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For example, in PHP, how would I reverse the bits of the byte 11011111 to 11111011?
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You may use a table that store the reverse value of each byte with (i * 0x0202020202ULL & 0x010884422010ULL) % 0x3ff, initialized through a lambda (you will need to compile it with g++ -std=c++1z since it only works since C++17), and then return the value in the table will give you the accordingly reversed bit:
As for the leap that reversing an 8 bit integer is the same as reversing 4 bit parts then swapping them; I claim experience and intuition (or magic). If you are talking about a single byte, a table-lookup is probably the best bet, unless for some reason you don't have 256 bytes available.
Probably the most logical, consists in rotating the byte while applying a mask on the first bit (n & 1): unsigned char reverse_bits (unsigned char b) { unsigned char r = 0; unsigned byte_len = 8; while (byte_len--) { r = (r << 1) | (b & 1); b >>= 1; } return r; }
In IBM 360 Assembler, assuming the byte you wish to reverse the bits of is at a memory location called “BYTE” and the result replaces the original (…’s to show spacing only). Only 4 instructions and no loop. It really depends how many bits you are wanting to reverse.
If you have already the bits in the form of a string, use strrev.
If not, convert first the byte to its binary representation by using decbin, then reverse using strrev, then go back to byte (if necessary) by using bindec.
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