gecko-dev/dom/bindings/PrimitiveConversions.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this file,
 * You can obtain one at http://mozilla.org/MPL/2.0/. */

/**
 * Conversions from jsval to primitive values
 */

#ifndef mozilla_dom_PrimitiveConversions_h
#define mozilla_dom_PrimitiveConversions_h

#include <limits>
#include <math.h>
#include <stdint.h>

#include "js/Conversions.h"
#include "js/RootingAPI.h"
#include "mozilla/Assertions.h"
#include "mozilla/FloatingPoint.h"
#include "mozilla/dom/BindingCallContext.h"

namespace mozilla::dom {

template <typename T>
struct TypeName {};

template <>
struct TypeName<int8_t> {
  static const char* value() { return "byte"; }
};
template <>
struct TypeName<uint8_t> {
  static const char* value() { return "octet"; }
};
template <>
struct TypeName<int16_t> {
  static const char* value() { return "short"; }
};
template <>
struct TypeName<uint16_t> {
  static const char* value() { return "unsigned short"; }
};
template <>
struct TypeName<int32_t> {
  static const char* value() { return "long"; }
};
template <>
struct TypeName<uint32_t> {
  static const char* value() { return "unsigned long"; }
};
template <>
struct TypeName<int64_t> {
  static const char* value() { return "long long"; }
};
template <>
struct TypeName<uint64_t> {
  static const char* value() { return "unsigned long long"; }
};

enum ConversionBehavior { eDefault, eEnforceRange, eClamp };

template <typename T, ConversionBehavior B>
struct PrimitiveConversionTraits {};

template <typename T>
struct DisallowedConversion {
  typedef int jstype;
  typedef int intermediateType;

 private:
  static inline bool converter(JSContext* cx, JS::Handle<JS::Value> v,
                               const char* sourceDescription, jstype* retval) {
    MOZ_CRASH("This should never be instantiated!");
  }
};

struct PrimitiveConversionTraits_smallInt {
  // The output of JS::ToInt32 is determined as follows:
  //   1) The value is converted to a double
  //   2) Anything that's not a finite double returns 0
  //   3) The double is rounded towards zero to the nearest integer
  //   4) The resulting integer is reduced mod 2^32.  The output of this
  //      operation is an integer in the range [0, 2^32).
  //   5) If the resulting number is >= 2^31, 2^32 is subtracted from it.
  //
  // The result of all this is a number in the range [-2^31, 2^31)
  //
  // WebIDL conversions for the 8-bit, 16-bit, and 32-bit integer types
  // are defined in the same way, except that step 4 uses reduction mod
  // 2^8 and 2^16 for the 8-bit and 16-bit types respectively, and step 5
  // is only done for the signed types.
  //
  // C/C++ define integer conversion semantics to unsigned types as taking
  // your input integer mod (1 + largest value representable in the
  // unsigned type).  Since 2^32 is zero mod 2^8, 2^16, and 2^32,
  // converting to the unsigned int of the relevant width will correctly
  // perform step 4; in particular, the 2^32 possibly subtracted in step 5
  // will become 0.
  //
  // Once we have step 4 done, we're just going to assume 2s-complement
  // representation and cast directly to the type we really want.
  //
  // So we can cast directly for all unsigned types and for int32_t; for
  // the smaller-width signed types we need to cast through the
  // corresponding unsigned type.
  typedef int32_t jstype;
  typedef int32_t intermediateType;
  static inline bool converter(JSContext* cx, JS::Handle<JS::Value> v,
                               const char* sourceDescription, jstype* retval) {
    return JS::ToInt32(cx, v, retval);
  }
};
template <>
struct PrimitiveConversionTraits<int8_t, eDefault>
    : PrimitiveConversionTraits_smallInt {
  typedef uint8_t intermediateType;
};
template <>
struct PrimitiveConversionTraits<uint8_t, eDefault>
    : PrimitiveConversionTraits_smallInt {};
template <>
struct PrimitiveConversionTraits<int16_t, eDefault>
    : PrimitiveConversionTraits_smallInt {
  typedef uint16_t intermediateType;
};
template <>
struct PrimitiveConversionTraits<uint16_t, eDefault>
    : PrimitiveConversionTraits_smallInt {};
template <>
struct PrimitiveConversionTraits<int32_t, eDefault>
    : PrimitiveConversionTraits_smallInt {};
template <>
struct PrimitiveConversionTraits<uint32_t, eDefault>
    : PrimitiveConversionTraits_smallInt {};

template <>
struct PrimitiveConversionTraits<int64_t, eDefault> {
  typedef int64_t jstype;
  typedef int64_t intermediateType;
  static inline bool converter(JSContext* cx, JS::Handle<JS::Value> v,
                               const char* sourceDescription, jstype* retval) {
    return JS::ToInt64(cx, v, retval);
  }
};

template <>
struct PrimitiveConversionTraits<uint64_t, eDefault> {
  typedef uint64_t jstype;
  typedef uint64_t intermediateType;
  static inline bool converter(JSContext* cx, JS::Handle<JS::Value> v,
                               const char* sourceDescription, jstype* retval) {
    return JS::ToUint64(cx, v, retval);
  }
};

template <typename T>
struct PrimitiveConversionTraits_Limits {
  static inline T min() { return std::numeric_limits<T>::min(); }
  static inline T max() { return std::numeric_limits<T>::max(); }
};

template <>
struct PrimitiveConversionTraits_Limits<int64_t> {
  static inline int64_t min() { return -(1LL << 53) + 1; }
  static inline int64_t max() { return (1LL << 53) - 1; }
};

