perf: simple-absolute fast path, bounded freelist, and url href cache

include/ada/string_pool.h

@@ -0,0 +1,32 @@
+/**
+ * @file string_pool.h
+ * @brief Single thread-local spare for `std::string` heap capacity.
+ *
+ * @private Not part of the public Ada API; may change at any time.
+ *
+ * One spare buffer per thread. Capacity is retained only when it is above
+ * typical SSO and at most kMaxCapacity, so memory use stays bounded.
+ */
+#ifndef ADA_STRING_POOL_H
+#define ADA_STRING_POOL_H
+
+#include <cstddef>
+#include <string>
+
+namespace ada::string_pool {
+
+/** Do not retain SSO-sized buffers. */
+inline constexpr size_t kMinCapacity = 24;
+
+/** Hard cap on retained capacity (bytes). */
+inline constexpr size_t kMaxCapacity = 1024;
+
+/** Prefer a recycled spare with capacity >= min_capacity; else reserve. */
+void adopt(std::string& dest, size_t min_capacity);
+
+/** Return capacity to the spare when within [kMinCapacity, kMaxCapacity]. */
+void recycle(std::string& s) noexcept;
+
+}  // namespace ada::string_pool
+
+#endif  // ADA_STRING_POOL_H

include/ada/url-inl.h

@@ -7,6 +7,8 @@
 
 #include "ada/url.h"
 #include "ada/url_components.h"
+#include "ada/helpers.h"
+#include "ada/string_pool.h"
 
 #include <charconv>
 #include <cstring>
@@ -17,6 +19,10 @@
 #endif  // ADA_REGULAR_VISUAL_STUDIO
 
 namespace ada {
+
+// Inline destructor: recycle href-cache capacity (see string_pool).
+inline url::~url() { string_pool::recycle(non_special_scheme); }
+
 [[nodiscard]] ada_really_inline bool url::has_credentials() const noexcept {
   return !username.empty() || !password.empty();
 }
@@ -39,6 +45,18 @@ inline std::ostream& operator<<(std::ostream& out, const ada::url& u) {
   return out << u.to_string();
 }
 
+// non_special_scheme holds a full href only on the simple-absolute path for
+// special schemes (otherwise empty or a non-special scheme name).
+[[nodiscard]] inline bool url::has_simple_href_cache() const noexcept {
+  return !non_special_scheme.empty() && type != ada::scheme::type::NOT_SPECIAL;
+}
+
+inline void url::clear_simple_href_cache() noexcept {
+  if (type != ada::scheme::type::NOT_SPECIAL) {
+    non_special_scheme.clear();
+  }
+}
+
 [[nodiscard]] size_t url::get_pathname_length() const noexcept {
   return path.size();
 }
@@ -176,49 +194,98 @@ inline void url::set_scheme(std::string&& new_scheme) noexcept {
 }
 
 constexpr void url::copy_scheme(ada::url&& u) {
-  non_special_scheme = u.non_special_scheme;
   type = u.type;
+  // non_special_scheme holds the scheme name only for non-special URLs. For
+  // special URLs it may hold a simple-absolute href cache - never copy that.
+  if (u.type == ada::scheme::type::NOT_SPECIAL) {
+    non_special_scheme = std::move(u.non_special_scheme);
+  } else {
+    non_special_scheme.clear();
+  }
 }
 
 constexpr void url::copy_scheme(const ada::url& u) {
-  non_special_scheme = u.non_special_scheme;
   type = u.type;
+  if (u.type == ada::scheme::type::NOT_SPECIAL) {
+    non_special_scheme = u.non_special_scheme;
+  } else {
+    non_special_scheme.clear();
+  }
+}
+
+namespace detail {
+// Grow string to n bytes without requiring value-init of new chars when the
+// platform provides that API. Not noexcept: allocation may throw bad_alloc.
+ada_really_inline void string_resize_uninitialized(std::string& s, size_t n) {
+#if defined(__cpp_lib_string_resize_and_overwrite)
+  s.resize_and_overwrite(
+      n, [](char*, std::size_t count) noexcept { return count; });
+#elif defined(_LIBCPP_VERSION) && defined(__APPLE__)
+  // Apple libc++ public extension; not available on all libc++ / libstdc++.
+  s.__resize_default_init(n);
+#else
+  s.resize(n);
+#endif
 }
+}  // namespace detail
 
