bytes: implement std::hash using appending_hash

This is a preparation for the upcoming introduction of managed_bytes_view,
intended as a fragmented replacement for bytes_view.
To ease the transition, we want both types to give equal hashes for equal
contents.

bytes.hh

@@ -28,6 +28,7 @@
 #include <iosfwd>
 #include <functional>
 #include "utils/mutable_view.hh"
+#include <xxhash.h>
 
 using bytes = basic_sstring<int8_t, uint32_t, 31, false>;
 using bytes_view = std::basic_string_view<int8_t>;
@@ -47,17 +48,6 @@ inline bytes_view to_bytes_view(sstring_view view) {
     return {reinterpret_cast<const int8_t*>(view.data()), view.size()};
 }
 
-namespace std {
-
-template <>
-struct hash<bytes_view> {
-    size_t operator()(bytes_view v) const {
-        return hash<sstring_view>()({reinterpret_cast<const char*>(v.begin()), v.size()});
-    }
-};
-
-}
-
 struct fmt_hex {
     bytes_view& v;
     fmt_hex(bytes_view& v) noexcept : v(v) {}
@@ -98,6 +88,30 @@ struct appending_hash<bytes_view> {
     }
 };
 
+struct bytes_view_hasher : public hasher {
+    XXH64_state_t _state;
+    bytes_view_hasher(uint64_t seed = 0) noexcept {
+        XXH64_reset(&_state, seed);
+    }
+    void update(const char* ptr, size_t length) noexcept {
+        XXH64_update(&_state, ptr, length);
+    }
+    size_t finalize() {
+        return static_cast<size_t>(XXH64_digest(&_state));
+    }
+};
+
+namespace std {
+template <>
+struct hash<bytes_view> {
+    size_t operator()(bytes_view v) const {
+        bytes_view_hasher h;
+        appending_hash<bytes_view>{}(h, v);
+        return h.finalize();
+    }
+};
+} // namespace std
+
 inline int32_t compare_unsigned(bytes_view v1, bytes_view v2) {
   auto size = std::min(v1.size(), v2.size());
   if (size) {

test/cql/lwt_test.result

@@ -3978,7 +3978,7 @@ drop table lwt;
 -- non-frozen nested sets are not supported
 create table lwt (a int, b set<map<int,int>>, c list<set<int>>, primary key (a));
 {
-	"message" : "exceptions::invalid_request_exception (Non-frozen user types or collections are not allowed inside collections: list<set<int>>)",
+	"message" : "exceptions::invalid_request_exception (Non-frozen user types or collections are not allowed inside collections: set<map<int, int>>)",
 	"status" : "error"
 }
 -- frozen collection elements are however ok