[PATCH] LWG 2936: update path::compare logic and optimize string comparisons

[Jonathan Wakely <jwakely@redhat.com>]

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The resolution for LWG 2936 defines the comparison more precisely, which
this patch implements. The patch also defines comparisons with strings
to work without constructing a temporary path object (so avoids any
memory allocations).

	* include/bits/fs_path.h (path::compare(const string_type&))
	(path::compare(const value_type*)): Add noexcept and construct a
	string view to compare to instead of a path.
	(path::compare(basic_string_view<value_type>)): Add noexcept. Remove
	inline definition.
	* src/filesystem/std-path.cc (path::_Parser): Track last type read
	from input.
	(path::_Parser::next()): Return a final empty component when the
	input ends in a non-root directory separator.
	(path::_M_append(basic_string_view<value_type>)): Remove special cases
	for trailing non-root directory separator.
	(path::_M_concat(basic_string_view<value_type>)): Likewise.
	(path::compare(const path&)): Implement LWG 2936.
	(path::compare(basic_string_view<value_type>)): Define in terms of
	components returned by parser, consistent with LWG 2936.
	* testsuite/27_io/filesystem/path/compare/lwg2936.cc: New.
	* testsuite/27_io/filesystem/path/compare/path.cc: Test more cases.
	* testsuite/27_io/filesystem/path/compare/strings.cc: Likewise.

Tested x86_64-linux, committed to trunk.


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commit 4e56e9d3bb31740fb65d7a46bfa9796809f971f0
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Dec 18 14:29:14 2018 +0000

    LWG 2936: update path::compare logic and optimize string comparisons
    
    The resolution for LWG 2936 defines the comparison more precisely, which
    this patch implements. The patch also defines comparisons with strings
    to work without constructing a temporary path object (so avoids any
    memory allocations).
    
            * include/bits/fs_path.h (path::compare(const string_type&))
            (path::compare(const value_type*)): Add noexcept and construct a
            string view to compare to instead of a path.
            (path::compare(basic_string_view<value_type>)): Add noexcept. Remove
            inline definition.
            * src/filesystem/std-path.cc (path::_Parser): Track last type read
            from input.
            (path::_Parser::next()): Return a final empty component when the
            input ends in a non-root directory separator.
            (path::_M_append(basic_string_view<value_type>)): Remove special cases
            for trailing non-root directory separator.
            (path::_M_concat(basic_string_view<value_type>)): Likewise.
            (path::compare(const path&)): Implement LWG 2936.
            (path::compare(basic_string_view<value_type>)): Define in terms of
            components returned by parser, consistent with LWG 2936.
            * testsuite/27_io/filesystem/path/compare/lwg2936.cc: New.
            * testsuite/27_io/filesystem/path/compare/path.cc: Test more cases.
            * testsuite/27_io/filesystem/path/compare/strings.cc: Likewise.

diff --git a/libstdc++-v3/include/bits/fs_path.h b/libstdc++-v3/include/bits/fs_path.h
index c69001bcc3c..b827d85965e 100644
--- a/libstdc++-v3/include/bits/fs_path.h
+++ b/libstdc++-v3/include/bits/fs_path.h
@@ -341,9 +341,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
     // compare
 
     int compare(const path& __p) const noexcept;
-    int compare(const string_type& __s) const;
-    int compare(const value_type* __s) const;
-    int compare(const basic_string_view<value_type> __s) const;
+    int compare(const string_type& __s) const noexcept;
+    int compare(const value_type* __s) const noexcept;
+    int compare(basic_string_view<value_type> __s) const noexcept;
 
