From 06ca1979b0353ad61a98cb46b1674865bcaa4151 Mon Sep 17 00:00:00 2001
From: Niels Lohmann <mail@nlohmann.me>
Date: Mon, 3 Jan 2022 20:45:26 +0100
Subject: [PATCH] :recycle: remove stringstream

---
 include/nlohmann/detail/output/serializer.hpp | 23 +++++++++++++------
 single_include/nlohmann/json.hpp              | 23 +++++++++++++------
 2 files changed, 32 insertions(+), 14 deletions(-)

diff --git a/include/nlohmann/detail/output/serializer.hpp b/include/nlohmann/detail/output/serializer.hpp
index 7cce7d5c7c..cc63ab1d3d 100644
--- a/include/nlohmann/detail/output/serializer.hpp
+++ b/include/nlohmann/detail/output/serializer.hpp
@@ -10,7 +10,6 @@
 #include <limits> // numeric_limits
 #include <string> // string, char_traits
 #include <iomanip> // setfill, setw
-#include <sstream> // stringstream
 #include <type_traits> // is_same
 #include <utility> // move
 
@@ -501,9 +500,7 @@ class serializer
                     {
                         case error_handler_t::strict:
                         {
-                            std::stringstream ss;
-                            ss << std::uppercase << std::setfill('0') << std::setw(2) << std::hex << (byte | 0);
-                            JSON_THROW(type_error::create(316, "invalid UTF-8 byte at index " + std::to_string(i) + ": 0x" + ss.str(), BasicJsonType()));
+                            JSON_THROW(type_error::create(316, "invalid UTF-8 byte at index " + std::to_string(i) + ": 0x" + hex_bytes(byte | 0), BasicJsonType()));
                         }
 
                         case error_handler_t::ignore:
@@ -595,9 +592,7 @@ class serializer
             {
                 case error_handler_t::strict:
                 {
-                    std::stringstream ss;
-                    ss << std::uppercase << std::setfill('0') << std::setw(2) << std::hex << (static_cast<std::uint8_t>(s.back()) | 0);
-                    JSON_THROW(type_error::create(316, "incomplete UTF-8 string; last byte: 0x" + ss.str(), BasicJsonType()));
+                    JSON_THROW(type_error::create(316, "incomplete UTF-8 string; last byte: 0x" + hex_bytes(static_cast<std::uint8_t>(s.back() | 0)), BasicJsonType()));
                 }
 
                 case error_handler_t::ignore:
@@ -664,6 +659,20 @@ class serializer
         }
     }
 
+    /*!
+     * @brief convert a byte to a uppercase hex representation
+     * @param[in] byte byte to represent
+     * @return representation ("00".."FF")
+     */
+    inline std::string hex_bytes(std::uint8_t byte)
+    {
+        std::string result = "FF";
+        constexpr const char* nibble_to_hex = "0123456789ABCDEF";
+        result[0] = nibble_to_hex[byte / 16];
+        result[1] = nibble_to_hex[byte % 16];
+        return result;
+    }
+
     // templates to avoid warnings about useless casts
     template <typename NumberType, enable_if_t<std::is_signed<NumberType>::value, int> = 0>
     bool is_negative_number(NumberType x)
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index bfa5c823c4..e573adc135 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -14935,7 +14935,6 @@ class binary_writer
 #include <limits> // numeric_limits
 #include <string> // string, char_traits
 #include <iomanip> // setfill, setw
-#include <sstream> // stringstream
 #include <type_traits> // is_same
 #include <utility> // move
 
@@ -16544,9 +16543,7 @@ class serializer
                     {
                         case error_handler_t::strict:
                         {
-                            std::stringstream ss;
-                            ss << std::uppercase << std::setfill('0') << std::setw(2) << std::hex << (byte | 0);
-                            JSON_THROW(type_error::create(316, "invalid UTF-8 byte at index " + std::to_string(i) + ": 0x" + ss.str(), BasicJsonType()));
+                            JSON_THROW(type_error::create(316, "invalid UTF-8 byte at index " + std::to_string(i) + ": 0x" + hex_bytes(byte | 0), BasicJsonType()));
                         }
 
                         case error_handler_t::ignore:
@@ -16638,9 +16635,7 @@ class serializer
             {
                 case error_handler_t::strict:
                 {
-                    std::stringstream ss;
-                    ss << std::uppercase << std::setfill('0') << std::setw(2) << std::hex << (static_cast<std::uint8_t>(s.back()) | 0);
-                    JSON_THROW(type_error::create(316, "incomplete UTF-8 string; last byte: 0x" + ss.str(), BasicJsonType()));
+                    JSON_THROW(type_error::create(316, "incomplete UTF-8 string; last byte: 0x" + hex_bytes(static_cast<std::uint8_t>(s.back() | 0)), BasicJsonType()));
                 }
 
                 case error_handler_t::ignore:
@@ -16707,6 +16702,20 @@ class serializer
         }
     }
 
+    /*!
+     * @brief convert a byte to a uppercase hex representation
+     * @param[in] byte byte to represent
+     * @return representation ("00".."FF")
+     */
+    inline std::string hex_bytes(std::uint8_t byte)
+    {
+        std::string result = "FF";
+        constexpr const char* nibble_to_hex = "0123456789ABCDEF";
+        result[0] = nibble_to_hex[byte / 16];
+        result[1] = nibble_to_hex[byte % 16];
+        return result;
+    }
+
     // templates to avoid warnings about useless casts
     template <typename NumberType, enable_if_t<std::is_signed<NumberType>::value, int> = 0>
     bool is_negative_number(NumberType x)