[Issue 3876] New: std.range.Take back/popBack methods don't work correctly

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=3876

           Summary: std.range.Take back/popBack methods don't work
                    correctly
           Product: D
           Version: 2.040
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody at puremagic.com
        ReportedBy: philippe.sigaud at gmail.com


--- Comment #0 from Philippe Sigaud <philippe.sigaud at gmail.com> 2010-03-04 15:24:27 CET ---
std.range.Take back and popBack methods do not work correctly:

----
import std.range;

void main() {
    auto r = [0,1,2,3,4];
    auto t = take(3,r);

    assert(equal(t, [0,1,2][])); // OK.

    assert(equal(retro(t), [4,3,2,1,1,0,0][])); // Something is wrong.
}
----


Here is a version that seems to work, though the code may not be as clean as it
should (and Take's code wasn't pretty to begin with...)
It demands rather strict conditions on the input range: back/popBack need a
random-access range with a length. 

----
struct Take(R) if (isInputRange!(R))
{
private:
    size_t _maxAvailable;
    R _input;
    enum bool byRef = is(typeof(&(R.init[0])));

public:
    alias R Source;

    static if (byRef)
        alias ref .ElementType!(R) ElementType;
    else
        alias .ElementType!(R) ElementType;

    bool empty()
    {
        return _maxAvailable == 0 || _input.empty;
    }

    void popFront()
    {
        enforce(_maxAvailable > 0);
        _input.popFront;
        --_maxAvailable;
    }

    // @@@@@@@@@@@ UGLY @@@@@@@@@@@@@@@
    mixin(
        (byRef ? "ref " : "")~
        q{ElementType front()
        {
            enforce(_maxAvailable > 0);
            return _input.front;
        }});

    static if (isInfinite!(R))
    {
        size_t length() const
        {
            return _maxAvailable;
        }

        void popBack()
        {
            enforce(_maxAvailable);
            --_maxAvailable;
        }
    }
    else static if (hasLength!(R))
    {
        size_t length()
        {
            return min(_maxAvailable, _input.length);
        }

        static if (isRandomAccessRange!(R))
        {
            void popBack()
            {
                if (_maxAvailable < _input.length) // changed from > to <
                {
                    --_maxAvailable;
                }
                else
                {
                    _input.popBack;
                }
            }
        }
    }

    static if (isRandomAccessRange!(R))
    {
        mixin(
            (byRef ? "ref " : "")~
            q{ElementType opIndex(uint index)
                {
                    enforce(_maxAvailable > index);
                    return _input[index];
                }
            });
    }

/+    static if (isBidirectionalRange!(R))
    {
        mixin(
            (byRef ? "ref " : "")~
            q{ElementType back()
                {
                    return _input[_maxAvailable];
                }
            });
    }
    else+/ static if (isRandomAccessRange!(R) /+&& isInfinite!(R)+/)
    {
        // Random access but not bidirectional could happen in the
        // case of e.g. some infinite ranges
        mixin(
            (byRef ? "ref " : "")~
            q{ElementType back()
                {
                    return _input[this.length() - 1];
                }
            });
    }

    Take opSlice() { return this; }
}
----

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