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113 lines
2.9 KiB
C++
113 lines
2.9 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/*
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* Reusable template meta-functions on types and compile-time values. Meta-
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* functions are placed inside the 'tl' namespace to avoid conflict with non-
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* meta functions of the same name (e.g., mozilla::tl::FloorLog2 vs.
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* mozilla::FloorLog2).
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*
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* When constexpr support becomes universal, we should probably use that instead
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* of some of these templates, for simplicity.
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*/
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#ifndef mozilla_TemplateLib_h
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#define mozilla_TemplateLib_h
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#include <limits.h>
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#include <stddef.h>
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namespace mozilla {
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namespace tl {
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/** Compute min/max. */
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template<size_t I, size_t J>
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struct Min
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{
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static const size_t value = I < J ? I : J;
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};
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template<size_t I, size_t J>
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struct Max
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{
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static const size_t value = I > J ? I : J;
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};
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/** Compute floor(log2(i)). */
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template<size_t I>
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struct FloorLog2
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{
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static const size_t value = 1 + FloorLog2<I / 2>::value;
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};
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template<> struct FloorLog2<0> { /* Error */ };
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template<> struct FloorLog2<1> { static const size_t value = 0; };
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/** Compute ceiling(log2(i)). */
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template<size_t I>
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struct CeilingLog2
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{
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static const size_t value = FloorLog2<2 * I - 1>::value;
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};
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/** Round up to the nearest power of 2. */
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template<size_t I>
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struct RoundUpPow2
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{
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static const size_t value = size_t(1) << CeilingLog2<I>::value;
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};
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template<>
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struct RoundUpPow2<0>
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{
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static const size_t value = 1;
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};
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/** Compute the number of bits in the given unsigned type. */
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template<typename T>
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struct BitSize
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{
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static const size_t value = sizeof(T) * CHAR_BIT;
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};
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/**
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* Produce an N-bit mask, where N <= BitSize<size_t>::value. Handle the
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* language-undefined edge case when N = BitSize<size_t>::value.
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*/
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template<size_t N>
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struct NBitMask
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{
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// Assert the precondition. On success this evaluates to 0. Otherwise it
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// triggers divide-by-zero at compile time: a guaranteed compile error in
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// C++11, and usually one in C++98. Add this value to |value| to assure
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// its computation.
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static const size_t checkPrecondition =
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0 / size_t(N < BitSize<size_t>::value);
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static const size_t value = (size_t(1) << N) - 1 + checkPrecondition;
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};
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template<>
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struct NBitMask<BitSize<size_t>::value>
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{
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static const size_t value = size_t(-1);
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};
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/**
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* For the unsigned integral type size_t, compute a mask M for N such that
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* for all X, !(X & M) implies X * N will not overflow (w.r.t size_t)
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*/
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template<size_t N>
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struct MulOverflowMask
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{
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static const size_t value =
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~NBitMask<BitSize<size_t>::value - CeilingLog2<N>::value>::value;
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};
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template<> struct MulOverflowMask<0> { /* Error */ };
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template<> struct MulOverflowMask<1> { static const size_t value = 0; };
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} // namespace tl
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} // namespace mozilla
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#endif /* mozilla_TemplateLib_h */
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