1.1 --- /dev/null Thu Jan 01 00:00:00 1970 +0000
1.2 +++ b/mfbt/TemplateLib.h Wed Dec 31 06:09:35 2014 +0100
1.3 @@ -0,0 +1,111 @@
1.4 +/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
1.5 +/* vim: set ts=8 sts=2 et sw=2 tw=80: */
1.6 +/* This Source Code Form is subject to the terms of the Mozilla Public
1.7 + * License, v. 2.0. If a copy of the MPL was not distributed with this
1.8 + * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
1.9 +
1.10 +/*
1.11 + * Reusable template meta-functions on types and compile-time values. Meta-
1.12 + * functions are placed inside the 'tl' namespace to avoid conflict with non-
1.13 + * meta functions of the same name (e.g., mozilla::tl::FloorLog2 vs.
1.14 + * mozilla::FloorLog2).
1.15 + *
1.16 + * When constexpr support becomes universal, we should probably use that instead
1.17 + * of some of these templates, for simplicity.
1.18 + */
1.19 +
1.20 +#ifndef mozilla_TemplateLib_h
1.21 +#define mozilla_TemplateLib_h
1.22 +
1.23 +#include <limits.h>
1.24 +#include <stddef.h>
1.25 +
1.26 +namespace mozilla {
1.27 +
1.28 +namespace tl {
1.29 +
1.30 +/** Compute min/max. */
1.31 +template<size_t I, size_t J>
1.32 +struct Min
1.33 +{
1.34 + static const size_t value = I < J ? I : J;
1.35 +};
1.36 +template<size_t I, size_t J>
1.37 +struct Max
1.38 +{
1.39 + static const size_t value = I > J ? I : J;
1.40 +};
1.41 +
1.42 +/** Compute floor(log2(i)). */
1.43 +template<size_t I>
1.44 +struct FloorLog2
1.45 +{
1.46 + static const size_t value = 1 + FloorLog2<I / 2>::value;
1.47 +};
1.48 +template<> struct FloorLog2<0> { /* Error */ };
1.49 +template<> struct FloorLog2<1> { static const size_t value = 0; };
1.50 +
1.51 +/** Compute ceiling(log2(i)). */
1.52 +template<size_t I>
1.53 +struct CeilingLog2
1.54 +{
1.55 + static const size_t value = FloorLog2<2 * I - 1>::value;
1.56 +};
1.57 +
1.58 +/** Round up to the nearest power of 2. */
1.59 +template<size_t I>
1.60 +struct RoundUpPow2
1.61 +{
1.62 + static const size_t value = size_t(1) << CeilingLog2<I>::value;
1.63 +};
1.64 +template<>
1.65 +struct RoundUpPow2<0>
1.66 +{
1.67 + static const size_t value = 1;
1.68 +};
1.69 +
1.70 +/** Compute the number of bits in the given unsigned type. */
1.71 +template<typename T>
1.72 +struct BitSize
1.73 +{
1.74 + static const size_t value = sizeof(T) * CHAR_BIT;
1.75 +};
1.76 +
1.77 +/**
1.78 + * Produce an N-bit mask, where N <= BitSize<size_t>::value. Handle the
1.79 + * language-undefined edge case when N = BitSize<size_t>::value.
1.80 + */
1.81 +template<size_t N>
1.82 +struct NBitMask
1.83 +{
1.84 + // Assert the precondition. On success this evaluates to 0. Otherwise it
1.85 + // triggers divide-by-zero at compile time: a guaranteed compile error in
1.86 + // C++11, and usually one in C++98. Add this value to |value| to assure
1.87 + // its computation.
1.88 + static const size_t checkPrecondition = 0 / size_t(N < BitSize<size_t>::value);
1.89 + static const size_t value = (size_t(1) << N) - 1 + checkPrecondition;
1.90 +};
1.91 +template<>
1.92 +struct NBitMask<BitSize<size_t>::value>
1.93 +{
1.94 + static const size_t value = size_t(-1);
1.95 +};
1.96 +
1.97 +/**
1.98 + * For the unsigned integral type size_t, compute a mask M for N such that
1.99 + * for all X, !(X & M) implies X * N will not overflow (w.r.t size_t)
1.100 + */
1.101 +template<size_t N>
1.102 +struct MulOverflowMask
1.103 +{
1.104 + static const size_t value =
1.105 + ~NBitMask<BitSize<size_t>::value - CeilingLog2<N>::value>::value;
1.106 +};
1.107 +template<> struct MulOverflowMask<0> { /* Error */ };
1.108 +template<> struct MulOverflowMask<1> { static const size_t value = 0; };
1.109 +
1.110 +} // namespace tl
1.111 +
1.112 +} // namespace mozilla
1.113 +
1.114 +#endif /* mozilla_TemplateLib_h */
