summaryrefslogtreecommitdiff
path: root/mfbt/TemplateLib.h
blob: 415bba4581c3b829520156cbaaf3fa6eebcf39b7 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* 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>

#include "mozilla/TypeTraits.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; };

/**
 * And<bool...> computes the logical 'and' of its argument booleans.
 *
 * Examples:
 *   mozilla::t1::And<true, true>::value is true.
 *   mozilla::t1::And<true, false>::value is false.
 *   mozilla::t1::And<>::value is true.
 */

template<bool...>
struct And;

template<>
struct And<> : public TrueType { };

template<bool C1, bool... Cn>
struct And<C1, Cn...>
  : public Conditional<C1, And<Cn...>, FalseType>::Type { };

} // namespace tl

} // namespace mozilla

#endif /* mozilla_TemplateLib_h */