gecko-dev/js/public/TemplateLib.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: set ts=8 sts=4 et sw=4 tw=99:
 * 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/. */

#ifndef js_TemplateLib_h
#define js_TemplateLib_h

#include "jstypes.h"

/*
 * Library of reusable template meta-functions (that is, functions on types and
 * compile-time values). Meta-functions are placed inside the 'tl' namespace to
 * avoid conflict with non-meta functions that logically have the same name
 * (e.g., js::tl::Min vs. js::Min).
 */

namespace js {
namespace tl {

/* Compute min/max/clamp. */
template <size_t i, size_t j> struct Min {
    static const size_t result = i < j ? i : j;
};
template <size_t i, size_t j> struct Max {
    static const size_t result = i > j ? i : j;
};
template <size_t i, size_t min, size_t max> struct Clamp {
    static const size_t result = i < min ? min : (i > max ? max : i);
};

/* Compute x^y. */
template <size_t x, size_t y> struct Pow {
    static const size_t result = x * Pow<x, y - 1>::result;
};
template <size_t x> struct Pow<x,0> {
    static const size_t result = 1;
};

/* Compute floor(log2(i)). */
template <size_t i> struct FloorLog2 {
    static const size_t result = 1 + FloorLog2<i / 2>::result;
};
template <> struct FloorLog2<0> { /* Error */ };
template <> struct FloorLog2<1> { static const size_t result = 0; };

/* Compute ceiling(log2(i)). */
template <size_t i> struct CeilingLog2 {
    static const size_t result = FloorLog2<2 * i - 1>::result;
};

/* Round up to the nearest power of 2. */
template <size_t i> struct RoundUpPow2 {
    static const size_t result = size_t(1) << CeilingLog2<i>::result;
};
template <> struct RoundUpPow2<0> {
    static const size_t result = 1;
};

/* Compute the number of bits in the given unsigned type. */
template <class T> struct BitSize {
    static const size_t result = sizeof(T) * JS_BITS_PER_BYTE;
};

/*
 * Produce an N-bit mask, where N <= BitSize<size_t>::result.  Handle the
 * language-undefined edge case when N = BitSize<size_t>::result.
 */
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 |result| to assure
    // its computation.
    static const size_t checkPrecondition = 0 / size_t(N < BitSize<size_t>::result);
    static const size_t result = (size_t(1) << N) - 1 + checkPrecondition;
};
template <> struct NBitMask<BitSize<size_t>::result> {
    static const size_t result = 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 result =
        ~NBitMask<BitSize<size_t>::result - CeilingLog2<N>::result>::result;
};
template <> struct MulOverflowMask<0> { /* Error */ };
template <> struct MulOverflowMask<1> { static const size_t result = 0; };

/*
 * Generate a mask for T such that if (X & sUnsafeRangeSizeMask), an X-sized
 * array of T's is big enough to cause a ptrdiff_t overflow when subtracting
 * a pointer to the end of the array from the beginning.
 */
template <class T> struct UnsafeRangeSizeMask {
    /*
     * The '2' factor means the top bit is clear, sizeof(T) converts from
     * units of elements to bytes.
     */
    static const size_t result = MulOverflowMask<2 * sizeof(T)>::result;
};

template <bool cond, typename T, T v1, T v2> struct If        { static const T result = v1; };
template <typename T, T v1, T v2> struct If<false, T, v1, v2> { static const T result = v2; };

/*
 * Traits class for identifying types that are implicitly barriered.
 */
template <class T> struct IsRelocatableHeapType { static const bool result = true; };

} /* namespace tl */
} /* namespace js */

#endif  /* js_TemplateLib_h */