bit_utils.c

#include <stdint.h>

// treated as "independent", not as part of a larger integer
// i.e., for the "2nd" ("index 1") bit from the left will be output as `1`, not `2`
static uint32_t _read_bits(uint32_t u32, uint8_t where, uint8_t len) {
    return ((u32 & (((1 << len) - 1) << where)) >> where);
}

static void _write_bits(uint32_t *var, uint8_t where, uint32_t bits, uint8_t len) {
    *var = ((*var & ~(len << where)) | bits << where);
}

#define convert_le_off_to_be_off_in_u32(off) ((((31 - off) / 8) * 8) + (off % 8))
static uint32_t be_read_bits(uint32_t u32, uint8_t where, uint8_t len) {
    return _read_bits(u32, convert_le_off_to_be_off_in_u32(where), len);
}

static void be_write_bits(uint32_t *var, uint8_t where, uint32_t bits, uint8_t len) {
    return _write_bits(var, convert_le_off_to_be_off_in_u32(where), bits, len);
}

uint32_t (*read_bits)(uint32_t var, uint8_t where, uint8_t len);
void (*write_bits)(uint32_t *var, uint8_t where, uint32_t bits, uint8_t len);

void init_bit_rw_ptrs(void) {
    uint8_t bytes[] = { 0x00, 0x01 };
    if (*(uint16_t *)&bytes & 1) {
        read_bits = &be_read_bits;
        write_bits = &be_write_bits;
    }
    read_bits = &_read_bits;
    write_bits = &_write_bits;
}