[Issue 8754] New: Function commonPrefix returns invalid string when passing two cyrillic utf-8 strings

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=8754

           Summary: Function commonPrefix returns invalid string when
                    passing two cyrillic utf-8 strings
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody at puremagic.com
        ReportedBy: lxyd.dlang at lxyd.net


--- Comment #0 from Alexey Dubinin <lxyd.dlang at lxyd.net> 2012-10-04 07:53:02 PDT ---
Run this demo:
--------
import std.algorithm, std.stdio, std.encoding;
void main() {
    // ciryllic letters 'б' and 'в' consist of two bytes. First one is common
    auto p = commonPrefix("б", "в");
    writeln(p.length); // 1 code unit. Must be 0
    assert(isValid(p)); // fails: incomplete code point
}
--------
I'm just studying D and, so I'm not sure this is a real bug, but commonPrefix
seems to be designed to treat strings special way and this way seems to be
wrong for strings :)

Let me suggest this separate implementation of commonPrefix for strings (tried
to mimic original code):
--------
import std.functional, std.traits, std.range;

auto commonPrefix(alias pred = "a == b", R1, R2)(R1 r1, R2 r2)
    if (isSomeString!R1 && isSomeString!R2) {
    auto result = r1.save;
    for (; !r1.empty && !r2.empty && binaryFun!pred(r1.front, r2.front);
         r1.popFront(), r2.popFront()){}
    return result[0..$-r1.length];
}
--------
Once again, I'm just studying D and I'm not sure if this code is fully correct,
but it seems to work fine with strings (also, not sure if this separate
implementation sould be trusted and pure).

BTW: documentation has a mistake too:
"The type of the result is the same as $(D takeExactly(r1, n))".
But takeExactly always returns takeExactly.Result, and commonPrefix can return
slice.

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