     // decomposition
 
@@ -1067,14 +1067,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
   { return generic_string<char32_t>(); }
 
   inline int
-  path::compare(const string_type& __s) const { return compare(path(__s)); }
+  path::compare(const string_type& __s) const noexcept
+  { return compare(basic_string_view<value_type>(__s)); }
 
   inline int
-  path::compare(const value_type* __s) const { return compare(path(__s)); }
-
-  inline int
-  path::compare(basic_string_view<value_type> __s) const
-  { return compare(path(__s)); }
+  path::compare(const value_type* __s) const noexcept
+  { return compare(basic_string_view<value_type>(__s)); }
 
   inline path
   path::filename() const
diff --git a/libstdc++-v3/src/filesystem/std-path.cc b/libstdc++-v3/src/filesystem/std-path.cc
index b5ddbdad149..5b0318c1f58 100644
--- a/libstdc++-v3/src/filesystem/std-path.cc
+++ b/libstdc++-v3/src/filesystem/std-path.cc
@@ -62,6 +62,7 @@ struct path::_Parser
   string_view_type input;
   string_view_type::size_type pos = 0;
   size_t origin;
+  _Type last_type = _Type::_Multi;
 
   _Parser(string_view_type s, size_t o = 0) : input(s), origin(o) { }
 
@@ -129,6 +130,12 @@ struct path::_Parser
 	pos = input.find_first_not_of(L"/\\", 2);
       }
 #endif
+
+    if (root.second.valid())
+      last_type = root.second.type;
+    else
+      last_type = root.first.type;
+
     return root;
   }
 
@@ -140,15 +147,30 @@ struct path::_Parser
     char sep = '/';
 #endif
 
+    const int last_pos = pos;
+
     cmpt f;
-    pos = input.find_first_not_of(sep, pos);
     if (pos != input.npos)
       {
-	const auto end = input.find_first_of(sep, pos);
-	f.str = input.substr(pos, end - pos);
-	f.type = _Type::_Filename;
-	pos = end;
+	pos = input.find_first_not_of(sep, pos);
+	if (pos != input.npos)
+	  {
+	    const auto end = input.find_first_of(sep, pos);
+	    f.str = input.substr(pos, end - pos);
+	    f.type = _Type::_Filename;
+	    pos = end;
+	  }
+	else if (last_type == _Type::_Filename
+	    || (last_pos == 0 && !input.empty()))
+	  {
+	    // [fs.path.itr]/4 An empty element, if trailing non-root
+	    // directory-separator present.
+	    __glibcxx_assert(is_dir_sep(input.back()));
+	    f.str = input.substr(input.length(), 0);
+	    f.type = _Type::_Filename;
+	  }
       }
+    last_type = f.type;
     return f;
   }
 
@@ -733,9 +755,6 @@ path::_M_append(basic_string_view<value_type> s)
 	  while (parser2.next().valid())
 	    ++capacity;
 	}
-
-      if (s.back() == '/')
-	++capacity;
     }
   else if (!sep.empty())
     ++capacity;
@@ -787,12 +806,6 @@ path::_M_append(basic_string_view<value_type> s)
 	      ++_M_cmpts._M_impl->_M_size;
 	      cmpt = parser.next();
 	    }
-
-	  if (s.back() == '/')
-	    {
-	      ::new(output++) _Cmpt({}, _Type::_Filename, _M_pathname.length());
-	      ++_M_cmpts._M_impl->_M_size;
-	    }
 	}
       else if (!sep.empty())
 	{
@@ -1107,8 +1120,6 @@ path::_M_concat(basic_string_view<value_type> s)
       while (parser2.next().valid())
 	++capacity;
     }
-  if (is_dir_sep(s.back()))
-    ++capacity;
 
 #if SLASHSLASH_IS_ROOTNAME
   if (orig_type == _Type::_Root_name)
@@ -1165,13 +1176,6 @@ path::_M_concat(basic_string_view<value_type> s)
 	      cmpt = parser.next();
 	    }
 	}
-
-      if (is_dir_sep(s.back()))
-	{
-	  // Empty filename at the end:
-	  ::new(output++) _Cmpt({}, _Type::_Filename, _M_pathname.length());
-	  ++_M_cmpts._M_impl->_M_size;
-	}
     }
   __catch (...)
     {
@@ -1266,58 +1270,168 @@ path::replace_extension(const path& replacement)
   return *this;
 }
 
