From: "François Dumont" <frs.dumont@gmail.com>
To: "libstdc++@gcc.gnu.org" <libstdc++@gcc.gnu.org>
Subject: Add three way lower_bound
Date: Wed, 23 Jun 2021 22:34:13 +0200 [thread overview]
Message-ID: <e6c92783-399a-1e23-eaef-fe3bea91f275@gmail.com> (raw)
[-- Attachment #1: Type: text/plain, Size: 665 bytes --]
Hi
Following the message to propose an alternative lower_bound and the
reply to use three way comparison I try to implement this.
Before going further I wonder if this is something possible ?
The purpose of the:
if constexpr (three_way_comparable<>)
is to make sure that we use it only if there is a proper <=> operator
defined. Afai understood what is in <compare> we can have the
__synth3way for any type as long as < exist. But I think that if <=> is
implemented in terms of < then it might be too expensive, the actual
lower_bound might already be implemented this way.
My main concerns is of course Standard conformity, could it be ok ?
François
[-- Attachment #2: lower_bound_three_way.patch --]
[-- Type: text/x-patch, Size: 2081 bytes --]
diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h
index d001b5f9dae..3ccb3c0301b 100644
--- a/libstdc++-v3/include/bits/stl_algobase.h
+++ b/libstdc++-v3/include/bits/stl_algobase.h
@@ -1473,6 +1473,48 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
return __first;
}
+#if __cpp_lib_three_way_comparison
+ template<typename _ForwardIterator, typename _Tp, typename _Compare>
+ _GLIBCXX20_CONSTEXPR
+ _ForwardIterator
+ __lower_bound_three_way(_ForwardIterator __first, _ForwardIterator __last,
+ const _Tp& __val, _Compare __comp)
+ {
+ auto __len = std::distance(__first, __last);
+
+ while (__len > 0)
+ {
+ auto __half = __len >> 1;
+ if (__half == 0)
+ {
+ if (auto __c = __comp(*__first, __val); __c < 0)
+ return std::next(__first);
+ return __first;
+ }
+
+ _ForwardIterator __prev_mid = __first;
+ std::advance(__prev_mid, __half - 1);
+ _ForwardIterator __middle = std::next(__prev_mid);
+ if (auto __c = __comp(*__middle, __val); __c != 0)
+ {
+ if (__c < 0)
+ {
+ __first = __middle;
+ ++__first;
+ __len = __len - __half - 1;
+ }
+ else
+ __len = __half;
+ }
+ else if (__c = __comp(*__prev_mid, __val); __c != 0)
+ return __middle;
+ else // __c == 0.
+ __len = __half - 1;
+ }
+ return __first;
+ }
+#endif
+
/**
* @brief Finds the first position in which @a val could be inserted
* without changing the ordering.
@@ -1496,6 +1538,13 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
typename iterator_traits<_ForwardIterator>::value_type, _Tp>)
__glibcxx_requires_partitioned_lower(__first, __last, __val);
+#if __cpp_lib_three_way_comparison
+ if constexpr (three_way_comparable_with<
+ typename iterator_traits<_ForwardIterator>::reference,
+ const _Tp&>)
+ return std::__lower_bound_three_way(__first, __last, __val,
+ compare_three_way{});
+#endif
return std::__lower_bound(__first, __last, __val,
__gnu_cxx::__ops::__iter_less_val());
}
next reply other threads:[~2021-06-23 20:34 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-23 20:34 François Dumont [this message]
2022-09-01 5:01 ` François Dumont
2022-09-01 6:47 ` Jonathan Wakely
2022-09-01 6:47 ` Jonathan Wakely
2022-09-07 4:53 ` François Dumont
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=e6c92783-399a-1e23-eaef-fe3bea91f275@gmail.com \
--to=frs.dumont@gmail.com \
--cc=libstdc++@gcc.gnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).