[Issue 3638] New: stable sort assertion failure for repeated key sequence

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=3638

           Summary: stable sort assertion failure for repeated key
                    sequence
           Product: D
           Version: unspecified
          Platform: Other
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody at puremagic.com
        ReportedBy: andrew.talbot at talbotville.com


--- Comment #0 from Andrew Talbot <andrew.talbot at talbotville.com> 2009-12-21 13:40:32 PST ---
Performing a stable sort on a repeated pair of keys that are already in sorted
order causes the following error message.

core.exception.AssertError at std.algorithm(3736): Assertion failure

However, doing so with repeated reverse-sorted keys works fine.

In other words, a stable sort of the array
[ "B", "A", "B", A" ] will succeed, but one of ["A", "B", "A", "B" ] will fail.

Illustrative code:

import std.algorithm;
import std.stdio;

void main()
{
    auto ar1 = [ "B", "A", "B", "A" ];
    sort!("a <  b", SwapStrategy.stable)(ar1); // ok
    writeln(ar1);

    auto ar2 = [ "A", "B", "A", "B" ];
    sort!("a <  b", SwapStrategy.stable)(ar2); // fails
    writeln(ar2);
}

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