The Quantum compression format is a somewhat obscure method invented by a company called Cinematronics (written by an author identified as David Stafford, "davids a& cruzio.com"). Rights to it were purchased at some point by Microsoft, and Quantum is one of the possible compression methods in a CAB archive. This document does not describe the Quantum header nor the CAB details, only the compressed data.
The Quantum arithmetic coder is similar to the bitwise coder described under "Practical Arithmetic Coding with Renormalization" at Charles Bloom's site. I will not describe it further here. What differentiates Quantum is the statistical models it uses as input to the coder.
All the models used by the Quantum compressor share the same structure and update technique. All have a table with some number of entries, each entry composed of a symbol and an associated cumulative frequency. The cumulative frequency is the frequency of all the symbols which are at a higher index in the table than that symbol — thus the last entry in the table has a cumulative frequency of 0. Each model also has a total frequency (the frequency of all the symbols in the table), and a number time_to_reorder.
All the models are initialized with the symbols in symbol order in the table, and with every symbol in the table having a frequency of 1. Thus the initial total frequency is equal to the number of entries in the table. The initial time_to_reorder value is 4.The way the models are updated is as follows: Each time a symbol is encoded or decoded, all the cumulative frequencies up to (but not including) that symbol have 8 added to them. If this causes the total frequency to exceed 3800, time_to_reorder is decremented.
If time_to_reorder has not reached zero, each cumulative frequency in the table, starting with the last, is divided (with truncation) by 2. If this causes the cumulative frequency to become less than or equal to the cumulative frequency of the next entry, the entry cumulative frequency is instead set to the next entry cumulative frequency plus 1.
If time_to_reorder HAS reached zero, the frequencies (not cumulative frequencies) for each entry in the table are computed. Each frequency is divided by 2 with rounding. The table is then sorted by frequency (highest first) using an inplace selection sort (not stable!) and the cumulative frequencies recomputed. The time_to_reorder is set to 50.
To start decompressing a Quantum archive, read a symbol from the code stream using the selector model. This is a very simple model of 7 entries consisting of the symbols 06. What to do next depends on the value of the selector. If the selector is 0,1,2, or 3, a symbol is read using a literal model. These models encode symbols to be directly output. They are, respectively:
Selector  Number of entries  Starting symbol 

0  64  0 
1  64  64 
2  64  128 
3  64  192 
Selectors 4,5, and 6 implement LempelZiv matches. Selector 4 encodes 3 character matches, selector 5 implements 4 character matches, and selector 6 encodes 5+ character matches. Two character matches are encoded as literals. All three selectors use a position_slot and extra_bits table given below, and selector 6 uses a length_slot and length_extra_bits table.


The match selectors use several models to encode match length slots and position slots. The size of each position model depends on the compression table size. For example, if the table size is 1K, there is no need to encode position slots of 1K or greater. The number of position slots for each table size, and thus the position model size, is given in the Appendix. The 4 and 5 selectors also have a smaller maximum position slot regardless of table size; this implies that small matches are not emitted beyond a certain point. (In fact, the original compressor often doesn't emit small matches even within the range of its position slot table, but that is an implementation detail.)
Selector  Number of entries  Starting symbol 

