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digitalmars.D.learn - Lexicographic comparison of arrays (of chars)

reply Per =?UTF-8?B?Tm9yZGzDtnc=?= <per.nordlow gmail.com> writes:
I've implement a small size optimized `string` at

https://github.com/nordlow/phobos-next/blob/c35fa4052738af0cd7ad39a9fa715b5ec29c7bba/src/nxt/sso_string.d

I'm now wondering whether or not its definition of comparison at

https://github.com/nordlow/phobos-next/blob/c35fa4052738af0cd7ad39a9fa715b5ec29c7bba/src/nxt/sso_string.d#L248

is a suitable definition of `opCmp` for `SSOString` in terms of 
independence of Phobos' `std.algorithm.comparison.cmp`.

I'm asking because comparison of `string`s can be done directly as

     assert("a" < "b")

without any dependence on Phobos.

Is there a builtin definition of array (of `char`) comparison 
defined somewhere in druntime?

Further, does

     "a" < "b"

perform autodecoding of UTF-8 sequences?
Jan 22 2020
parent reply Per =?UTF-8?B?Tm9yZGzDtnc=?= <per.nordlow gmail.com> writes:
On Wednesday, 22 January 2020 at 08:30:55 UTC, Per Nordlöw wrote:

 is a suitable definition of `opCmp` for `SSOString` in terms of 
 independence of Phobos' `std.algorithm.comparison.cmp`.


I just found

     import core.internal.array.comparison : __cmp;

I presume that is a better alternative if Phobos' independence is 
desired.
Jan 22 2020
parent reply Jacob Carlborg <doob me.com> writes:
On Wednesday, 22 January 2020 at 08:44:15 UTC, Per Nordlöw wrote:


 I just found

     import core.internal.array.comparison : __cmp;

 I presume that is a better alternative if Phobos' independence 
 is desired.


That looks like it's for internal use. There is a `compare` 
method in the `TypeInfo` of each type.

https://github.com/dlang/druntime/blob/2fa694319da397d72ab09cb336f3d588107278c1/src/object.d#L541

--
/Jacob Carlborg
Jan 22 2020
parent reply Per =?UTF-8?B?Tm9yZGzDtnc=?= <per.nordlow gmail.com> writes:
On Wednesday, 22 January 2020 at 10:19:38 UTC, Jacob Carlborg 
wrote:

 That looks like it's for internal use. There is a `compare` 
 method in the `TypeInfo` of each type.


Will that incur an extra runtime cost compared to __cmp?
Jan 22 2020
next sibling parent reply Jacob Carlborg <doob me.com> writes:
On Wednesday, 22 January 2020 at 14:50:01 UTC, Per Nordlöw wrote:


 Will that incur an extra runtime cost compared to __cmp?


I haven't looked at how `__cmp` is implemented but I would guess 
there's some extra overhead. Need to get type info and then there 
will be several virtual method calls involved. Seems to be one of 
the initial call to `compare` and then one for each element of 
the array.

BTW, why don't you implement `opCmp` with the built-in comparison 
operators. Those are going to get lower to a call to `__cmp`. 
Something like this:

int opCmp()(const scope typeof(this) that) const  nogc
{
     auto a = this[];
     auto b = that[];
     return a < b ? -1 : (a > b);
}

--
/Jacob Carlborg
Jan 22 2020
next sibling parent Per =?UTF-8?B?Tm9yZGzDtnc=?= <per.nordlow gmail.com> writes:
On Wednesday, 22 January 2020 at 15:11:09 UTC, Jacob Carlborg 
wrote:

 int opCmp()(const scope typeof(this) that) const  nogc
 {
     auto a = this[];
     auto b = that[];
     return a < b ? -1 : (a > b);
 }

 --
 /Jacob Carlborg


I see. Good to know. Thanks
Jan 22 2020
prev sibling parent Per =?UTF-8?B?Tm9yZGzDtnc=?= <per.nordlow gmail.com> writes:
On Wednesday, 22 January 2020 at 15:11:09 UTC, Jacob Carlborg 
wrote:

 BTW, why don't you implement `opCmp` with the built-in 
 comparison operators. Those are going to get lower to a call to 
 `__cmp`. Something like this:

 int opCmp()(const scope typeof(this) that) const  nogc
 {
     auto a = this[];
     auto b = that[];
     return a < b ? -1 : (a > b);
 }

 --
 /Jacob Carlborg


I presume

int opCmp()(const scope typeof(this) that) const  nogc
{
     scope const a = this[];
     scope const b = that[];
     return a < b ? -1 : (a > b);
}

is preferred.
Aug 02 2020
prev sibling parent Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:
On Wednesday, January 22, 2020 7:50:01 AM MST Per Nordlöw via Digitalmars-d-
learn wrote:

 On Wednesday, 22 January 2020 at 10:19:38 UTC, Jacob Carlborg

 wrote:

 That looks like it's for internal use. There is a `compare`
 method in the `TypeInfo` of each type.


 Will that incur an extra runtime cost compared to __cmp?


Regardless of the overhead involved, you really shouldn't be calling
functions that start with __ or any that are in an internal package. They're
not intended to be called directly by anything outside of druntime or
Phobos, and they could change at any time. In the case of
core.internal.array, the only reason that any of it is even exposed is
because it had to be when it was changed to a template.

- Jonathan M Davis
Jan 22 2020