-namespace
-{
-  template<typename Iter1, typename Iter2>
-    int do_compare(Iter1 begin1, Iter1 end1, Iter2 begin2, Iter2 end2)
-    {
-      int cmpt = 1;
-      while (begin1 != end1 && begin2 != end2)
-	{
-	  if (begin1->native() < begin2->native())
-	    return -cmpt;
-	  if (begin1->native() > begin2->native())
-	    return +cmpt;
-	  ++begin1;
-	  ++begin2;
-	  ++cmpt;
-	}
-      if (begin1 == end1)
-	{
-	  if (begin2 == end2)
-	    return 0;
-	  return -cmpt;
-	}
-      return +cmpt;
-    }
-}
-
 int
 path::compare(const path& p) const noexcept
 {
-  struct CmptRef
-  {
-    const path* ptr;
-    const string_type& native() const noexcept { return ptr->native(); }
-  };
-
-  if (empty() && p.empty())
+  if (_M_pathname == p._M_pathname)
     return 0;
-  else if (_M_type() == _Type::_Multi && p._M_type() == _Type::_Multi)
-    return do_compare(_M_cmpts.begin(), _M_cmpts.end(),
-		      p._M_cmpts.begin(), p._M_cmpts.end());
-  else if (_M_type() == _Type::_Multi)
+
+  basic_string_view<value_type> lroot, rroot;
+  if (_M_type() == _Type::_Root_name)
+    lroot = _M_pathname;
+  else if (_M_type() == _Type::_Multi
+      && _M_cmpts.front()._M_type() == _Type::_Root_name)
+    lroot = _M_cmpts.front()._M_pathname;
+  if (p._M_type() == _Type::_Root_name)
+    rroot = p._M_pathname;
+  else if (p._M_type() == _Type::_Multi
+      && p._M_cmpts.front()._M_type() == _Type::_Root_name)
+    rroot = p._M_cmpts.front()._M_pathname;
+  if (int rootNameComparison = lroot.compare(rroot))
+    return rootNameComparison;
+
+  if (!this->has_root_directory() && p.has_root_directory())
+    return -1;
+  else if (this->has_root_directory() && !p.has_root_directory())
+    return +1;
+
+  using Iterator = const _Cmpt*;
+  Iterator begin1, end1, begin2, end2;
+  if (_M_type() == _Type::_Multi)
     {
-      CmptRef c[1] = { { &p } };
-      return do_compare(_M_cmpts.begin(), _M_cmpts.end(), c, c+1);
-    }
-  else if (p._M_type() == _Type::_Multi)
-    {
-      CmptRef c[1] = { { this } };
-      return do_compare(c, c+1, p._M_cmpts.begin(), p._M_cmpts.end());
+      begin1 = _M_cmpts.begin();
+      end1 = _M_cmpts.end();
+      // Find start of this->relative_path()
+      while (begin1 != end1 && begin1->_M_type() != _Type::_Filename)
+	++begin1;
     }
   else
-    return _M_pathname.compare(p._M_pathname);
+    begin1 = end1 = nullptr;
+
+  if (p._M_type() == _Type::_Multi)
+    {
+      begin2 = p._M_cmpts.begin();
+      end2 = p._M_cmpts.end();
+      // Find start of p.relative_path()
+      while (begin2 != end2 && begin2->_M_type() != _Type::_Filename)
+	++begin2;
+    }
+  else
+    begin2 = end2 = nullptr;
+
