group.stock.dat

File group.stock.dat was seen to contain a shape in the data when unpacked as signed or unsigned shorts (16 bit integers), as seen in Figure 1. The key point in the figure (as the group sees it) is that the difference between two consecutive shorts is usually rather small.

Figure 1. File group.stock unpacked as 1250 unsigned shorts.

Naturally, the regularity seen in the Figure 1. be taken into account when compressing. We used a simple method where we only marked the starting number of the sequence and then the difference between each number and its predecessor. This sequence was then concatenated as a string, e.g. suppose that the three first numbers are 3000, 2500 and 2700, then, the yielded string would be 3000-500+200.

Acquired string was Huffman coded using each character in the string as an individual symbol and written to a file. The (symbol, code) -mapping was then added into decompressor in order to obtain the original data.