tensor_predictors/tensorPredictors/src/bit_utils.h

#ifndef INCLUDE_GUARD_BIT_UTILS_H
#define INCLUDE_GUARD_BIT_UTILS_H

#include <stdint.h> // uint32_t, uint64_t

#if (defined(__GNUC__) && defined(__BMI2__))
    #include <x86intrin.h>
    #include <bmi2intrin.h>     // _pdep_u32
#endif

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

/**
 * Computes the parity of a word (0 for even bit count and 1 otherwise)
 */
#ifdef __GNUC__
static inline int bitParity32(uint32_t x) { return __builtin_parity(x); }
static inline int bitParity64(uint64_t x) { return __builtin_parityll(x); }
#else
static inline int bitParity32(uint32_t x) {
    int p = (x != 0);
    while (x &= x - 1) { p = !p; }
    return p;
}
static inline int bitParity64(uint64_t x) {
    int p = (x != 0);
    while (x &= x - 1) { p = !p; }
    return p;
}
#endif

/**
 * Counts the number of set bits (`1`s in binary) in the number `x`
 */
#ifdef __GNUC__                                   /* POPulation COUNT */
static inline int bitCount32(uint32_t x) { return __builtin_popcount(x); }
static inline int bitCount64(uint64_t x) { return __builtin_popcountll(x); }
#else
static inline int bitCount32(uint32_t x) {
    int count = 0; // counts set bits
    for (; x; count++) { x &= x - 1; } // increment count until there are no bits set in x
    return count;
}
static inline int bitCount64(uint64_t x) {
    int count = 0; // counts set bits
    for (; x; count++) { x &= x - 1; } // increment count until there are no bits set in x
    return count;
}
#endif

/**
 * Gets the index of the LSB (least significant bit)
 *
 * @condition `x != 0`, for `x == 0` undefined behaviour
 */
#ifdef __GNUC__
static inline int bitScanLS32(uint32_t x) { return __builtin_ctz(x); } // Count Trailing Zeros
static inline int bitScanLS64(uint64_t x) { return __builtin_ctzll(x); }
#elif _MSC_VER
static inline int bitScanLS32(uint32_t x) {
    unsigned long bsr;
    _BitScanReverse(&bsr, x);
    return 31 - bsr;
}
static inline int bitScanLS64(uint64_t x) {
    unsigned long bsr;
    _BitScanReverse64(&bsr, x);
    return 63 - bsr;
}
#else
static inline int bitScanLS32(uint32_t x) {
    int ctz = 0; // result storing the Count of Trailing Zeros
    bool empty;  // boolean variable storing if a bit has not found (search area is empty)

    // logarithmic search for LSB bit index (-1)
    ctz += (empty = !(x & (uint32_t)(65535))) << 4;
    x >>= 16 * empty;
    ctz += (empty = !(x & (uint32_t)(  255))) << 3;
    x >>=  8 * empty;
    ctz += (empty = !(x & (uint32_t)(   15))) << 2;
    x >>=  4 * empty;
    ctz += (empty = !(x & (uint32_t)(    3))) << 1;
    x >>=  2 * empty;
    ctz += (empty = !(x & (uint32_t)(    1)));

    return ctz;
}
static inline int bitScanLS64(uint64_t x) {
    int ctz = 0;
    bool empty;

    // logarithmic search for LSB bit index (-1)
    ctz += (empty = !(x & (uint64_t)(4294967295))) << 5;
    x >>= 32 * empty;
    ctz += (empty = !(x & (uint64_t)(     65535))) << 4;
    x >>= 16 * empty;
    ctz += (empty = !(x & (uint64_t)(       255))) << 3;
    x >>=  8 * empty;
    ctz += (empty = !(x & (uint64_t)(        15))) << 2;
    x >>=  4 * empty;
    ctz += (empty = !(x & (uint64_t)(         3))) << 1;
    x >>=  2 * empty;
    ctz += (empty = !(x & (uint64_t)(         1)));

    return ctz;
}
#endif

/**
 * Parallel DEPosit (aka PDEP)
 *
 * Writes the `val` bits into the positions of the set bits in `mask`.
 *
 * Example:
 *  val:   **** **** **** 1.1.
 * mask:   1... 1... 1... 1...
 *  res:   1... .... 1... ....
 */
#if (defined(__GNUC__) && defined(__BMI2__))
static inline uint32_t bitDeposit32(uint32_t val, uint32_t mask) {
    return _pdep_u32(val, mask);
}
static inline uint64_t bitDeposit64(uint64_t val, uint64_t mask) {
    return _pdep_u64(val, mask);
}
#else
static inline uint32_t bitDeposit32(uint32_t val, uint32_t mask) {
    uint32_t res = 0;
    for (uint32_t pos = 1; mask; pos <<= 1) {
        if (val & pos) {
            res |= mask & -mask;
        }
        mask &= mask - 1;
    }
    return res;
}
static inline uint64_t bitDeposit64(uint64_t val, uint64_t mask) {
    uint64_t res = 0;
    for (uint64_t pos = 1; mask; pos <<= 1) {
        if (val & pos) {
            res |= mask & -mask;
        }
        mask &= mask - 1;
    }
    return res;
}
#endif

/**
 * Gets the next lexicographically ordered permutation of an n-bit word.
 * 
 * Let `val` be a bit-word with `n` bits set, then this procedire computes a
 * `n` bit word wich is the next element in the lexicographically ordered
 * sequence of `n` bit words. For example
 * 
 *           val -> bitNextPerm(val)
 *      00010011 -> 00010101
 *      00010101 -> 00010110
 *      00010110 -> 00011001
 *      00011001 -> 00011010
 *      00011010 -> 00011100
 *      00011100 -> 00100011
 *
 * @condition `x != 0`, for `x == 0` undefined behaviour due to `bitScanLS`
 *
 * see: https://graphics.stanford.edu/~seander/bithacks.html#NextBitPermutation
 */
static inline uint32_t bitNextPerm32(uint32_t val) {
    // Sets all least significant 0-bits of val to 1
    uint32_t t = val | (val - 1);
    // Next set to 1 the most significant bit to change,
    // set to 0 the least significant ones, and add the necessary 1 bits.
    return (t + 1) | (((~t & -~t) - 1) >> (bitScanLS32(val) + 1));
}
static inline uint64_t bitNextPerm64(uint64_t val) {
    // Sets all least significant 0-bits of val to 1
    uint64_t t = val | (val - 1);
    // Next set to 1 the most significant bit to change,
    // set to 0 the least significant ones, and add the necessary 1 bits.
    return (t + 1) | (((~t & -~t) - 1) >> (bitScanLS64(val) + 1));
}

#endif /* BIT_UTILS_INCLUDE_GUARD_H */