be/src/gutil/bits.cc

// Copyright 2002 and onwards Google Inc.
//
// Derived from code by Moses Charikar

#include "gutil/bits.h"

#include <assert.h>

// this array gives the number of bits for any number from 0 to 255
// (We could make these ints.  The tradeoff is size (eg does it overwhelm
// the cache?) vs efficiency in referencing sub-word-sized array elements)
const char Bits::num_bits[] = {
        0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4, 1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3,
        4, 4, 5, 1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4,
        4, 5, 4, 5, 5, 6, 1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5, 2, 3, 3, 4, 3, 4, 4,
        5, 3, 4, 4, 5, 4, 5, 5, 6, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 3, 4, 4, 5,
        4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7, 1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5, 2,
        3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5,
        5, 6, 3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4,
        5, 4, 5, 5, 6, 3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7, 3, 4, 4, 5, 4, 5, 5, 6,
        4, 5, 5, 6, 5, 6, 6, 7, 4, 5, 5, 6, 5, 6, 6, 7, 5, 6, 6, 7, 6, 7, 7, 8};

int Bits::Count(const void* m, int num_bytes) {
    int nbits = 0;
    const uint8* s = (const uint8*)m;
    for (int i = 0; i < num_bytes; i++) nbits += num_bits[*s++];
    return nbits;
}

int Bits::Difference(const void* m1, const void* m2, int num_bytes) {
    int nbits = 0;
    const uint8* s1 = (const uint8*)m1;
    const uint8* s2 = (const uint8*)m2;
    for (int i = 0; i < num_bytes; i++) nbits += num_bits[(*s1++) ^ (*s2++)];
    return nbits;
}

int Bits::CappedDifference(const void* m1, const void* m2, int num_bytes, int cap) {
    int nbits = 0;
    const uint8* s1 = (const uint8*)m1;
    const uint8* s2 = (const uint8*)m2;
    for (int i = 0; i < num_bytes && nbits <= cap; i++) nbits += num_bits[(*s1++) ^ (*s2++)];
    return nbits;
}

int Bits::Log2Floor_Portable(uint32 n) {
    if (n == 0) return -1;
    int log = 0;
    uint32 value = n;
    for (int i = 4; i >= 0; --i) {
        int shift = (1 << i);
        uint32 x = value >> shift;
        if (x != 0) {
            value = x;
            log += shift;
        }
    }
    assert(value == 1);
    return log;
}

int Bits::Log2Ceiling(uint32 n) {
    int floor = Log2Floor(n);
    if (n == (n & ~(n - 1))) // zero or a power of two
        return floor;
    else
        return floor + 1;
}

int Bits::Log2Ceiling64(uint64 n) {
    int floor = Log2Floor64(n);
    if (n == (n & ~(n - 1))) // zero or a power of two
        return floor;
    else
        return floor + 1;
}

int Bits::FindLSBSetNonZero_Portable(uint32 n) {
    int rc = 31;
    for (int i = 4, shift = 1 << 4; i >= 0; --i) {
        const uint32 x = n << shift;
        if (x != 0) {
            n = x;
            rc -= shift;
        }
        shift >>= 1;
    }
    return rc;
}