Extract util::Xor, Add key_offset option, Add bench

Author: MarcoFalke

src/Makefile.bench.include

@@ -52,7 +52,8 @@ bench_bench_bitcoin_SOURCES = \
   bench/streams_findbyte.cpp \
   bench/strencodings.cpp \
   bench/util_time.cpp \
-  bench/verify_script.cpp
+  bench/verify_script.cpp \
+  bench/xor.cpp
 
 nodist_bench_bench_bitcoin_SOURCES = $(GENERATED_BENCH_FILES)
 

src/bench/bench.h

@@ -14,7 +14,7 @@
 #include <string>
 #include <vector>
 
-#include <bench/nanobench.h>
+#include <bench/nanobench.h> // IWYU pragma: export
 
 /*
  * Usage:

src/bench/xor.cpp

@@ -0,0 +1,24 @@
+// Copyright (c) The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or https://opensource.org/license/mit/.
+
+#include <bench/bench.h>
+
+#include <random.h>
+#include <streams.h>
+
+#include <cstddef>
+#include <vector>
+
+static void Xor(benchmark::Bench& bench)
+{
+    FastRandomContext frc{/*fDeterministic=*/true};
+    auto data{frc.randbytes<std::byte>(1024)};
+    auto key{frc.randbytes<std::byte>(31)};
+
+    bench.batch(data.size()).unit("byte").run([&] {
+        util::Xor(data, key);
+    });
+}
+
+BENCHMARK(Xor, benchmark::PriorityLevel::HIGH);

src/streams.h

@@ -23,6 +23,27 @@
 #include <utility>
 #include <vector>
 
+namespace util {
+inline void Xor(Span<std::byte> write, Span<const std::byte> key, size_t key_offset = 0)
+{
+    if (key.size() == 0) {
+        return;
+    }
+    key_offset %= key.size();
+
+    for (size_t i = 0, j = key_offset; i != write.size(); i++) {
+        write[i] ^= key[j++];
+
+        // This potentially acts on very many bytes of data, so it's
+        // important that we calculate `j`, i.e. the `key` index in this
+        // way instead of doing a %, which would effectively be a division
+        // for each byte Xor'd -- much slower than need be.
+        if (j == key.size())
+            j = 0;
+    }
+}
+} // namespace util
+
 template<typename Stream>
 class OverrideStream
 {
@@ -316,20 +337,7 @@ class DataStream
      */
     void Xor(const std::vector<unsigned char>& key)
     {
-        if (key.size() == 0) {
-            return;
-        }
-
-        for (size_type i = 0, j = 0; i != size(); i++) {
-            vch[i] ^= std::byte{key[j++]};
-
-            // This potentially acts on very many bytes of data, so it's
-            // important that we calculate `j`, i.e. the `key` index in this
-            // way instead of doing a %, which would effectively be a division
-            // for each byte Xor'd -- much slower than need be.
-            if (j == key.size())
-                j = 0;
-        }
+        util::Xor(MakeWritableByteSpan(*this), MakeByteSpan(key));
     }
 };
 
@@ -469,7 +477,6 @@ class BitStreamWriter
     }
 };
 
-
 /** Non-refcounted RAII wrapper for FILE*
  *
  * Will automatically close the file when it goes out of scope if not null.