如何操作Smalltalk中的位？

Question

I am currently working on a file compressor based on Huffman decoding. So I have a decoding tree like so:

and I have to encode this tree on an output file by following a certain criteria:

"for each leaf, write out a 0 bit, followed by the 8 bits of the corresponding character. Write out the bits in the order bit 7, bit 6, . . ., bit 0, that is high bit first. As a special case, if the byte is 0, write out bit 8, which will be a 0 for a byte value of 0, and 1 for a byte value of 256 (the EOF marker)." For an internal node, just write a bit 1.

So what I plan to do is to create a bit array and add to it the corresponding bits in the specified format. The problem is that I don't know how to convert a number to binary in smalltalk.

For example, if I want to encode the first leaf, I would want to do something like 01101011 ie 0 followed by the bit representation of k and then add every bit one by one into the array.

Answer 1

I don't know which dialect you are using exactly, but generally, you can access the bits of Integer. They are modelled as if the representation was in two-complement, with an infinite sequence of bits.

 2 is ....0000000000010
 1 is ....0000000000001    
 0 is ....0000000000000 with infinitely many 0 on the left
-1 is ....1111111111111 with infinitely many 1 on the left
-2 is ....1111111111110

This is also true for LargeIntegers, even though they are generally implemented as sign magnitude (the class encodes the sign), two-complement will be emulated.

Then you can operate with bitAnd: bitOr: bitXor: bitInvert bitShift:, and in some flavours bitAt:put:

You can access the bits with (2 bitAt: index) where the index starts at 1 for least significant bit, or grows higher. If it's missing, implement it with bitAnd: and bitShift:...

For positive, you can ask for the rank of high bit (2 highBit).

All these operations should create a new integer (there's no in place modification possible).

Conceptually, a ByteArray is a collection of unsigned integers on 8 bits (between 0 and 255), so you can implement a bit Array with them (if it does not already exist in the dialect). Or you can use an Integer (but won't be able to control size which will be infinite, nor in place mofifications, operations will cost a copy).