ctz.hpp

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/*******************************************************************************
 * tlx/math/ctz.hpp
 *
 * ctz() count trailing zeros - mainly for portability.
 *
 * Part of tlx - http://panthema.net/tlx
 *
 * Copyright (C) 2019 Timo Bingmann <tb@panthema.net>
 *
 * All rights reserved. Published under the Boost Software License, Version 1.0
 ******************************************************************************/

#ifndef TLX_MATH_CTZ_HEADER
#define TLX_MATH_CTZ_HEADER

#ifdef _MSC_VER
#include <intrin.h>
#endif

namespace tlx {

//! \addtogroup tlx_math
//! \{

/******************************************************************************/
// ctz() - count trailing zeros

//! ctz (count trailing zeros) - generic implementation
template <typename Integral>
static inline unsigned ctz_template(Integral x) {
    if (x == 0) return 8 * sizeof(x);
    unsigned r = 0;
    while ((x & static_cast<Integral>(1)) == 0)
        x >>= 1, ++r;
    return r;
}

/******************************************************************************/

template <typename Integral>
inline unsigned ctz(Integral x);

#if defined(__GNUC__) || defined(__clang__)

//! ctz (count trailing zeros)
template <>
inline unsigned ctz<unsigned>(unsigned i) {
    if (i == 0) return 8 * sizeof(i);
    return static_cast<unsigned>(__builtin_ctz(i));
}

//! ctz (count trailing zeros)
template <>
inline unsigned ctz<int>(int i) {
    return ctz(static_cast<unsigned>(i));
}

//! ctz (count trailing zeros)
template <>
inline unsigned ctz<unsigned long>(unsigned long i) {
    if (i == 0) return 8 * sizeof(i);
    return static_cast<unsigned>(__builtin_ctzl(i));
}

//! ctz (count trailing zeros)
template <>
inline unsigned ctz<long>(long i) {
    return ctz(static_cast<unsigned long>(i));
}

//! ctz (count trailing zeros)
template <>
inline unsigned ctz<unsigned long long>(unsigned long long i) {
    if (i == 0) return 8 * sizeof(i);
    return static_cast<unsigned>(__builtin_ctzll(i));
}

//! ctz (count trailing zeros)
template <>
inline unsigned ctz<long long>(long long i) {
    return ctz(static_cast<unsigned long long>(i));
}

#elif defined(_MSC_VER)

//! ctz (count trailing zeros)
template <typename Integral>
inline unsigned ctz<unsigned>(Integral i) {
    unsigned long trailing_zeros = 0;
    if (sizeof(i) > 4) {
#if defined(_WIN64)
        if (_BitScanForward64(&trailing_zeros, i))
            return trailing_zeros;
        else
            return 8 * sizeof(i);
#else
        return ctz_template(i);
#endif
    }
    else {
        if (_BitScanForward(&trailing_zeros, static_cast<unsigned>(i)))
            return trailing_zeros;
        else
            return 8 * sizeof(i);
    }
}

#else

//! ctz (count trailing zeros)
template <>
inline unsigned ctz<int>(int i) {
    return ctz_template(i);
}

//! ctz (count trailing zeros)
template <>
inline unsigned ctz<unsigned>(unsigned i) {
    return ctz_template(i);
}

//! ctz (count trailing zeros)
template <>
inline unsigned ctz<long>(long i) {
    return ctz_template(i);
}

//! ctz (count trailing zeros)
template <>
inline unsigned ctz<unsigned long>(unsigned long i) {
    return ctz_template(i);
}

//! ctz (count trailing zeros)
template <>
inline unsigned ctz<long long>(long long i) {
    return ctz_template(i);
}

//! ctz (count trailing zeros)
template <>
inline unsigned ctz<unsigned long long>(unsigned long long i) {
    return ctz_template(i);
}

#endif

//! \}

} // namespace tlx

#endif // !TLX_MATH_CTZ_HEADER

/******************************************************************************/