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comparison: mfbt/TemplateLib.h
mfbt/TemplateLib.h
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- TOR_BUG_9701
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- 97036ab72558
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| -1:000000000000 | 0:97cc6a61447d |
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| 1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */ | |
| 2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */ | |
| 3 /* This Source Code Form is subject to the terms of the Mozilla Public | |
| 4 * License, v. 2.0. If a copy of the MPL was not distributed with this | |
| 5 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */ | |
| 6 | |
| 7 /* | |
| 8 * Reusable template meta-functions on types and compile-time values. Meta- | |
| 9 * functions are placed inside the 'tl' namespace to avoid conflict with non- | |
| 10 * meta functions of the same name (e.g., mozilla::tl::FloorLog2 vs. | |
| 11 * mozilla::FloorLog2). | |
| 12 * | |
| 13 * When constexpr support becomes universal, we should probably use that instead | |
| 14 * of some of these templates, for simplicity. | |
| 15 */ | |
| 16 | |
| 17 #ifndef mozilla_TemplateLib_h | |
| 18 #define mozilla_TemplateLib_h | |
| 19 | |
| 20 #include <limits.h> | |
| 21 #include <stddef.h> | |
| 22 | |
| 23 namespace mozilla { | |
| 24 | |
| 25 namespace tl { | |
| 26 | |
| 27 /** Compute min/max. */ | |
| 28 template<size_t I, size_t J> | |
| 29 struct Min | |
| 30 { | |
| 31 static const size_t value = I < J ? I : J; | |
| 32 }; | |
| 33 template<size_t I, size_t J> | |
| 34 struct Max | |
| 35 { | |
| 36 static const size_t value = I > J ? I : J; | |
| 37 }; | |
| 38 | |
| 39 /** Compute floor(log2(i)). */ | |
| 40 template<size_t I> | |
| 41 struct FloorLog2 | |
| 42 { | |
| 43 static const size_t value = 1 + FloorLog2<I / 2>::value; | |
| 44 }; | |
| 45 template<> struct FloorLog2<0> { /* Error */ }; | |
| 46 template<> struct FloorLog2<1> { static const size_t value = 0; }; | |
| 47 | |
| 48 /** Compute ceiling(log2(i)). */ | |
| 49 template<size_t I> | |
| 50 struct CeilingLog2 | |
| 51 { | |
| 52 static const size_t value = FloorLog2<2 * I - 1>::value; | |
| 53 }; | |
| 54 | |
| 55 /** Round up to the nearest power of 2. */ | |
| 56 template<size_t I> | |
| 57 struct RoundUpPow2 | |
| 58 { | |
| 59 static const size_t value = size_t(1) << CeilingLog2<I>::value; | |
| 60 }; | |
| 61 template<> | |
| 62 struct RoundUpPow2<0> | |
| 63 { | |
| 64 static const size_t value = 1; | |
| 65 }; | |
| 66 | |
| 67 /** Compute the number of bits in the given unsigned type. */ | |
| 68 template<typename T> | |
| 69 struct BitSize | |
| 70 { | |
| 71 static const size_t value = sizeof(T) * CHAR_BIT; | |
| 72 }; | |
| 73 | |
| 74 /** | |
| 75 * Produce an N-bit mask, where N <= BitSize<size_t>::value. Handle the | |
| 76 * language-undefined edge case when N = BitSize<size_t>::value. | |
| 77 */ | |
| 78 template<size_t N> | |
| 79 struct NBitMask | |
| 80 { | |
| 81 // Assert the precondition. On success this evaluates to 0. Otherwise it | |
| 82 // triggers divide-by-zero at compile time: a guaranteed compile error in | |
| 83 // C++11, and usually one in C++98. Add this value to |value| to assure | |
| 84 // its computation. | |
| 85 static const size_t checkPrecondition = 0 / size_t(N < BitSize<size_t>::value); | |
| 86 static const size_t value = (size_t(1) << N) - 1 + checkPrecondition; | |
| 87 }; | |
| 88 template<> | |
| 89 struct NBitMask<BitSize<size_t>::value> | |
| 90 { | |
| 91 static const size_t value = size_t(-1); | |
| 92 }; | |
| 93 | |
| 94 /** | |
| 95 * For the unsigned integral type size_t, compute a mask M for N such that | |
| 96 * for all X, !(X & M) implies X * N will not overflow (w.r.t size_t) | |
| 97 */ | |
| 98 template<size_t N> | |
| 99 struct MulOverflowMask | |
| 100 { | |
| 101 static const size_t value = | |
| 102 ~NBitMask<BitSize<size_t>::value - CeilingLog2<N>::value>::value; | |
| 103 }; | |
| 104 template<> struct MulOverflowMask<0> { /* Error */ }; | |
| 105 template<> struct MulOverflowMask<1> { static const size_t value = 0; }; | |
| 106 | |
| 107 } // namespace tl | |
| 108 | |
| 109 } // namespace mozilla | |
| 110 | |
| 111 #endif /* mozilla_TemplateLib_h */ |
