Introduce base64 encode & decode in xeus

CMakeLists.txt

@@ -91,6 +91,7 @@ endif ()
 # ============
 
 set(XEUS_HEADERS
+    ${XEUS_INCLUDE_DIR}/xeus/xbase64.hpp
     ${XEUS_INCLUDE_DIR}/xeus/xbasic_fixed_string.hpp
     ${XEUS_INCLUDE_DIR}/xeus/xcomm.hpp
     ${XEUS_INCLUDE_DIR}/xeus/xcontrol_messenger.hpp

include/xeus/xbase64.hpp

@@ -0,0 +1,75 @@
+/***************************************************************************
+* Copyright (c) Sylvain Corlay and Johan Mabille and Wolf Vollprecht       *
+* Copyright (c) QuantStack                                                 *
+*                                                                          *
+* Distributed under the terms of the BSD 3-Clause License.                 *
+*                                                                          *
+* The full license is in the file LICENSE, distributed with this software. *
+****************************************************************************/
+
+#ifndef XEUS_BASE64_HPP
+#define XEUS_BASE64_HPP
+
+#include <array>
+#include <cstddef>
+#include <string>
+
+namespace xeus
+{
+    inline std::string base64decode(const std::string& input)
+    {
+        std::array<int, 256> T;
+        T.fill(-1);
+        for (std::size_t i = 0; i < 64; ++i)
+        {
+            T[std::size_t("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"[i])] = int(i);
+        }
+
+        std::string output;
+        int val = 0;
+        int valb = -8;
+        for (char c : input)
+        {
+            if (T[std::size_t(c)] == -1)
+            {
+                break;
+            }
+            val = (val << 6) + T[std::size_t(c)];
+            valb += 6;
+            if (valb >= 0)
+            {
+                output.push_back(char((val >> valb) & 0xFF));
+                valb -= 8;
+            }
+        }
+        return output;
+    }
+
+    inline std::string base64encode(const std::string& input)
+    {
+        std::string output;
+        int val = 0;
+        int valb = -6;
+        for (char sc : input)
+        {
+            unsigned char c = static_cast<unsigned char>(sc);
+            val = (val << 8) + c;
+            valb += 8;
+            while (valb >= 0)
+            {
+                output.push_back("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"[(val >> valb) & 0x3F]);
+                valb -= 6;
+            }
+        }
+        if (valb > -6)
+        {
+            output.push_back("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"[((val << 8) >> (valb + 8)) & 0x3F]);
+        }
+        while (output.size() % 4)
+        {
+            output.push_back('=');
+        }
+        return output;
+    }
+}
+#endif