[Issue 3037] New: Off-by-one error in Stride.length

[d-bugmail at puremagic.com]

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 at digitalmars.com
        ReportedBy: dsimcha at 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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