[NFC] Factor fuzzer randomness into a separate utility (#4304)

In preparation for using it from a separate file specifically for generating
random HeapTypes that has no need to depend on all of fuzzing.h.

1 files changed, 62 insertions, 0 deletions

diff --git a/src/tools/fuzzing/random.h b/src/tools/fuzzing/random.h
new file mode 100644
index 000000000..156f248ca
--- /dev/null
+++ b/src/tools/fuzzing/random.h
@@ -0,0 +1,62 @@
+/*
+ * Copyright 2021 WebAssembly Community Group participants
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef wasm_tools_fuzzing_random_h
+#define wasm_tools_fuzzing_random_h
+
+#include <cstdint>
+#include <vector>
+
+namespace wasm {
+
+class Random {
+  // The input seed bytes.
+  std::vector<char> bytes;
+  // The current position in `bytes`.
+  size_t pos = 0;
+  // Whether we already cycled through all the input (which might mean we should
+  // try to finish things off).
+  bool finishedInput = false;
+  // After we finish the input, we start going through it again, but xoring
+  // so it's not identical.
+  int xorFactor = 0;
+
+public:
+  Random(std::vector<char>&& bytes);
+
+  // Methods for getting random data.
+  int8_t get();
+  int16_t get16();
+  int32_t get32();
+  int64_t get64();
+  float getFloat();
+  double getDouble();
+
+  // Choose an integer value in [0, x). This doesn't use a perfectly uniform
+  // distribution, but it's fast and reasonable.
+  uint32_t upTo(uint32_t x);
+  bool oneIn(uint32_t x) { return upTo(x) == 0; }
+
+  // Apply upTo twice, generating a skewed distribution towards
+  // low values.
+  uint32_t upToSquared(uint32_t x) { return upTo(upTo(x)); }
+
+  bool finished() { return finishedInput; }
+};
+
+} // namespace wasm
+
+#endif // wasm_tools_fuzzing_random_h