src/bitfield/8byte.c


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#include <bitfield/bitfield.h>
#include <bitfield/8byte.h>
#include <stddef.h>
#include <limits.h>
#include <string.h>

#define EIGHTBYTE_BIT (8 * sizeof(uint64_t))

uint64_t bitmask(const uint8_t bit_count) {
    return (((uint64_t)0x1) << bit_count) - 1;
}

static uint16_t bits_to_bytes(uint32_t bits) {
    uint8_t byte_count = bits / CHAR_BIT;
    if(bits % CHAR_BIT != 0) {
        ++byte_count;
    }
    return byte_count;
}

uint64_t get_bit_field(uint64_t source, const uint16_t offset,
        const uint16_t bit_count, bool big_endian) {
    int startByte = offset / CHAR_BIT;
    int endByte = (offset + bit_count - 1) / CHAR_BIT;

    if(!big_endian) {
        source = __builtin_bswap64(source);
    }

    uint8_t* bytes = (uint8_t*)&source;
    uint64_t ret = bytes[startByte];
    if(startByte != endByte) {
        // The lowest byte address contains the most significant bit.
        int i;
        for(i = startByte + 1; i <= endByte; i++) {
            ret = ret << 8;
            ret = ret | bytes[i];
        }
    }

    ret >>= 8 - find_end_bit(offset + bit_count);
    return ret & bitmask(bit_count);
}

bool set_bit_field(uint64_t* destination, uint64_t value, const uint16_t offset,
        const uint16_t bit_count) {
    if(value > bitmask(bit_count)) {
        return false;
    }

    int shiftDistance = EIGHTBYTE_BIT - offset - bit_count;
    value <<= shiftDistance;
    *destination &= ~(bitmask(bit_count) << shiftDistance);
    *destination |= value;
    return true;
}

uint8_t nth_byte(const uint64_t source, const uint16_t byte_index) {
    return (source >> (EIGHTBYTE_BIT - ((byte_index + 1) * CHAR_BIT))) & 0xFF;
}