template <>
struct PrimitiveConversionTraits_Limits<uint64_t> {
  static inline uint64_t min() { return 0; }
  static inline uint64_t max() { return (1LL << 53) - 1; }
};

template <typename T, typename U,
          bool (*Enforce)(U cx, const char* sourceDescription, const double& d,
                          T* retval)>
struct PrimitiveConversionTraits_ToCheckedIntHelper {
  typedef T jstype;
  typedef T intermediateType;

  static inline bool converter(U cx, JS::Handle<JS::Value> v,
                               const char* sourceDescription, jstype* retval) {
    double intermediate;
    if (!JS::ToNumber(cx, v, &intermediate)) {
      return false;
    }

    return Enforce(cx, sourceDescription, intermediate, retval);
  }
};

template <typename T>
inline bool PrimitiveConversionTraits_EnforceRange(
    BindingCallContext& cx, const char* sourceDescription, const double& d,
    T* retval) {
  static_assert(std::numeric_limits<T>::is_integer,
                "This can only be applied to integers!");

  if (!std::isfinite(d)) {
    return cx.ThrowErrorMessage<MSG_ENFORCE_RANGE_NON_FINITE>(
        sourceDescription, TypeName<T>::value());
  }

  bool neg = (d < 0);
  double rounded = floor(neg ? -d : d);
  rounded = neg ? -rounded : rounded;
  if (rounded < PrimitiveConversionTraits_Limits<T>::min() ||
      rounded > PrimitiveConversionTraits_Limits<T>::max()) {
    return cx.ThrowErrorMessage<MSG_ENFORCE_RANGE_OUT_OF_RANGE>(
        sourceDescription, TypeName<T>::value());
  }

  *retval = static_cast<T>(rounded);
  return true;
}

template <typename T>
struct PrimitiveConversionTraits<T, eEnforceRange>
    : public PrimitiveConversionTraits_ToCheckedIntHelper<
          T, BindingCallContext&, PrimitiveConversionTraits_EnforceRange<T> > {
};

template <typename T>
inline bool PrimitiveConversionTraits_Clamp(JSContext* cx,
                                            const char* sourceDescription,
                                            const double& d, T* retval) {
  static_assert(std::numeric_limits<T>::is_integer,
                "This can only be applied to integers!");

  if (std::isnan(d)) {
    *retval = 0;
    return true;
  }
  if (d >= PrimitiveConversionTraits_Limits<T>::max()) {
    *retval = PrimitiveConversionTraits_Limits<T>::max();
    return true;
  }
  if (d <= PrimitiveConversionTraits_Limits<T>::min()) {
    *retval = PrimitiveConversionTraits_Limits<T>::min();
    return true;
  }

  MOZ_ASSERT(std::isfinite(d));

  // Banker's rounding (round ties towards even).
  // We move away from 0 by 0.5f and then truncate.  That gets us the right
  // answer for any starting value except plus or minus N.5.  With a starting
  // value of that form, we now have plus or minus N+1.  If N is odd, this is
  // the correct result.  If N is even, plus or minus N is the correct result.
  double toTruncate = (d < 0) ? d - 0.5 : d + 0.5;

  T truncated = static_cast<T>(toTruncate);

  if (truncated == toTruncate) {
    /*
     * It was a tie (since moving away from 0 by 0.5 gave us the exact integer
     * we want). Since we rounded away from 0, we either already have an even
     * number or we have an odd number but the number we want is one closer to
     * 0. So just unconditionally masking out the ones bit should do the trick
     * to get us the value we want.
     */
    truncated &= ~1;
  }

  *retval = truncated;
  return true;
}

template <typename T>
struct PrimitiveConversionTraits<T, eClamp>
    : public PrimitiveConversionTraits_ToCheckedIntHelper<
          T, JSContext*, PrimitiveConversionTraits_Clamp<T> > {};

template <ConversionBehavior B>
struct PrimitiveConversionTraits<bool, B> : public DisallowedConversion<bool> {
};

template <>
struct PrimitiveConversionTraits<bool, eDefault> {
  typedef bool jstype;
  typedef bool intermediateType;
  static inline bool converter(JSContext* /* unused */, JS::Handle<JS::Value> v,
                               const char* sourceDescription, jstype* retval) {
    *retval = JS::ToBoolean(v);
    return true;
  }
};

template <ConversionBehavior B>
struct PrimitiveConversionTraits<float, B>
    : public DisallowedConversion<float> {};

template <ConversionBehavior B>
struct PrimitiveConversionTraits<double, B>
    : public DisallowedConversion<double> {};

struct PrimitiveConversionTraits_float {
  typedef double jstype;
  typedef double intermediateType;
  static inline bool converter(JSContext* cx, JS::Handle<JS::Value> v,
                               const char* sourceDescription, jstype* retval) {
    return JS::ToNumber(cx, v, retval);
  }
};

template <>
struct PrimitiveConversionTraits<float, eDefault>
    : PrimitiveConversionTraits_float {};
template <>
struct PrimitiveConversionTraits<double, eDefault>
    : PrimitiveConversionTraits_float {};

template <typename T, ConversionBehavior B, typename U>
bool ValueToPrimitive(U& cx, JS::Handle<JS::Value> v,
                      const char* sourceDescription, T* retval) {
  typename PrimitiveConversionTraits<T, B>::jstype t;
  if (!PrimitiveConversionTraits<T, B>::converter(cx, v, sourceDescription, &t))
    return false;

  *retval = static_cast<T>(
      static_cast<typename PrimitiveConversionTraits<T, B>::intermediateType>(
          t));
  return true;
}

}  // namespace mozilla::dom

#endif /* mozilla_dom_PrimitiveConversions_h */