Improvements for epee binary to hex functions:

- Performance improvements
  - Added `span` for zero-copy pointer+length arguments
  - Added `std::ostream` overload for direct writing to output buffers
  - Removal of unused `string_tools::buff_to_hex`

contrib/epee/src/CMakeLists.txt

@@ -26,7 +26,7 @@
 # STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
 # THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-add_library(epee STATIC http_auth.cpp mlog.cpp string_tools.cpp)
+add_library(epee STATIC hex.cpp http_auth.cpp mlog.cpp string_tools.cpp)
 # Build and install libepee if we're building for GUI
 if (BUILD_GUI_DEPS)
     if(IOS)

contrib/epee/src/hex.cpp

@@ -0,0 +1,82 @@
+// Copyright (c) 2017, The Monero Project
+//
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "hex.h"
+
+#include <iterator>
+#include <limits>
+#include <ostream>
+#include <stdexcept>
+
+namespace epee
+{
+  namespace
+  {
+    template<typename T>
+    void write_hex(T&& out, const view<std::uint8_t> src)
+    {
+      static constexpr const char hex[] = u8"0123456789abcdef";
+      static_assert(sizeof(hex) == 17, "bad string size");
+      for (const std::uint8_t byte : src)
+      {
+        *out = hex[byte >> 4];
+        ++out;
+        *out = hex[byte & 0x0F];
+        ++out;
+      }
+    }
+  }
+
+  std::string to_hex::string(const view<std::uint8_t> src)
+  {
+    if (std::numeric_limits<std::size_t>::max() / 2 < src.size())
+      throw std::range_error("hex_view::to_string exceeded maximum size");
+
+    std::string out{};
+    out.resize(src.size() * 2);
+    buffer_unchecked(std::addressof(out[0]), src);
+    return out;
+  }
+
+  void to_hex::buffer(std::ostream& out, const view<std::uint8_t> src)
+  {
+    write_hex(std::ostreambuf_iterator<char>{out}, src);
+  }
+
+  void to_hex::formatted(std::ostream& out, const view<std::uint8_t> src)
+  {
+    out.put('<');
+    buffer(out, src);
+    out.put('>');
+  }
+
+  void to_hex::buffer_unchecked(char* out, const view<std::uint8_t> src) noexcept
+  {
+    return write_hex(out, src);
+  }
+}

contrib/epee/src/http_auth.cpp

@@ -67,6 +67,7 @@
 #include <type_traits>
 
 #include "crypto/crypto.h"
+#include "hex.h"
 #include "md5_l.h"
 #include "string_coding.h"
 
@@ -104,25 +105,6 @@ namespace
 
   //// Digest Algorithms
 
-  template<std::size_t N>
-  std::array<char, N * 2> to_hex(const std::array<std::uint8_t, N>& digest) noexcept
-  {
-    static constexpr const char alphabet[] = u8"0123456789abcdef";
-    static_assert(sizeof(alphabet) == 17, "bad alphabet size");
-
-    // TODO upgrade (improve performance) of to hex in epee string tools
-    std::array<char, N * 2> out{{}};
-    auto current = out.begin();
-    for (const std::uint8_t byte : digest)
-    {
-      *current = alphabet[byte >> 4];
-      ++current;
-      *current = alphabet[byte & 0x0F];
-      ++current;
-    }
-    return out;
-  }
-
   struct md5_
   {
     static constexpr const boost::string_ref name = ceref(u8"MD5");
@@ -156,7 +138,7 @@ namespace
 
       std::array<std::uint8_t, 16> digest{{}};
       md5::MD5Final(digest.data(), std::addressof(ctx));
-      return to_hex(digest);
+      return epee::to_hex::array(digest);
     }
   };
   constexpr const boost::string_ref md5_::name;