Huffman Tutorial Part 4

05 Jul 2016

Tags: c++ Huffman encoder compression bits

Ah! Here is the fun part! Actually writing each individual bit into a file! So, how does one go about doing it?

You Don’t

As mentioned before, you can’t do that.

Bit Manipulation

That’s why you need to manipulate the bits inside a byte! What you have to do is this: push every 8 bits into a byte (char), and write said byte into a file.

If you are very familiar with bitwise operators, you can just go right ahead. For those who aren’t, here is a refresher.

Note that I’ll refer to logic gates as ‘gates’, and bitwise operators as ‘operators’. Gates typically have 2 inputs and 1 output (either 0 or 1, which is 1 bit). Operators are just gates, only they are for bytes. They apply the gate to each individual bit in the byte.

There are a few basic bitwise operators you need to know: AND, OR, NOT, XOR come into mind.


AND

An AND gate only outputs 1 if all of it’s inputs are 1. Otherwise, it outputs 0. Here is a truth table:

a b out
0 0 0
0 1 0
1 0 0
1 1 1

Similarly, the AND operator applies an AND gate to each individual bit of the inputs, and spitting it in it’s place in the output. This is exactly the same for all the other ones, so it won’t be brought up again.

OR

An OR gate outputs 1 if at least 1 of the inputs are 1.

a b out
0 0 0
0 1 1
1 0 1
1 1 1

NOT

A NOT gate outputs a 1 if the input is a 0, and vice versa.

a out
0 1
1 0

XOR

An XOR gate (exclusive OR) outputs 1 if 1 and only 1 of the inputs are 1.

a b out
0 0 0
0 1 1
1 0 1
1 1 0

Back to bit manipulation. Assuming you have a long string of binary ones and zeros, you will need to take every 8 bits, and shove them into a byte. Here is how it should go:

  1. Iterate through all binary digits
  2. Is digit a 1?
    1. Use OR operation to place 1 into temporary variable (byte)
  3. Increment counter (for which bit you are in the temporary variable)
  4. Repeat until all binary digits are exhausted

Now, the question becomes how can you do that. You shift the 1 by whatever the counter is. Note that you may have to tinker around with how much you shift by.

string bs = "01001100";
char byte = 0;
for (unsigned i = 0; i < 8; i++) {
    if (bs[i] == '1') byte |= 1 << i;
}

That was a small snippet of how it is done. I didn’t use std::string, and instead opted to use a vector of bits (bool). For some systems, a std::vector<bool> is a “space efficient specialization of std::vector1.

Technically, you didn’t need to know most of the gates written above. So yeah. Made you look.

Writing to File

Well, that’s easy, now that you have an array/vector of bytes. You can just iterate through all of them and write them all to a file.

Assignment for This Part

(:star: denotes a challenging task. :star2: denotes an even more challenging task.)

  1. http://en.cppreference.com/w/cpp/container/vector_bool