On Friday, 22 June 2018 at 00:50:05 UTC, Steven Schveighoffer
wrote:
On 6/21/18 6:46 PM, Per Nordlöw wrote:
I've discovered the annoying fact that std.conv.to doesn't
scale for enum to string conversion when the enum has hundreds
of members. This because of a call to `NoDuplicates` which has
(at least) O(n*log(n) time and space complexity.
So I've come up with
/** Faster implementation of `std.conv.to`.
*/
string toString(T)(T value) @safe pure nothrow @nogc
if (is(T == enum))
{
final switch (value)
{
static foreach (member; __traits(allMembers, T))
{
case __traits(getMember, T, member):
return member;
}
}
}
///
@safe pure nothrow @nogc unittest
{
enum E { unknown, x, y, z, }
assert(E.x.toString == "x");
assert(E.y.toString == "y");
assert(E.z.toString == "z");
}
The question know is: How do I make this support enums with
enumerator aliases without needing to call `NoDuplicates`?
You can't. That's why it's in there. And I can't think of a
better algorithm than O(nlgn). That's what it would cost to
sort anyway.
The sucky thing is, the compiler is *already* doing a sort on
the items in the switch, and *already* doing the duplicate
check. It would be cool to be able to leverage this mechanism
to avoid the library solution, but I don't know how we can do
that, as the semantics for switch are well defined, and there's
no other way to hook this builtin functionality.
One thing that may be worthwhile is checking to see if a CTFE
solution is better than a template solution. In other words:
string[] dedup(string[] identifiers) { ... }
enum items = dedup(__traits(allMembers, T));
static foreach(member; items) ...
Just avoiding all the template symbol generation may make it
worth it, even if the dedup function is O(n^2) complexity.
-Steve
I do have my own ctfe utils for this
{https://forum.dlang.org/post/[email protected]}, however
without dedup (because I'd rather be alerted when it happens) what you have to
do is to sort the enum values.
(I'd suggest a non-recursive mergesort.)
However if there are no duplicates there's no need to
deduplicate, the reason it is deduplicated it because the
switch_statement forbids duplicates.
I may add that this performs very well with newCTFE I don't know
about the old engine, but I'd assume even there it's faster.
