zircon/system/ulib/affine/include/lib/affine/ratio.h

// Copyright 2019 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef LIB_AFFINE_RATIO_H_
#define LIB_AFFINE_RATIO_H_

#include <lib/affine/assert.h>
#include <stdint.h>
#include <zircon/compiler.h>

#include <limits>
#include <type_traits>

namespace affine {

class Ratio {
 public:
  enum class Exact { No, Yes };

  // Used to indicate overflow/underflow of scaling operations.
  static constexpr int64_t kOverflow = std::numeric_limits<int64_t>::max();
  static constexpr int64_t kUnderflow = std::numeric_limits<int64_t>::min();

  // Reduces the ratio of N/D
  //
  // Defined only for uint32_t and uint64_t
  template <typename T>
  static void Reduce(T* numerator, T* denominator);

  // Reduce the ratio instance, in-place.
  void Reduce() { Reduce(&numerator_, &denominator_); }

  // Produces the product two 32 bit ratios. If exact is true, ASSERTs on loss
  // of precision.
  static void Product(uint32_t a_numerator, uint32_t a_denominator, uint32_t b_numerator,
                      uint32_t b_denominator, uint32_t* product_numerator,
                      uint32_t* product_denominator, Exact exact = Exact::Yes);

  // Produces the product of a 32 bit ratio and the int64_t as an int64_t. Returns
  // a saturated value (either kOverflow or kUnderflow) on overflow/underflow.
  static int64_t Scale(int64_t value, uint32_t numerator, uint32_t denominator);

  // Returns the product of the ratios. If exact is true, ASSERTs on loss of
  // precision.
  static Ratio Product(Ratio a, Ratio b, Exact exact = Exact::Yes) {
    uint32_t result_numerator;
    uint32_t result_denominator;
    Product(a.numerator(), a.denominator(), b.numerator(), b.denominator(), &result_numerator,
            &result_denominator, exact);
    return Ratio(result_numerator, result_denominator);
  }

  Ratio() = default;

  // TODO(https://fxbug.dev/42111693) : Remove these __LOCAL annotations
  //
  // So, there is something wrong with GCC when building this library with -O0
  // (see the referenced bug).  It does not seem to be respecting the
  // -fvisibility-hidden or -fvisibility-inlines-hidden flags which are being
  // passed to it.
  //
  // As a result, when this library is used by a DSO, the symbols become
  // exported by the DSO and require some runtime fixup.  This generated a GOT
  // which is (by definition) a RW segment.  This is a problem when we attempt
  // to use this library in the VDSO image (libzircon.so) since the VDSO
  // _demands_ that there be no read write segments.
  //
  // The workaround here is to put the __LOCAL annotation on the two methods
  // that the VDSO images uses directly (the non-default constructor, and the
  // Scale method).  Under the hood, __LOCAL is attribute("hidden"), which the
  // compiler does seem to respect.  Once the symbols are no longer exported
  // from the VDSO image, the GOT goes away and the system is happy.
  //
  // Eventually, if the issue with GCC gets resolved, we can come back and
  // remove these explicit annotations.
  //
  Ratio(uint32_t numerator, uint32_t denominator) __LOCAL : numerator_(numerator),
                                                            denominator_(denominator) {
    internal::DebugAssert(denominator_ != 0);
  }

  uint32_t numerator() const { return numerator_; }
  uint32_t denominator() const { return denominator_; }
  bool invertible() const { return numerator_ != 0; }

  Ratio Inverse() const {
    internal::DebugAssert(invertible());
    return Ratio{denominator_, numerator_};
  }

  int64_t Scale(int64_t value) const __LOCAL { return Scale(value, numerator_, denominator_); }

 private:
  uint32_t numerator_ = 1;
  uint32_t denominator_ = 1;
};

// Returns the ratio of the two ratios.
inline Ratio operator/(Ratio a, Ratio b) { return Ratio::Product(a, b.Inverse()); }

// Returns the product of the two ratios.
inline Ratio operator*(Ratio a, Ratio b) { return Ratio::Product(a, b); }

// Returns the product of the rate and the int64_t.
inline int64_t operator*(Ratio a, int64_t b) { return a.Scale(b); }

// Returns the product of the rate and the int64_t.
inline int64_t operator*(int64_t a, Ratio b) { return b.Scale(a); }

// Returns the the int64_t divided by the rate.
inline int64_t operator/(int64_t a, Ratio b) { return b.Inverse().Scale(a); }

}  // namespace affine

#endif  // LIB_AFFINE_RATIO_H_