http://d.puremagic.com/issues/show_bug.cgi?id=7484

           Summary: std.algorithm.copy overlapping array copy
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: ellery-newcomer@utulsa.edu


--- Comment #0 from Ellery Newcomer <ellery-newcomer@utulsa.edu> 2012-02-11 11:52:25 PST ---
Way back in 2.054 you could use std.algorithm.copy in an overlapping copy, e.g.,

copy(a[5 .. 10], a[4 .. 9]);

Fast forward a couple releases, and copy seems to be delegating to

a[4 .. 9] = a[5 .. 10];

in the interest of speed, but here overlapping copy is prohibited by spec and implementation.

I maintain that std.algorithm.copy should permit overlapping copy in the case of arrays because

1: it is consistent with STL copy's semantics (see Note 2)

2: copy can still do an overlapping copy on non-array range types*, and failing to do so on arrays is an inconsistency

If this behavior is desired, the docs should at least make mention of it. Also note that if copy's behavior is reverted, it could be the solution for issue 1317.

*proof:

auto a = make!(SList!int) ([1,2,3,4,5,6,7,8,9,10]);
auto r1 = a[];
auto r2 = a[];
r2.popFront();
copy(r2,r1);
writeln(a[]); // prints [2, 3, 4, 5, 6, 7, 8, 9, 10, 10]

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David Simcha <dsimcha@yahoo.com> changed:

           What    |Removed                     |Added
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                 CC|                            |dsimcha@yahoo.com


--- Comment #1 from David Simcha <dsimcha@yahoo.com> 2012-02-11 14:47:10 PST ---
Agreed.  I implemented the special-casing a few releases back because the performance difference was huge, and the overlapping copy thing was an oversight, not a conscious decision.  I'll put in an extra test for overlap and fallback to naive copying if the arrays do overlap.

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bearophile_hugs@eml.cc changed:

           What    |Removed                     |Added
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                 CC|                            |bearophile_hugs@eml.cc


--- Comment #2 from bearophile_hugs@eml.cc 2012-02-11 14:54:13 PST ---
(In reply to comment #1)
> I'll put in an extra test for overlap and
> fallback to naive copying if the arrays do overlap.

Is using memmove() good there is overlap?

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--- Comment #3 from David Simcha <dsimcha@yahoo.com> 2012-02-11 15:21:17 PST ---
(In reply to comment #2)
> (In reply to comment #1)
> > I'll put in an extra test for overlap and
> > fallback to naive copying if the arrays do overlap.
> 
> Is using memmove() good there is overlap?

I don't think it would have the same semantics in all cases.  memmove() requires semantics identical to if an intermediate buffer were used.  Imagine copying a[4..9] to a[5..10].

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--- Comment #4 from github-bugzilla@puremagic.com 2012-02-11 16:36:39 PST ---
Commits pushed to master at https://github.com/D-Programming-Language/phobos

https://github.com/D-Programming-Language/phobos/commit/3f510e39f1bfddb16701834590ef3a8ba9e50578 Bug 7484:  Overlapping array copy.

https://github.com/D-Programming-Language/phobos/commit/cbe1d8e13d6e1b7506fd104beb039d670efc4bfc Merge pull request #430 from dsimcha/overlappingArrayCopy2

Bug 7484:  Overlapping array copy.

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Jonathan M Davis <jmdavisProg@gmx.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |jmdavisProg@gmx.com
         Resolution|                            |FIXED
           Severity|normal                      |regression


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--- Comment #5 from github-bugzilla@puremagic.com 2012-02-11 16:39:47 PST ---
Commit pushed to master at https://github.com/D-Programming-Language/phobos

https://github.com/D-Programming-Language/phobos/commit/0179562172613ca272017dc8d776da51bc75597f Added bug 7484 to changelog.

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--- Comment #6 from Ellery Newcomer <ellery-newcomer@utulsa.edu> 2012-02-12 08:10:17 PST ---
Awesome. May I additionally request that overlapping copy be mentioned in the documentation

1: in std.algorithm.copy

something like

copy(a[5 .. 10], a[4 .. 9]); //valid
copy(a[4 .. 9], a[5 .. 10]); //invalid
copy(retro(a[5 .. 10]), retro(a[4 .. 9])); // invalid
copy(retro(a[4 .. 9]), retro(a[5 .. 10])); // valid

2: in Language / Arrays / Array Copying

just point the reader to std.algorithm.copy for overlapping copy

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