if (items.Any(pair => pair.Value<0))
    throw new ArgumentException("Item weights cannot be less than zero.");

double sum = items.Sum(pair => pair.Value);
foreach (KeyValuePair<T, double> pair in items) {...}

哪里weight是Func<T, double>.

问题是我希望weight尽可能少地执行.这意味着每件物品最多应执行一次.我可以通过将它保存到数组来实现这一点.但是,如果任何权重返回负值,我不想继续执行.

有没有办法在LINQ框架内轻松完成这项工作？

Answer 1

当然,这是完全可行的:

public static Func<A, double> ThrowIfNegative<A, double>(this Func<A, double> f)
{
    return a=>
    { 
      double r = f(a);  
      // if r is NaN then this will throw.
      if ( !(r >= 0.0) )
        throw new Exception(); 
      return r;
    };
}

public static Func<A, R> Memoize<A, R>(this Func<A, R> f)
{
    var d = new Dictionary<A, R>();
    return a=>
    {
        R r;
        if (!d.TryGetValue(a, out r))
        {
          r = f(a);
          d.Add(a, r);
        }
        return r;
    };
}

现在...

Func<T, double> weight = whatever;
weight = weight.ThrowIfNegative().Memoize();

你完成了