Question

I was looking at hash functions the other day and came across a website that had an example of one. Most of the code was easy to grasp, however this macro function I can’t really wrap my head around.

Could someone breakdown what’s going on here?

#define get16bits(d) ((((uint32_t)(((const uint8_t *)(d))[1])) << 8) +(uint32_t)(((const uint8_t *)(d))[0]))

Answer 1

Basically it gets the lower 16 bit of the 32 bit integer d

lets break it down

#define get16bits(d) ((((uint32_t)(((const uint8_t *)(d))[1])) << 8) +(uint32_t)(((const uint8_t *)(d))[0]))
uint32_t a = 0x12345678;
uint16_t b = get16bits(&a); // b == 0x00005678

first we must pass the address of a to get16bits() or it will not work.

(((uint32_t)(const uint8_t *)(d))[1])) << 8

this first converts the 32 bit integer into an array of 8 bit integers and retrieves the 2 one.
It then shifts the value by 8 bit so it and adds the lower 8 bits to it

+ (uint32_t)(((const uint8_t *)(d))[0]))

In our example it will be

uint8_t tmp[4] = (uint8_t *)&a;
uint32_t result;
result = tmp[1] << 8; // 0x00005600
result += tmp[0]; //tmp[0] == 0x78
// result is now 0x00005678