Like "tail" in Unix. Given a range R r and a number size_t n, return a TakeExactly!R that's r at no more than n steps from its end:

TakeExactly!R advanceWithin(R)(R r, size_t n)
if (isForwardRange!R);

Invariant:

assert(r.advanceWithin(n).length <= n);

Implementation would send a scout range ahead, etc.

I didn't file an issue for it, but it's a great function to have.


Takers?

Andrei

On 2015-12-04 17:37, Andrei Alexandrescu wrote:
> Like "tail" in Unix. Given a range R r and a number size_t n, return a
> TakeExactly!R that's r at no more than n steps from its end:
>
> TakeExactly!R advanceWithin(R)(R r, size_t n)
> if (isForwardRange!R);
>
> Invariant:
>
> assert(r.advanceWithin(n).length <= n);
>
> Implementation would send a scout range ahead, etc.
>
> I didn't file an issue for it, but it's a great function to have.

retro + take?

-- 
/Jacob Carlborg

On Friday, 4 December 2015 at 20:01:10 UTC, Jacob Carlborg wrote:
> On 2015-12-04 17:37, Andrei Alexandrescu wrote:
>> Like "tail" in Unix. Given a range R r and a number size_t n, return a
>> TakeExactly!R that's r at no more than n steps from its end:
>>
>> TakeExactly!R advanceWithin(R)(R r, size_t n)
>> if (isForwardRange!R);
>>
>> Invariant:
>>
>> assert(r.advanceWithin(n).length <= n);
>>
>> Implementation would send a scout range ahead, etc.
>>
>> I didn't file an issue for it, but it's a great function to have.
>
> retro + take?

+ retro to turn it back the normal way?

Also, I think this'd work?
return r.takeExactly(r.walkLength - n);

It wouldn't be particularly efficient though I wouldn't think - as you'd need to walk the whole range to find it's length.

r.retro.take(n).retro seems like the easiest fit.

On 12/04/2015 03:01 PM, Jacob Carlborg wrote:
> On 2015-12-04 17:37, Andrei Alexandrescu wrote:
>> Like "tail" in Unix. Given a range R r and a number size_t n, return a
>> TakeExactly!R that's r at no more than n steps from its end:
>>
>> TakeExactly!R advanceWithin(R)(R r, size_t n)
>> if (isForwardRange!R);
>>
>> Invariant:
>>
>> assert(r.advanceWithin(n).length <= n);
>>
>> Implementation would send a scout range ahead, etc.
>>
>> I didn't file an issue for it, but it's a great function to have.
>
> retro + take?

Try it! -- Andrei

On 12/04/2015 04:26 PM, wobbles wrote:
> r.retro.take(n).retro seems like the easiest fit.

Doesn't work. Try it!

The right solution is to send a scout range ahead. Once the scout got n steps ahead the initial range, advance both until the scout reaches the end. Then return the initial range.

It's a nontrivial algorithm that can't be composed easily from existing ones.


Andrei

On Friday, 4 December 2015 at 22:53:01 UTC, Andrei Alexandrescu wrote:
> Doesn't work. Try it!

void main()
{
	import std.range : retro, take;
	import std.stdio : writeln;
	assert([1,2,3,4,5].retro.take(3).retro == [3,4,5]);
}

What exactly doesn't work?

On 12/04/2015 06:09 PM, Sebastiaan Koppe wrote:
> On Friday, 4 December 2015 at 22:53:01 UTC, Andrei Alexandrescu wrote:
>> Doesn't work. Try it!
>
> void main()
> {
>      import std.range : retro, take;
>      import std.stdio : writeln;
>      assert([1,2,3,4,5].retro.take(3).retro == [3,4,5]);
> }
>
> What exactly doesn't work?

Forward ranges.

On 2015-12-04 23:33, Andrei Alexandrescu wrote:

>> retro + take?
>
> Try it! -- Andrei

One would need another "retro" as well. But I see now that you have replied it doesn't work for forward ranges.

-- 
/Jacob Carlborg

On Saturday, 5 December 2015 at 01:03:05 UTC, Andrei Alexandrescu wrote:
>> What exactly doesn't work?
>
> Forward ranges.

I see; retro requires a bidirectional range.

I was thinking about

void main()
{
	import std.algorithm : count;
	import std.range : drop;
	import std.stdio : writeln;
	auto a = [1,2,3,4,5];
	writeln(a.drop(a.count-3));
}

But it has the downside that it calls front/empty/pop 2*n times.

What about using a rangified circular buffer of the same size you want the tail to be, and lazily fill it?

On Friday, 4 December 2015 at 16:37:36 UTC, Andrei Alexandrescu wrote:
> Takers?

https://github.com/D-Programming-Language/phobos/pull/3855