Short-circuit range counting algorithm?

[Steven Schveighoffer]

On 3/16/18 2:07 PM, Seb wrote:
> On Friday, 16 March 2018 at 17:50:37 UTC, Andrei Alexandrescu wrote:
>> On 3/16/18 1:41 PM, H. S. Teoh wrote:
>>> Given a forward range r, I want to test whether it has exactly n
>>> elements that satisfy some given predicate pred.  Is it possible to do
>>> this using current Phobos algorithms such that it does not traverse more
>>> members of the range than necessary?
>>>
>>> The naïve solution `r.count!pred == n` walks the entire range, even
>>> though it may already have seen n+1 elements that satisfies pred, and
>>> therefore should already know that the answer must be false.
>>
>> r.count!pred.walkLength(n) == n
> 
> Shouldn't this be using filter (count is eager)?
> 
> r.filter!pred.walkLength(n) == n

r.filter!pred.walkLength(n + 1) == n

-Steve