Move the flags class into sparta

Summary:
In Mariana Trench, we have a convenient utility class called `Flags` which implements a bitset on top of an enum where values are powers of 2. This makes it very efficient to store a small set of flags.

I would like to re-use this in other tools (namely, Angliru). My two options are:
* Copy/paste the code of that class.
* Move this in a library that "other" tools can/will use. This is what this diff does.

The downside of course is that sparta might become a dumping group for shared code, which is not what we want.
I believe this is small and generic enough that it could fit in sparta.

Reviewed By: anwesht

Differential Revision: D51589404

fbshipit-source-id: 87ad709a8b786126caa48e31fe58f797d2415bca

include/sparta/Flags.h

@@ -0,0 +1,152 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <initializer_list>
+#include <type_traits>
+
+namespace sparta {
+
+/**
+ * A set of flags.
+ *
+ * `Flags<Enum>` can be used to store an OR-combination of enum values, where
+ * `Enum` is an enum class type. `Enum` underlying values must be a power of 2.
+ */
+template <typename Enum>
+class Flags final {
+  static_assert(std::is_enum_v<Enum>, "Enum must be an enumeration type");
+  static_assert(
+      std::is_unsigned_v<std::underlying_type_t<Enum>>,
+      "The underlying type of Enum must be an unsigned arithmetic type");
+
+ public:
+  using EnumType = Enum;
+  using IntT = std::underlying_type_t<Enum>;
+
+ public:
+  Flags() = default;
+
+  /* implicit */ constexpr Flags(Enum flag) : value_(static_cast<IntT>(flag)) {}
+
+  /* implicit */ constexpr Flags(std::initializer_list<Enum> flags)
+      : value_(0) {
+    for (auto flag : flags) {
+      value_ |= static_cast<IntT>(flag);
+    }
+  }
+
+  constexpr Flags& operator&=(Enum flag) {
+    value_ &= static_cast<IntT>(flag);
+    return *this;
+  }
+
+  constexpr Flags& operator&=(Flags flags) {
+    value_ &= flags.value_;
+    return *this;
+  }
+
+  constexpr Flags& operator|=(Enum flag) {
+    value_ |= static_cast<IntT>(flag);
+    return *this;
+  }
+
+  constexpr Flags& operator|=(Flags flags) {
+    value_ |= flags.value_;
+    return *this;
+  }
+
+  constexpr Flags& operator^=(Enum flag) {
+    value_ ^= static_cast<IntT>(flag);
+    return *this;
+  }
+
+  constexpr Flags& operator^=(Flags flags) {
+    value_ ^= flags.value_;
+    return *this;
+  }
+
+  constexpr Flags operator&(Enum flag) const {
+    return Flags(value_ & static_cast<IntT>(flag));
+  }
+
+  constexpr Flags operator&(Flags flags) const {
+    return Flags(value_ & flags.value_);
+  }
+
+  constexpr Flags operator|(Enum flag) const {
+    return Flags(value_ | static_cast<IntT>(flag));
+  }
+
+  constexpr Flags operator|(Flags flags) const {
+    return Flags(value_ | flags.value_);
+  }
+
+  constexpr Flags operator^(Enum flag) const {
+    return Flags(value_ ^ static_cast<IntT>(flag));
+  }
+
+  constexpr Flags operator^(Flags flags) const {
+    return Flags(value_ ^ flags.value_);
+  }
+
+  constexpr Flags operator~() const { return Flags(~value_); }
+
+  explicit constexpr operator bool() const { return value_ != 0; }
+
+  constexpr bool operator!() const { return value_ == 0; }
+
+  constexpr bool operator==(Flags flags) const {
+    return value_ == flags.value_;
+  }
+
+  constexpr bool operator!=(Flags flags) const {
+    return value_ != flags.value_;
+  }
+
+  constexpr bool test(Enum flag) const {
+    if (static_cast<IntT>(flag) == 0) {
+      return value_ == 0;
+    } else {
+      return (value_ & static_cast<IntT>(flag)) == static_cast<IntT>(flag);
+    }
+  }
+
+  constexpr Flags& set(Enum flag, bool on = true) {
+    if (on) {
+      value_ |= static_cast<IntT>(flag);
+    } else {
+      value_ &= ~static_cast<IntT>(flag);
+    }
+    return *this;
+  }
+
+  constexpr bool empty() const { return value_ == 0; }
+
+  constexpr void clear() { value_ = 0; }
+
+  constexpr bool is_subset_of(Flags flags) const {
+    return (value_ | flags.value_) == flags.value_;
+  }
+
+  constexpr bool has_single_bit() const {
+    return (value_ && !(value_ & (value_ - 1)));
+  }
+
+  constexpr IntT encode() const { return value_; }
+
+  static constexpr Flags decode(IntT encoding) { return Flags(encoding); }
+
+ private:
+  explicit constexpr Flags(IntT value) : value_(value) {}
+
+ private:
+  IntT value_ = 0;
+};
+
+} // namespace sparta