[Issue 12188] New: std.algorithm.nextPermutation requires random access

[d-bugmail at puremagic.com]

https://d.puremagic.com/issues/show_bug.cgi?id=12188

           Summary: std.algorithm.nextPermutation requires random access
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody at puremagic.com
        ReportedBy: peter.alexander.au at gmail.com


--- Comment #0 from Peter Alexander <peter.alexander.au at gmail.com> 2014-02-17 12:20:14 PST ---
The constraints says it requires bidirectional, but it actually requires random
access due to this line:

reverse(takeExactly(retro(range), n));

'reverse' requires a bidirectional range, but 'takeExactly' on a bidirectional
range returns a forward range. You need to provide a random access range to
'takeExactly' to get bidirectional, so this becomes a requirement of
'nextPermutation'.

nextPermutation must be modified to support bidirectional ranges. Changing the
constraint to random access is not an acceptable fix.

Here's a simple test case where a bidirectional range fails:

---------------------
import std.algorithm, std.array;

struct MyRange
{
    int[] a = [1, 2, 3];
    @property
    {
        auto ref front() { return a.front; }
        auto ref back() { return a.back; }
        auto empty() { return a.empty; }
        auto save() { return MyRange(a); }
    }
    void popFront() { a.popFront(); }
    void popBack() { a.popBack(); }
}

void main()
{
    MyRange r;
    nextPermutation(r);
} 
---------------------

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