http://d.puremagic.com/issues/show_bug.cgi?id=3037

           Summary: Off-by-one error in Stride.length
           Product: D
           Version: 2.030
          Platform: PC
        OS/Version: Windows
            Status: NEW
          Severity: major
          Priority: P2
         Component: Phobos
        AssignedTo: bugzilla@digitalmars.com
        ReportedBy: dsimcha@yahoo.com


std.range.Stride has an off-by-one error in its length() function, which causes the reported length to be one less than the actual length whenever _input.length % _n != 0.

import std.stdio, std.range;

void main() {
    uint[] foo = [1,2,3,4,5];

    auto s = stride(foo, 2);
    writeln(s.length);  // 2

    uint realLength = 0;
    foreach(elem; s) {
        realLength++;
    }
    writeln(realLength);  // 3
}

This can be fixed by changing the length function in std.range.Stride to the following:

size_t length()
{
     return (_input.length % _n == 0) ?
            _input.length / _n :
            _input.length / _n + 1;
}


The fix can be verified by the following test case:

import std.stdio, std.range;

void main() {
    foreach(l; 0..10) {
        foreach(s; 1..l) {
            uint[] foo = new uint[l];
            auto st = stride(foo, s);
            auto len1 = st.length;
            uint len2 = 0;
            foreach(elem; st) {
                len2++;
            }
            assert(len1 == len2);
            writeln(len1, "\t", len2);
        }
    }
}

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Andrei Alexandrescu <andrei@metalanguage.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED




--- Comment #1 from Andrei Alexandrescu <andrei@metalanguage.com>  2009-08-27 22:31:31 PDT ---
I fixed length like this:

return (_input.length - 1) / _n + 1;

Thanks!

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--- Comment #2 from Andrei Alexandrescu <andrei@metalanguage.com>  2009-08-27 23:38:34 PDT ---
(In reply to comment #1)
> I fixed length like this:
> 
> return (_input.length - 1) / _n + 1;
> 
> Thanks!

In fact this doesn't work for _input.length == 0. So I rewrote it as:

return (_input.length + _n - 1) / _n

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