Convert two ASCII Hexadecimal Characters (Two ASCII bytes) in one byte

Question

I want to convert two ASCII bytes to one hexadecimal byte. eg.

0x30 0x43 => 0x0C , 0x34 0x46 => 0x4F ...

The ASCII bytes are a number bet

Answer 1

Converting 2 hex chars to a byte is done in two steps:

1. Convert char a and b to their number (e.g. 'F' -> 0xF), which is done in two big if else branches, that check if the char is in the range '0' to '9', 'A' to 'F' or 'a' to 'f'.

2. In the 2nd step the two numbers are joined by shifting a (largest value is 0xF (0b0000_FFFF)) 4 to the left (a << 4 -> 0b1111_0000) and then apply the bitwise or operation on a and b ((a << 4) | b):

a: 0000_1111
b: 1111_0000
-> 1111_1111

#include 
#include 

#define u8 uint8_t
#define u32 uint32_t

u8 to_hex_digit(char a, char b) {
    u8 result = 0;

    if (a >= 0x30 && a <= 0x39) {
        result = (a - 0x30) << 4;
    } else if (a >= 0x41 && a <= 0x46) {
        result = (a - 0x41 + 10) << 4;
    } else if (a >= 0x61 && a <= 0x7A) {
        result = (a - 0x61 + 10) << 4;
    } else {
        printf("invalid hex digit: '%c'\n", a);
    }

    if (b >= 0x30 && b <= 0x39) {
        result |= b - 0x30;
    } else if (b >= 0x41 && b <= 0x46) {
        result |= b - 0x41 + 10;
    } else if (b >= 0x61 && b <= 0x7A) {
        result |= b - 0x61 + 10;
    } else {
        printf("invalid hex digit: '%c'\n", b);
    }

    return result;
}

u32 main() {
    u8 result = to_hex_digit('F', 'F');
    printf("0x%X (%d)\n", result, result);

    return 0;
}