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.

From-SVN: r267235

libstdc++-v3/ChangeLog

 2018-12-18  Jonathan Wakely  <jwakely@redhat.com>
 
+	* 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.
+
 	* include/std/string_view [__cplusplus > 201703L]
 	(basic_string_view::starts_with(basic_string_view)): Implement
 	proposed resolution of LWG 3040 to avoid redundant length check.

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

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
+int
+path::compare(const path& p) const noexcept
 {
-  template<typename Iter1, typename Iter2>
-    int do_compare(Iter1 begin1, Iter1 end1, Iter2 begin2, Iter2 end2)
+  if (_M_pathname == p._M_pathname)
+    return 0;
+
+  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)
     {
-      int cmpt = 1;
-      while (begin1 != end1 && begin2 != end2)
+      begin1 = _M_cmpts.begin();
+      end1 = _M_cmpts.end();
+      // Find start of this->relative_path()
+      while (begin1 != end1 && begin1->_M_type() != _Type::_Filename)
+	++begin1;
+    }
+  else
+    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 (begin1->native() < begin2->native())
-	    return -cmpt;
-	  if (begin1->native() > begin2->native())
-	    return +cmpt;
-	  ++begin1;
-	  ++begin2;
-	  ++cmpt;
+	  if (int ret = native().compare(begin2->native()))
+	    return ret;
+	  else
+	    return ++begin2 == end2 ? 0 : -1;
 	}
-      if (begin1 == end1)
+      else
+	return +1;
+    }
+  else if (p._M_type() == _Type::_Filename)
+    {
+      if (begin1 != end1)
 	{
-	  if (begin2 == end2)
-	    return 0;
-	  return -cmpt;
+	  if (int ret = begin1->native().compare(p.native()))
+	    return ret;
+	  else
+	    return ++begin1 == end1 ? 0 : +1;
 	}
-      return +cmpt;
+      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(const path& p) const noexcept
+path::compare(basic_string_view<value_type> s) const noexcept
 {
-  struct CmptRef
-  {
-    const path* ptr;
-    const string_type& native() const noexcept { return ptr->native(); }
-  };
-
-  if (empty() && p.empty())
+  if (_M_pathname == s)
     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)
+
+  _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)
     {
-      CmptRef c[1] = { { &p } };
-      return do_compare(_M_cmpts.begin(), _M_cmpts.end(), c, c+1);
+      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 (p._M_type() == _Type::_Multi)
+  else if (_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();
+      while (begin1 != end1 && begin1->_M_type() != _Type::_Filename)
+	++begin1;
     }
   else
-    return _M_pathname.compare(p._M_pathname);
+    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())

libstdc++-v3/testsuite/27_io/filesystem/path/compare/lwg2936.cc

+// { 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();
+}

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();
 }

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();
 }