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113 lines
3.8 KiB
C++
113 lines
3.8 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
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* vim: set ts=8 sts=4 et sw=4 tw=99:
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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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#ifndef js_TemplateLib_h
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#define js_TemplateLib_h
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#include "jstypes.h"
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/*
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* Library of reusable template meta-functions (that is, functions on types and
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* compile-time values). Meta-functions are placed inside the 'tl' namespace to
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* avoid conflict with non-meta functions that logically have the same name
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* (e.g., js::tl::Min vs. js::Min).
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*/
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namespace js {
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namespace tl {
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/* Compute min/max/clamp. */
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template <size_t i, size_t j> struct Min {
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static const size_t result = i < j ? i : j;
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};
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template <size_t i, size_t j> struct Max {
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static const size_t result = i > j ? i : j;
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};
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template <size_t i, size_t min, size_t max> struct Clamp {
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static const size_t result = i < min ? min : (i > max ? max : i);
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};
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/* Compute x^y. */
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template <size_t x, size_t y> struct Pow {
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static const size_t result = x * Pow<x, y - 1>::result;
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};
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template <size_t x> struct Pow<x,0> {
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static const size_t result = 1;
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};
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/* Compute floor(log2(i)). */
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template <size_t i> struct FloorLog2 {
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static const size_t result = 1 + FloorLog2<i / 2>::result;
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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 result = 0; };
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/* Compute ceiling(log2(i)). */
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template <size_t i> struct CeilingLog2 {
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static const size_t result = FloorLog2<2 * i - 1>::result;
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};
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/* Round up to the nearest power of 2. */
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template <size_t i> struct RoundUpPow2 {
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static const size_t result = size_t(1) << CeilingLog2<i>::result;
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};
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template <> struct RoundUpPow2<0> {
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static const size_t result = 1;
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};
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/* Compute the number of bits in the given unsigned type. */
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template <class T> struct BitSize {
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static const size_t result = sizeof(T) * JS_BITS_PER_BYTE;
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};
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/*
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* Produce an N-bit mask, where N <= BitSize<size_t>::result. Handle the
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* language-undefined edge case when N = BitSize<size_t>::result.
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*/
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template <size_t N> struct NBitMask {
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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 |result| to assure
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// its computation.
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static const size_t checkPrecondition = 0 / size_t(N < BitSize<size_t>::result);
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static const size_t result = (size_t(1) << N) - 1 + checkPrecondition;
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};
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template <> struct NBitMask<BitSize<size_t>::result> {
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static const size_t result = 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> struct MulOverflowMask {
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static const size_t result =
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~NBitMask<BitSize<size_t>::result - CeilingLog2<N>::result>::result;
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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 result = 0; };
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/*
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* Generate a mask for T such that if (X & sUnsafeRangeSizeMask), an X-sized
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* array of T's is big enough to cause a ptrdiff_t overflow when subtracting
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* a pointer to the end of the array from the beginning.
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*/
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template <class T> struct UnsafeRangeSizeMask {
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/*
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* The '2' factor means the top bit is clear, sizeof(T) converts from
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* units of elements to bytes.
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*/
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static const size_t result = MulOverflowMask<2 * sizeof(T)>::result;
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};
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template <bool cond, typename T, T v1, T v2> struct If { static const T result = v1; };
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template <typename T, T v1, T v2> struct If<false, T, v1, v2> { static const T result = v2; };
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} /* namespace tl */
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} /* namespace js */
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#endif /* js_TemplateLib_h */
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