perf: SIMD percent-decode and form-urlencoded encode/decode

include/ada/unicode.h

@@ -198,6 +198,7 @@ ada_really_inline unsigned constexpr convert_hex_to_binary(char c) noexcept;
 std::string percent_decode(std::string_view input, size_t first_percent);
 
 /**
+ * @private
  * Decode an application/x-www-form-urlencoded component: map '+' to space,
  * then percent-decode. Single allocation; no intermediate string.
  *
@@ -207,6 +208,20 @@ std::string percent_decode(std::string_view input, size_t first_percent);
  */
 std::string form_urlencoded_decode(std::string_view input);
 
+/**
+ * @private
+ * application/x-www-form-urlencoded byte serializer: percent-encode with the
+ * form set, mapping 0x20 SPACE to '+' instead of '%20'.
+ * @see https://url.spec.whatwg.org/#concept-urlencoded-byte-serializer
+ */
+std::string form_urlencoded_encode(std::string_view input);
+
+/**
+ * @private
+ * Append a form-urlencoded component to `out` (no intermediate string).
+ */
+void form_urlencoded_encode_append(std::string_view input, std::string& out);
+
 /**
  * @private
  * Returns a percent-encoding string whether percent encoding was needed or not.

include/ada/url_search_params-inl.h

@@ -5,7 +5,6 @@
 #ifndef ADA_URL_SEARCH_PARAMS_INL_H
 #define ADA_URL_SEARCH_PARAMS_INL_H
 
-#include "ada/character_sets-inl.h"
 #include "ada/unicode.h"
 #include "ada/url_search_params.h"
 
@@ -120,22 +119,14 @@ inline bool url_search_params::has(std::string_view key,
 }
 
 inline std::string url_search_params::to_string() const {
-  auto character_set = ada::character_sets::WWW_FORM_URLENCODED_PERCENT_ENCODE;
   std::string out{};
   for (size_t i = 0; i < params.size(); i++) {
-    auto key = ada::unicode::percent_encode(params[i].first, character_set);
-    auto value = ada::unicode::percent_encode(params[i].second, character_set);
-
-    // Performance optimization: Move this inside percent_encode.
-    std::ranges::replace(key, ' ', '+');
-    std::ranges::replace(value, ' ', '+');
-
     if (i != 0) {
-      out += "&";
+      out += '&';
     }
-    out.append(key);
-    out += "=";
-    out.append(value);
+    unicode::form_urlencoded_encode_append(params[i].first, out);
+    out += '=';
+    unicode::form_urlencoded_encode_append(params[i].second, out);
   }
   return out;
 }