From fb8e0bcea3f6fae8f499fc3279739fdeb56d5826 Mon Sep 17 00:00:00 2001
From: John Maddock <john@johnmaddock.co.uk>
Date: Tue, 1 Nov 2005 17:36:42 +0000
Subject: [PATCH] Fix performance when using bidirectional iterators: don't use
 std::distance!

[SVN r31531]
---
 .../boost/regex/v4/perl_matcher_common.hpp    | 28 ++++++++++++++++---
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/include/boost/regex/v4/perl_matcher_common.hpp b/include/boost/regex/v4/perl_matcher_common.hpp
index af2139b6..54edaa15 100644
--- a/include/boost/regex/v4/perl_matcher_common.hpp
+++ b/include/boost/regex/v4/perl_matcher_common.hpp
@@ -682,12 +682,32 @@ bool perl_matcher<BidiIterator, Allocator, traits>::match_restart_continue()
 template <class BidiIterator, class Allocator, class traits>
 bool perl_matcher<BidiIterator, Allocator, traits>::match_backstep()
 {
-   std::ptrdiff_t maxlen = ::boost::re_detail::distance(backstop, position);
-   if(maxlen < static_cast<const re_brace*>(pstate)->index)
-      return false;
-   std::advance(position, -static_cast<const re_brace*>(pstate)->index);
+#ifdef BOOST_MSVC
+#pragma warning(push)
+#pragma warning(disable:4127)
+#endif
+   if( ::boost::is_random_access_iterator<BidiIterator>::value)
+   {
+      std::ptrdiff_t maxlen = ::boost::re_detail::distance(backstop, position);
+      if(maxlen < static_cast<const re_brace*>(pstate)->index)
+         return false;
+      std::advance(position, -static_cast<const re_brace*>(pstate)->index);
+   }
+   else
+   {
+      int c = static_cast<const re_brace*>(pstate)->index;
+      while(c--)
+      {
+         if(position == backstop)
+            return false;
+         --position;
+      }
+   }
    pstate = pstate->next.p;
    return true;
+#ifdef BOOST_MSVC
+#pragma warning(pop)
+#endif
 }
 
 template <class BidiIterator, class Allocator, class traits>