Copy the tokens of a pointer without escapes straight into to_string()

to_string() appended a slash and every escaped token with library calls.
Tokens that contain neither '~' nor '/' (nearly all) are copied into a
string of the final size instead; the others are escaped as before.

json_pointer::to_string() over the keys of citm_catalog (instructions on macOS,
Linux Ir GCC 13 / Clang 18): 47.5M -> 30.6M, 33.6M -> 25.8M / 35.9M -> 22.8M;
twitter 18.8M -> 12.9M, 13.7M -> 11.2M / 14.5M -> 9.9M.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/json_pointer.hpp

@@ -82,12 +82,35 @@ class json_pointer
     {
         // one buffer: every token adds its escaped form (at least its length) and a slash
         std::size_t length = 0;
+        bool plain = true;
         for (const auto& reference_token : reference_tokens)
         {
             length += reference_token.size() + 1;
+            const char* const data = reference_token.data();
+            for (std::size_t i = 0, size = reference_token.size(); plain && i < size; ++i)
+            {
+                plain = data[i] != '~' && data[i] != '/';
+            }
         }
 
         string_t result;
+        if (JSON_HEDLEY_LIKELY(plain))
+        {
+            // nothing to escape: the tokens are copied into the string
+            result.resize(length);
+            if (length != 0)
+            {
+                char* out = &result[0];
+                for (const auto& reference_token : reference_tokens)
+                {
+                    *out++ = '/';
+                    std::memcpy(out, reference_token.data(), reference_token.size());
+                    out += reference_token.size();
+                }
+            }
+            return result;
+        }
+
         result.reserve(length);
         for (const auto& reference_token : reference_tokens)
         {

single_include/nlohmann/json.hpp

@@ -22432,12 +22432,35 @@ class json_pointer
     {
         // one buffer: every token adds its escaped form (at least its length) and a slash
         std::size_t length = 0;
+        bool plain = true;
         for (const auto& reference_token : reference_tokens)
         {
             length += reference_token.size() + 1;
+            const char* const data = reference_token.data();
+            for (std::size_t i = 0, size = reference_token.size(); plain && i < size; ++i)
+            {
+                plain = data[i] != '~' && data[i] != '/';
+            }
         }
 
         string_t result;
+        if (JSON_HEDLEY_LIKELY(plain))
+        {
+            // nothing to escape: the tokens are copied into the string
+            result.resize(length);
+            if (length != 0)
+            {
+                char* out = &result[0];
+                for (const auto& reference_token : reference_tokens)
+                {
+                    *out++ = '/';
+                    std::memcpy(out, reference_token.data(), reference_token.size());
+                    out += reference_token.size();
+                }
+            }
+            return result;
+        }
+
         result.reserve(length);
         for (const auto& reference_token : reference_tokens)
         {