std.string.reverse() for mutable array of chars

[bearophile]

Reversing an array of chars/wchars is a common enough operation (mutable arrays often come from precedent operations that have built it). Currently std.algorithm.reverse() can't be used:


import std.algorithm;
void main() {
    dchar[] s1 = "hello"d.dup;
    s1.reverse(); // OK
    wchar[] s2 = "hello"w.dup;
    s2.reverse(); // error
    char[] s3 = "hello".dup;
    s3.reverse(); // error
}


I suggest to add a char[]/wchar[] specialization to std.algorithm.reverse() (or to add a std.string.reverse()), to make it work on those types too. Generally std.algorithms don't work on UTF8/UTF16 because of the variable length of its items, but for this specific algorithm I think this is not a problem because:

1) Reversing an array is an O(n) operation, and decoding UTF adds a constant overhead, so the computational complexity of reverse doesn't change.
2) If you reverse an char[] or wchar[] the result will fit in the input array (is this always true? Please tell me if this isn't true). It "just" needs to correctly swap the bytes of multi-byte chars, and swap if there are combined codepoints too.

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And I think std.algorithm.reverse() is sometimes buggy on a dchar[] (UTF32):


import std.algorithm: reverse;
void main() {
    dchar[] txt = "\U00000041\U00000308\U00000042"d.dup;
    txt.reverse();
    assert(txt == "\U00000042\U00000308\U00000041"d);
}


txt contains LATIN CAPITAL LETTER A, COMBINING DIAERESIS, LATIN CAPITAL LETTER B (see bug 7084 for more details).

A correct output for reversing txt is (LATIN CAPITAL LETTER B, LATIN CAPITAL LETTER A, COMBINING DIAERESIS):

"\U00000042\U00000041\U00000308"d


See for some code:
http://stackoverflow.com/questions/199260/how-do-i-reverse-a-utf-8-string-in-place

See also:
http://d.puremagic.com/issues/show_bug.cgi?id=7085

Regarding the printing of unicode strings see also:
http://d.puremagic.com/issues/show_bug.cgi?id=7084

Bye,
bearophile