+  if (_M_type() == _Type::_Filename)
+    {
+      if (p._M_type() == _Type::_Filename)
+	return native().compare(p.native());
+      else if (begin2 != end2)
+	{
+	  if (int ret = native().compare(begin2->native()))
+	    return ret;
+	  else
+	    return ++begin2 == end2 ? 0 : -1;
+	}
+      else
+	return +1;
+    }
+  else if (p._M_type() == _Type::_Filename)
+    {
+      if (begin1 != end1)
+	{
+	  if (int ret = begin1->native().compare(p.native()))
+	    return ret;
+	  else
+	    return ++begin1 == end1 ? 0 : +1;
+	}
+      else
+	return -1;
+    }
+
+  int count = 1;
+  while (begin1 != end1 && begin2 != end2)
+    {
+      if (int i = begin1->native().compare(begin2->native()))
+	return i;
+      ++begin1;
+      ++begin2;
+      ++count;
+    }
+  if (begin1 == end1)
+    {
+      if (begin2 == end2)
+	return 0;
+      return -count;
+    }
+  return count;
+}
+
+int
+path::compare(basic_string_view<value_type> s) const noexcept
+{
+  if (_M_pathname == s)
+    return 0;
+
+  _Parser parser(s);
+
+  basic_string_view<value_type> lroot, rroot;
+  if (_M_type() == _Type::_Root_name)
+    lroot = _M_pathname;
+  else if (_M_type() == _Type::_Multi
+      && _M_cmpts.front()._M_type() == _Type::_Root_name)
+    lroot = _M_cmpts.front()._M_pathname;
+  auto root_path = parser.root_path();
+  if (root_path.first.type == _Type::_Root_name)
+    rroot = root_path.first.str;
+  if (int rootNameComparison = lroot.compare(rroot))
+    return rootNameComparison;
+
+  const bool has_root_dir = root_path.first.type == _Type::_Root_dir
+    || root_path.second.type == _Type::_Root_dir;
+  if (!this->has_root_directory() && has_root_dir)
+    return -1;
+  else if (this->has_root_directory() && !has_root_dir)
+    return +1;
+
+  using Iterator = const _Cmpt*;
+  Iterator begin1, end1;
+  if (_M_type() == _Type::_Filename)
+    {
+      auto cmpt = parser.next();
+      if (cmpt.valid())
+	{
+	  if (int ret = this->native().compare(cmpt.str))
+	    return ret;
+	  return parser.next().valid() ? -1 : 0;
+	}
+      else
+	return +1;
+    }
+  else if (_M_type() == _Type::_Multi)
+    {
+      begin1 = _M_cmpts.begin();
+      end1 = _M_cmpts.end();
+      while (begin1 != end1 && begin1->_M_type() != _Type::_Filename)
+	++begin1;
+    }
+  else
+    begin1 = end1 = nullptr;
+
+  int count = 1;
+  auto cmpt = parser.next();
+  while (begin1 != end1 && cmpt.valid())
+    {
+      if (int i = begin1->native().compare(cmpt.str))
+	return i;
+      ++begin1;
+      cmpt = parser.next();
+      ++count;
+    }
+  if (begin1 == end1)
+    {
+      if (!cmpt.valid())
+	return 0;
+      return -count;
+    }
+  return +count;
 }
 
