Why the hell doesn't foreach decode strings

[Steven Schveighoffer]

On Mon, 24 Oct 2011 16:18:57 -0400, Dmitry Olshansky  
<dmitry.olsh at gmail.com> wrote:

> On 24.10.2011 23:41, Steven Schveighoffer wrote:
>> On Mon, 24 Oct 2011 11:58:15 -0400, Simen Kjaeraas
>> <simen.kjaras at gmail.com> wrote:
>>
>>> On Mon, 24 Oct 2011 16:02:24 +0200, Steven Schveighoffer
>>> <schveiguy at yahoo.com> wrote:
>>>
>>>> On Sat, 22 Oct 2011 05:20:41 -0400, Walter Bright
>>>> <newshound2 at digitalmars.com> wrote:
>>>>
>>>>> On 10/22/2011 2:21 AM, Peter Alexander wrote:
>>>>>> Which operations do you believe would be less efficient?
>>>>>
>>>>> All of the ones that don't require decoding, such as searching,
>>>>> would be less efficient if decoding was done.
>>>>
>>>> Searching that does not do decoding is fundamentally incorrect. That
>>>> is, if you want to find a substring in a string, you cannot just
>>>> compare chars.
>>>
>>> Assuming both string are valid UTF-8, you can. Continuation bytes can
>>> never
>>> be confused with the first byte of a code point, and the first byte
>>> always
>>> identifies how many continuation bytes there should be.
>>>
>>
>> As others have pointed out in the past to me (and I thought as you did
>> once), the same characters can be encoded in *different ways*. They must
>> be normalized to accurately compare.
>>
>
> Assuming language support stays on stage of "codepoint is a character"  
> it's totaly expected to ignore modifiers and compare identically  
> normalized UTF without decoding. Yes, it risks to hit certain issues.

Again, the "risk" is that it fails to achieve the goal you ask of it!

D-language: Here, use this search algorithm, it works most of the time,  
but may not work correctly in some cases.  If you run into one of those  
cases, you'll have to run a specialized search algorithm for strings.
User: How do I know I hit one of those cases?
D-language: You'll have to run the specialized version to find out.
User: Why wouldn't I just run the specialized version first?
D-language: Well, because it's slower!
User: But don't I have to use both algorithms to make sure I find the data?
D-language: Only if you "care" about accuracy!

Call me ludicrous, but is this really what we want to push on someone as a  
"unicode-aware" language?

>
>> Plus, a combining character (such as an umlaut or accent) is part of a
>> character, but may be a separate code point. If that's on the last
>> character in the word such as fiancé, then searching for fiance will
>> result in a match without proper decoding!
>
> Now if you are going to do real characters... If source/needle are  
> normalized you still can avoid lots of work by searching without  
> decoding. All you need to decode is one codepoint on each successful  
> match to see if there is a modifier at end of matched portion.
> But it depends on how you want to match if it's case-insensitive search  
> it will be a lot more complicated, but anyway it boils down to this:
> 1) do inexact search, get likely match ( false positives are OK,  
> negatives not) no decoding here
> 2) once found check it (or parts of it) with proper decoding
>
> There are cultural subtleties, that complicate these steps if you take  
> them into account, but it's doable.

I agree with you that simple searches using only byte (or dchar)  
comparison does not work, and can be optimized based on several factors.   
The easiest thing is to find a code unit sequence that only has one valid  
form, then search for that without decoding.  Then when found, decode the  
characters around it.  Or if that isn't possible, create all the  
un-normalized forms for one grapheme (based on how likely it is to occur),  
and search for one of those in the undecoded stream.

This can all be built into a specialized string type.  There's actually  
some really interesting problems to solve in this space I think.  I've  
created a basic string type that has lamented in my unfinished pile of  
stuff to do.  I think it can be done in a way that is close to as  
efficient as arrays for the most common operations (slicing, indexing,  
etc.), but is *correct* before it is efficient.  You should always be able  
to drop into "array mode" and deal with the code-units.

> Or if fiancé uses a
>> precomposed é, it won't match. So two valid representations of the word
>> either match or they don't. It's just a complete mess without proper
>> unicode decoding.
>
> It's a complete mess even with proper decoding ;)

Yes, all the more reason to solve the problem correctly so the hapless  
unicode novice user doesn't have to!

-Steve