add fnv1a hasher

fnv1a_hasher.hh

@@ -0,0 +1,54 @@
+/*
+ * Copyright (C) 2017 ScyllaDB
+ */
+
+/*
+ * This file is part of Scylla.
+ *
+ * Scylla is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Scylla is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Scylla.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+// FIXME: FNV-1a is quite slow, consider something faster, CityHash seems to be
+// a good choice.
+
+template<unsigned>
+struct fnv1a_constants { };
+
+template<>
+struct fnv1a_constants<8> {
+    enum : uint64_t {
+        offset = 0xcbf29ce484222325ull,
+        prime = 0x100000001b3ull,
+    };
+};
+
+class fnv1a_hasher {
+    using constants = fnv1a_constants<sizeof(size_t)>;
+    size_t _hash = constants::offset;
+public:
+    void update(const char* ptr, size_t length) {
+        auto end = ptr + length;
+        while (ptr != end) {
+            _hash ^= *ptr;
+            _hash *= constants::prime;
+            ++ptr;
+        }
+    }
+
+    size_t finalize() const {
+        return _hash;
+    }
+};