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/* -*- 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/. */
/*
* Reusable template meta-functions on types and compile-time values. Meta-
* functions are placed inside the 'tl' namespace to avoid conflict with non-
* meta functions of the same name (e.g., mozilla::tl::FloorLog2 vs.
* mozilla::FloorLog2).
*
* When constexpr support becomes universal, we should probably use that instead
* of some of these templates, for simplicity.
*/
#ifndef mozilla_TemplateLib_h
#define mozilla_TemplateLib_h
#include <limits.h>
#include <stddef.h>
namespace mozilla {
namespace tl {
/** Compute min/max. */
template<size_t I, size_t J>
struct Min
{
static const size_t value = I < J ? I : J;
};
template<size_t I, size_t J>
struct Max
{
static const size_t value = I > J ? I : J;
};
/** Compute floor(log2(i)). */
template<size_t I>
struct FloorLog2
{
static const size_t value = 1 + FloorLog2<I / 2>::value;
};
template<> struct FloorLog2<0> { /* Error */ };
template<> struct FloorLog2<1> { static const size_t value = 0; };
/** Compute ceiling(log2(i)). */
template<size_t I>
struct CeilingLog2
{
static const size_t value = FloorLog2<2 * I - 1>::value;
};
/** Round up to the nearest power of 2. */
template<size_t I>
struct RoundUpPow2
{
static const size_t value = size_t(1) << CeilingLog2<I>::value;
};
template<>
struct RoundUpPow2<0>
{
static const size_t value = 1;
};
/** Compute the number of bits in the given unsigned type. */
template<typename T>
struct BitSize
{
static const size_t value = sizeof(T) * CHAR_BIT;
};
/**
* Produce an N-bit mask, where N <= BitSize<size_t>::value. Handle the
* language-undefined edge case when N = BitSize<size_t>::value.
*/
template<size_t N>
struct NBitMask
{
// Assert the precondition. On success this evaluates to 0. Otherwise it
// triggers divide-by-zero at compile time: a guaranteed compile error in
// C++11, and usually one in C++98. Add this value to |value| to assure
// its computation.
static const size_t checkPrecondition =
0 / size_t(N < BitSize<size_t>::value);
static const size_t value = (size_t(1) << N) - 1 + checkPrecondition;
};
template<>
struct NBitMask<BitSize<size_t>::value>
{
static const size_t value = size_t(-1);
};
/**
* For the unsigned integral type size_t, compute a mask M for N such that
* for all X, !(X & M) implies X * N will not overflow (w.r.t size_t)
*/
template<size_t N>
struct MulOverflowMask
{
static const size_t value =
~NBitMask<BitSize<size_t>::value - CeilingLog2<N>::value>::value;
};
template<> struct MulOverflowMask<0> { /* Error */ };
template<> struct MulOverflowMask<1> { static const size_t value = 0; };
} // namespace tl
} // namespace mozilla
#endif /* mozilla_TemplateLib_h */
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