LZW decompression algorithm

Question

I\'m writing a program for an assignment which has to implement LZW compression/decompression. I\'m using the following algorithms for this:

-compression

Answer 1

From http://en.wikipedia.org/wiki/Lempel%E2%80%93Ziv%E2%80%93Welch are you falling into this case?

What happens if the decoder receives a code Z that is not yet in its dictionary? Since the decoder is always just one code behind the encoder, Z can be in the encoder's dictionary only if the encoder just generated it, when emitting the previous code X for χ. Thus Z codes some ω that is χ + ?, and the decoder can determine the unknown character as follows:

1) The decoder sees X and then Z.
2) It knows X codes the sequence χ and Z codes some unknown sequence ω.
3) It knows the encoder just added Z to code χ + some unknown character,
4) and it knows that the unknown character is the first letter z of ω.
5) But the first letter of ω (= χ + ?) must then also be the first letter of χ.
6) So ω must be χ + x, where x is the first letter of χ.
7) So the decoder figures out what Z codes even though it's not in the table,
8) and upon receiving Z, the decoder decodes it as χ + x, and adds χ + x to the table as the value of Z.

This situation occurs whenever the encoder encounters input of the form cScSc, where c is a single character, S is a string and cS is already in the dictionary, but cSc is not. The encoder emits the code for cS, putting a new code for cSc into the dictionary. Next it sees cSc in the input (starting at the second c of cScSc) and emits the new code it just inserted. The argument above shows that whenever the decoder receives a code not in its dictionary, the situation must look like this.

Although input of form cScSc might seem unlikely, this pattern is fairly common when the input stream is characterized by significant repetition. In particular, long strings of a single character (which are common in the kinds of images LZW is often used to encode) repeatedly generate patterns of this sort.

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For this specific case, the wikipedia thing fits, you have X+? where X is (o), Z is unknown so far so the first letter is X giving (oo) add (oo) to the table as 257. I am just going on what I read at wikipedia, let us know how this turns out if that is not the solution.