On Fri, 03 Dec 2010 08:13:50 -0500, Lars T. Kyllingstad
<[email protected]> wrote:
On Thu, 02 Dec 2010 16:18:52 -0500, Steven Schveighoffer wrote:
On Thu, 02 Dec 2010 02:09:51 -0500, Lars T. Kyllingstad
<[email protected]> wrote:
On Wed, 01 Dec 2010 16:44:42 -0500, Steven Schveighoffer wrote:
On Tue, 30 Nov 2010 18:34:11 -0500, Lars T. Kyllingstad
<[email protected]> wrote:
On Tue, 30 Nov 2010 13:52:20 -0500, Steven Schveighoffer wrote:
On Tue, 30 Nov 2010 13:34:50 -0500, Jonathan M Davis
<[email protected]> wrote:
[...]
4. Indexing is no longer O(1), which violates the guarantees of the
index operator.
Indexing is still O(1).
5. Slicing (other than a full slice) is no longer O(1), which
violates the
guarantees of the slicing operator.
Slicing is still O(1).
[...]
It feels extremely weird that the indices refer to code units and not
code points. If I write
auto str = mystring("hæ?");
writeln(str[1], " ", str[2]);
I expect it to print "æ ?", not "æ æ" like it does now.
I don't think it's possible to do that with any implementation without
making indexing not O(1). This just isn't possible, unless you want
to use dchar[].
But your point is well taken. I think what I'm going to do is throw
an exception when accessing an invalid index. While also surprising,
it doesn't result in "extra data". I feel it's probably very rare to
just access hard-coded indexes like that unless you are sure of the
data in the string. Or to use a for-loop to access characters, etc.
As soon as you add opIndex(), your interface becomes that of a random-
access range, something which narrow strings are not. In fact, the
distinction between random access and bidirectional range access for
strings is in many ways the reason we're having this discussion.
How about dropping opIndex() for UTF-8 and UTF-16 strings, and instead
adding a characterAt(i) function that retrieves the i'th code point,
and which is not required to be O(1)? Then, if someone wants O(1)
indexing they are forced to use string_t!dchar or just plain ol'
arrays, both of which have clear, predictable indexing semantics.
Then substring (slicing) becomes an O(n) operation. It just doesn't
work well.
What I meant wast that opSlice() should be disabled in the same way as
opIndex(). What you have now is
struct string_t(T)
{
dchar opIndex(size_t); // Must be O(1)
string_t opSlice(size_t, size_t); // Must be O(1)
}
and what I'm suggesting is
struct string_t(T)
{
dchar character(size_t); // May be O(n)
string_t substring(size_t, size_t); // May be O(n)
static if (is(T == dchar))
{
alias character opIndex; // Must be O(1)
alias substring opSlice; // Must be O(1)
}
}
O(n) slicing and indexing, called by different names, is unacceptable. It
is possible to have O(1) indexing and slicing, I don't see why we should
disallow it. If nothing else, it cripples this implementation to the
point where it won't be accepted, simply because it performs worse than
the current solution.
Now, if one wants to have that feature in *addition* to O(1)
indexing/slicing, I don't think that's an issue.
It seems to be awkward at first thought, but the more I
think about it, the more I think it's right. When do you ever depend on
specific indexes in a string being valid, or to be incrementing always
by 1?
That is a good question indeed. But I'm still not convinced. :)
Another thing to consider is that by having opIndex() in there, your
string satisfies isRandomAccessRange. Then, some algorithms which work
with both bidirectional and random access may choose to go with the
latter. This is a quote from the std.algorithm.find() docs:
Specializations taking advantage of bidirectional or
random access (where present) may accelerate search [...]
I just looked it up, isRandomAccess requires hasLength, which my string
type does not. So my string type is not a true random access range (this
was intentional).
-Steve