4  max 24  0 
5  max 36  0 
6 (position)  max 42  0 
6 (length)  27  0 
To decompress a selector 4 or 5 match, read a symbol from the code stream using the appropriate model. Look up the symbol in the extra_bits table, and read that many bits. Add the position_base from the position_slot table to the extra bits, and add one. This is the match offset. The match length is 3 (selector 4) or 4 (selector 5).
To decompress a selector 6 match, read a symbol from the code stream using the selector 6 length model. Look up the symbol in the length_extra_bits table, and read that many bits. Add the length_base from the length_slot table to the extra bits, and add 5. This is the match length. Now read a symbol from the code stream using the selector 6 position model. Look up the symbol in the extra_bits table, and read that many bits. Add the position_base from the position_slot table to the extra bits just read, and add 1. This is the match offset.
This paragraph is applicable only to the CAB implementation
In order to ensure that there are an integral number of uncompressed
bytes per CAB block, no LZ match will cause the uncompressed stream to
cross an $8000byte boundary. Further, the arithmetic coder is
stopped at each $8000 byte boundary, with any remaining bits in the
block set to zero. The models and LZ window are not affected by
crossing such a boundary.
This paragraph is applicable only to the Cinematronics implementation
Each time you output a byte, whether from a literal or a match, XOR it
into a checksum accumulator (inital value 0), then rotate the
accumulator one bit left. There is no EOF symbol, so keep reading
until you've output the number of bytes you know are in the file.
Then read 16 bits from the stream. These 16 bits should match your
checksum accumulator. You're done.
Note that because of the way the decoder works, it has already taken in 16 + bit_plus_follow bits past the point the encoder had stopped. Some of these bits are significant. The method used in the CACM87 coder to handle this does not seem to work. Instead, what seems to work is: put a 0. Put bitplusfollow 1s. Put the bottom 15 bits of the low end of the current range. Then put the checksum. (this is not the actual method Quantum uses)
The method used in CACM87 is, after the last symbol is processed through the coder, the bitplusfollow counter is incremented and the low end range value is checked. If it is below 1/4 (0x3FFF), a one (with follow) is output. Otherwise, a zero (with follow) is output. I'm not sure whether this is valid or works 'by accident' in their application.I hit upon my method by considering what would happen if more symbols had been sent, with very low cumulative frequency. In that case, the high end of the range gets set equal (or nearly, anyway) to the low end of the range, and the bits output are exactly those from the low end of the range, with the bitplusfollow bits from the last real symbol, if any, after the high bit of the low end of the range. (the constant 0 bit is because CS_L is always less than 1/2 at the end of the encoding loop).
Hmm... new method based on the MACM96 encoder. That one outputs the code with the most trailing zeros. Not helpful efficiencywise for Quantum, but it cleans things up a bit. The important thing to realize is that all that is necessary is to send a code between CS_L and CS_H. But at termination, CS_H & 0x8000 = 1 and CS_L & 0x8000 = 0. So the code to be sent is simply 0x8000. Since we're sending a 1 bit first, the follow bits are 0 and therefore we send 0x8000 + bit_plus_follow zeros. This is also not the method Quantum uses; they appear to send a zero bit first.
If anyone knows the "correct" method, or the one Quantum uses, please let me know.
static int position_slot[] =
{
0x00000, 0x00001, 0x00002, 0x00003, 0x00004, 0x00006, 0x00008, 0x0000c,
0x00010, 0x00018, 0x00020, 0x00030, 0x00040, 0x00060, 0x00080, 0x000c0,
0x00100, 0x00180, 0x00200, 0x00300, 0x00400, 0x00600, 0x00800, 0x00c00,
0x01000, 0x01800, 0x02000, 0x03000, 0x04000, 0x06000, 0x08000, 0x0c000,
0x10000, 0x18000, 0x20000, 0x30000, 0x40000, 0x60000, 0x80000, 0xc0000,
0x100000, 0x180000
};
static int extra_bits[] = /* position extra_bits */
{
0, 0, 0, 0, 1, 1, 2, 2,
3, 3, 4, 4, 5, 5, 6, 6,
7, 7, 8, 8, 9, 9, 10, 10,
11, 11, 12, 12, 13, 13, 14, 14,
15, 15, 16, 16, 17, 17, 18, 18,
19, 19
};
static int length_slot[] =
{
0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x08,
0x0a, 0x0c, 0x0e, 0x12, 0x16, 0x1a, 0x1e, 0x26,
0x2e, 0x36, 0x3e, 0x4e, 0x5e, 0x6e, 0x7e, 0x9e,
0xbe, 0xde, 0xfe
};
static int length_extra_bits[] =
{
0, 0, 0, 0, 0, 0, 1, 1,
1, 1, 2, 2, 2, 2, 3, 3,
3, 3, 4, 4, 4, 4, 5, 5,
5, 5, 0
};
static int num_position_slots[] = /* number of position slots for (tsize  10) */
{
20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42
};
ushort getfreq(ushort totfreq)
/* get frequency from code */
{
ULONG range;
ULONG freq;
range = ((CS_H  CS_L)&0xFFFF) + 1;
freq = ((CS_C  CS_L + 1) * totfreq  1)/range;
return freq & 0xFFFF;
}
int getcode(int cumfreqm1, int cumfreq, int totfreq)
/* the decoder renormalization loop */
{
ULONG range;
int uf;
range = (CS_H  CS_L) + 1;
CS_H = CS_L + ((cumfreqm1 * range)/totfreq)  1;
CS_L = CS_L + (cumfreq * range)/totfreq;
while (1) {
if ((CS_L & 0x8000) != (CS_H & 0x8000)) {
if ((CS_L&0x4000) && !(CS_H&0x4000)) {
/* underflow case */
CS_C ^= 0x4000;
CS_L &= 0x3FFF;
CS_H = 0x4000;
}
else
break;
}
CS_L <<= 1;
CS_H = (CS_H << 1)  1;
CS_C = (CS_C << 1)  getbit();
}
struct modelsym {
ushort sym;
ushort cumfreq;
};
struct model {
int entries;
struct modelsym syms[1];
};
getsym(struct model *model) {
int freq;
int i;
int sym;
freq = getfreq(model>syms[0].cumfreq);
for (i = 1; i < model>entries; i++) {
if (model>syms[i].cumfreq <= freq)
break;
}
sym = model>syms[i1].sym;
getcode(model>syms[i1].cumfreq,
model>syms[i].cumfreq,
model>syms[0].cumfreq);
update_model(model, i);
return sym;
}