Bits: Search&Replace bit sequences

Being a high-level programmer myself, I struggle a lot with bitwise operations. I hope what I'm trying to achieve is doable?

Let's say I have one unsigned 8-bits integer - it can be any value. Let's work on two examples: 0xe4 (11100100) and 0xa5 (10100101).

The hardware interprets those as 4 chunks: 11 10 01 00 and 10 10 01 01.

I'm trying to write 3 methods in pure C:

• Replace all 00 chunks with 01 chunks. (result: 11 10 01 01 and 10 10 01 01)
• Replace all 01 chunks with 10 chunks. (result: 11 10 10 10 and 10 10 10 10)
• Replace all 10 chunks with 11 chunks. (result: 11 11 11 11 and 11 11 11 11)

Tried to search for bit-replacing methods, but couldn't bend any to solve this particular requirement. Where should I start?

Here are some fast, non-looping one-liners that do what you want:

unsigned set_00_to_01(unsigned x) {
    return x + ((~x >> 1) & ~x & 0x55);
}

unsigned set_01_to_10(unsigned x) {
    return x + ((~x >> 1) & x & 0x55);
}

unsigned set_10_to_11(unsigned x) {
    return x + ((x >> 1) & ~x & 0x55);
}

Note that they only operate on the low-order 8 bits, but they could easily be changed to operate on more by changing the 0x55 values to 0x5555 or 0x55555555 for instance.

Each function is hard-wired to its specific task, so you can't pass in arbitrary values. Their main advantage is their speed.