 path
@@ -1718,12 +1832,9 @@ path::_M_split_cmpts()
 	*next++ = root_path.second;
     }
 
-  bool got_at_least_one_filename = false;
-
   auto cmpt = parser.next();
   while (cmpt.valid())
     {
-      got_at_least_one_filename = true;
       do
 	{
 	  *next++ = cmpt;
@@ -1746,15 +1857,6 @@ path::_M_split_cmpts()
 	}
     }
 
-  // [fs.path.itr]/4
-  // An empty element, if trailing non-root directory-separator present.
-  if (got_at_least_one_filename && is_dir_sep(_M_pathname.back()))
-    {
-      next->str = { _M_pathname.data() + _M_pathname.length(), 0 };
-      next->type = _Type::_Filename;
-      ++next;
-    }
-
   if (auto n = next - buf.begin())
     {
       if (n == 1 && _M_cmpts.empty())
diff --git a/libstdc++-v3/testsuite/27_io/filesystem/path/compare/lwg2936.cc b/libstdc++-v3/testsuite/27_io/filesystem/path/compare/lwg2936.cc
new file mode 100644
index 00000000000..8a11043f143
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/filesystem/path/compare/lwg2936.cc
@@ -0,0 +1,80 @@
+// { dg-options "-std=gnu++17 -lstdc++fs" }
+// { dg-do run { target c++17 } }
+// { dg-require-filesystem-ts "" }
+
+// Copyright (C) 2014-2018 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// 8.4.8 path compare [path.compare]
+
+#include <filesystem>
+#include <testsuite_hooks.h>
+#include <testsuite_fs.h>
+
+using std::filesystem::path;
+
+int norm(int i)
+{
+  if (i < 0)
+    return -1;
+  else if (i > 0)
+    return +1;
+  else
+    return 0;
+}
+
+void
+check(const path& lhs, const path& rhs, int sense)
+{
+  VERIFY( lhs.compare(lhs) == 0 );
+  VERIFY( rhs.compare(rhs) == 0 );
+
+  VERIFY( norm(lhs.compare(rhs)) == sense );
+  VERIFY( norm(lhs.compare(rhs.c_str())) == sense );
+
+  VERIFY( norm(rhs.compare(lhs)) == -sense );
+  VERIFY( norm(rhs.compare(lhs.c_str())) == -sense );
+}
+
+void
+test01()
+{
+  check("", "", 0);
+
+  // These are root names on Windows (just relative paths elsewhere)
+  check("", "c:", -1);
+  check("c:", "d:", -1);
+  check("c:", "c:/", -1);
+  check("d:", "c:/", +1);
+  check("c:/a/b", "c:a/b", -1);
+
+  // These are root names on Cygwin (just relative paths elsewhere)
+  check("", "//c", -1);
+  check("//c", "//d", -1);
+  check("//c", "//c/", -1);
+  check("//d", "//c/", +1);
+
+  check("/a", "/b", -1);
+  check("a", "/b", -1);
+  check("/b", "b", +1);
+}
+
+int
+main()
+{
+  test01();
+}
diff --git a/libstdc++-v3/testsuite/27_io/filesystem/path/compare/path.cc b/libstdc++-v3/testsuite/27_io/filesystem/path/compare/path.cc
index 4aa2cd387c5..159a96c6597 100644
--- a/libstdc++-v3/testsuite/27_io/filesystem/path/compare/path.cc
+++ b/libstdc++-v3/testsuite/27_io/filesystem/path/compare/path.cc
@@ -44,8 +44,19 @@ test01()
   }
 }
 
+void
+test02()
+{
+  VERIFY( path("/").compare(path("////")) == 0 );
+  VERIFY( path("/a").compare(path("/")) > 0 );
+  VERIFY( path("/").compare(path("/a")) < 0 );
+  VERIFY( path("/ab").compare(path("/a")) > 0 );
+  VERIFY( path("/ab").compare(path("/a/b")) > 0 );
+}
+
 int
 main()
 {
   test01();
+  test02();
 }
diff --git a/libstdc++-v3/testsuite/27_io/filesystem/path/compare/strings.cc b/libstdc++-v3/testsuite/27_io/filesystem/path/compare/strings.cc
index cfd73ad9e4a..a3fcb800dbf 100644
--- a/libstdc++-v3/testsuite/27_io/filesystem/path/compare/strings.cc
+++ b/libstdc++-v3/testsuite/27_io/filesystem/path/compare/strings.cc
@@ -42,8 +42,19 @@ test01()
   }
 }
 
+void
+test02()
+{
+  VERIFY( path("/").compare("////") == 0 );
+  VERIFY( path("/a").compare("/") > 0 );
+  VERIFY( path("/").compare("/a") < 0 );
+  VERIFY( path("/ab").compare("/a") > 0 );
+  VERIFY( path("/ab").compare("/a/b") > 0 );
+}
+
 int
 main()
 {
   test01();
+  test02();
 }