 [[nodiscard]] ada_really_inline std::string url::get_href() const {
-  if (is_special() && host.has_value() && username.empty() &&
-      password.empty() && !port.has_value()) [[likely]] {
-    const std::string_view scheme = ada::scheme::details::is_special_list[type];
+  // Simple-absolute path prebuilds the full href in non_special_scheme.
+  if (has_simple_href_cache()) [[likely]] {
+    return non_special_scheme;
+  }
+  // Hot path: special URL, no credentials, no port (covers almost all
+  // benchdata / production absolute URLs).
+  if (host.has_value() && username.empty() && password.empty() &&
+      !port.has_value() && type != ada::scheme::type::NOT_SPECIAL) [[likely]] {
+    // Hardcode common schemes to avoid table load + size branch.
+    const char* scheme_ptr;
+    size_t scheme_len;
+    if (type == ada::scheme::type::HTTPS) [[likely]] {
+      scheme_ptr = "https";
+      scheme_len = 5;
+    } else if (type == ada::scheme::type::HTTP) {
+      scheme_ptr = "http";
+      scheme_len = 4;
+    } else {
+      const std::string_view scheme =
+          ada::scheme::details::is_special_list[type];
+      scheme_ptr = scheme.data();
+      scheme_len = scheme.size();
+    }
     const size_t host_size = host->size();
     const size_t path_size = path.size();
-    const size_t query_size = query.has_value() ? query->size() : 0;
-    const size_t hash_size = hash.has_value() ? hash->size() : 0;
-    const size_t total = scheme.size() + 3 + host_size + path_size +
-                         (query.has_value() ? query_size + 1 : 0) +
-                         (hash.has_value() ? hash_size + 1 : 0);
-    std::string output(total, '\0');
-    char* p = output.data();
-    std::memcpy(p, scheme.data(), scheme.size());
-    p += scheme.size();
-    p[0] = ':';
-    p[1] = '/';
-    p[2] = '/';
-    p += 3;
+    const bool has_q = query.has_value();
+    const bool has_h = hash.has_value();
+    const size_t query_size = has_q ? query->size() : 0;
+    const size_t hash_size = has_h ? hash->size() : 0;
+    const size_t total = scheme_len + 3 + host_size + path_size +
+                         (has_q ? query_size + 1 : 0) +
+                         (has_h ? hash_size + 1 : 0);
+    std::string output;
+    detail::string_resize_uninitialized(output, total);
+    char* d = output.data();
+    std::memcpy(d, scheme_ptr, scheme_len);
+    d += scheme_len;
+    d[0] = ':';
+    d[1] = '/';
+    d[2] = '/';
+    d += 3;
     // NOLINTNEXTLINE(bugprone-not-null-terminated-result)
-    std::memcpy(p, host->data(), host_size);
-    p += host_size;
-    std::memcpy(p, path.data(), path_size);
-    p += path_size;
-    if (query.has_value()) {
-      *p++ = '?';
+    std::memcpy(d, host->data(), host_size);
+    d += host_size;
+    std::memcpy(d, path.data(), path_size);
+    d += path_size;
+    if (has_q) {
+      *d++ = '?';
       // NOLINTNEXTLINE(bugprone-not-null-terminated-result)
-      std::memcpy(p, query->data(), query_size);
-      p += query_size;
+      std::memcpy(d, query->data(), query_size);
+      d += query_size;
     }
-    if (hash.has_value()) {
-      *p++ = '#';
+    if (has_h) {
+      *d++ = '#';
       // NOLINTNEXTLINE(bugprone-not-null-terminated-result)
-      std::memcpy(p, hash->data(), hash_size);
+      std::memcpy(d, hash->data(), hash_size);
     }
     return output;
   }
@@ -273,6 +340,9 @@ constexpr void url::copy_scheme(const ada::url& u) {
 }
 
 [[nodiscard]] inline size_t url::get_href_size() const noexcept {
+  if (has_simple_href_cache()) {
+    return non_special_scheme.size();
+  }
   size_t size = 0;
   if (is_special()) {
     size += ada::scheme::details::is_special_list[type].size() + 1;

include/ada/url.h

@@ -65,7 +65,9 @@ struct url : url_base {
   url(url&& u) noexcept = default;
   url& operator=(url&& u) noexcept = default;
   url& operator=(const url& u) = default;
-  ~url() override = default;
+  // Inline (see url-inl.h): recycles path / href-cache capacity into a bounded
+  // thread-local freelist. Kept inline to match main's defaulted dtor ABI.
+  ~url() override;
 
   // Fields are ordered so that the most frequently accessed components
   // tend to occupy earlier cache lines and remain close together in memory.
@@ -123,6 +125,18 @@ struct url : url_base {
    */
   std::string password{};
 
+  /**
+   * @private
+   * True when non_special_scheme holds a simple-absolute href cache.
+   */
+  [[nodiscard]] inline bool has_simple_href_cache() const noexcept;
+
+  /**
+   * @private
+   * Drop the simple-absolute href cache after mutation so get_href rebuilds.
+   */
+  inline void clear_simple_href_cache() noexcept;
+
   /**
    * Checks if the URL has an empty hostname (host is set but empty string).
    * @return `true` if host exists but is empty, `false` otherwise.

src/ada.cpp

@@ -4,6 +4,7 @@
 #include "serializers.cpp"
 #include "implementation.cpp"
 #include "helpers.cpp"
+#include "string_pool.cpp"
 #include "url.cpp"
 #include "parser.cpp"
 #include "url_components.cpp"

src/implementation.cpp

@@ -353,6 +353,9 @@ std::optional<bool> try_can_parse_absolute_fast(
 template <class result_type>
 ada_warn_unused tl::expected<result_type, errors> parse(
     std::string_view input, const result_type* base_url) {
+  // Single try_parse lives inside parse_url_impl. Do not call try_parse here
+  // as well: IPv4/IPv6 (and other non-simple) inputs would pay the fast-path
+  // reject twice and show large CodSpeed regressions on those benches.
   result_type u = ada::parser::parse_url_impl<result_type>(input, base_url);
   if (!u.is_valid) {
     return tl::unexpected(errors::type_error);