While developing jPSXdec for the last 4 years, I've run across three different methods of decoding bitstreams.

*If you'd like to learn more about what part this plays in MPEG and PlayStation .STR decoding, check out my thorough document on the subject: PlayStation_STR_format.txt*

#### Approach 1: Brute force

This is the most obvious approach. For each code, peek the next n-bits until the bits match something.

In the worst case, this approach requires 111 conditional checks to identify a bit code. To be honest, I've never actually seen this implemented anywhere besides by me years ago when first learning about bitstream parsing.

#### Approach 2: Binary tree

I actually ran across this approach implemented in the Serial Experiments Lain PlayStation game. You have a tree of conditionals testing the value of each bit until a match is found.

The branching can be optimized a bit for most leaves: once the length of the bit code is clear, the remaining bits can be used as the index in several small lookup tables. The jPSXdec implementation only requires (in the worst case) 12 branches to determine the longest bit codes.

#### Approach 3: Array lookup

I believe this type of approach is used in ffmpeg and the Q-gears decoder. Thanks to the unspoken tradition of never documenting anything, I was unable to understand what it was doing. It wasn't until I reverse-engineered the .iki bitstream parsing that I finally saw how this approach works.

At least for MPEG-1 (and PSX STR), you can take advantage of its particular set of variable length bit codes. Only the first code ('`11s`

') and the end-of-block code ('`10`

') need special parsing. The rest of the codes fall under one of three groups. The group a code belongs to can be determined by looking at how many initial zeros it has.

- Group one starts with between 1 and 4 zeros (this also includes the escape code
`000001`

). - Group two starts with between 6 and 8 zeros.
- Group three starts with between 9 and 11 zeros.

All codes in their groups:

[Special handling]

01 // end-of-block

11s

---- [Group 1] ----

0 11s

0 100s

0 101s

0 0101s

0 0110s

0 0111s

0 00100s

0 00101s

0 00110s

0 00111s

0 000100s

0 000101s

0 000110s

0 000111s

0 00001 // escape code

0 0100000s

0 0100001s

0 0100010s

0 0100011s

0 0100100s

0 0100101s

0 0100110s

0 0100111s

---- [Group 2] ----

000000 1000s

000000 1001s

000000 1010s

000000 1011s

000000 1100s

000000 1101s

000000 1110s

000000 1111s

000000 010000s

000000 010001s

000000 010010s

000000 010011s

000000 010100s

000000 010101s

000000 010110s

000000 010111s

000000 011000s

000000 011001s

000000 011010s

000000 011011s

000000 011100s

000000 011101s

000000 011110s

000000 011111s

000000 0010000s

000000 0010001s

000000 0010010s

000000 0010011s

000000 0010100s

000000 0010101s

000000 0010110s

000000 0010111s

000000 0011000s

000000 0011001s

000000 0011010s

000000 0011011s

000000 0011100s

000000 0011101s

000000 0011110s

000000 0011111s

---- [Group 3] ----

000000000 10000s

000000000 10001s

000000000 10010s

000000000 10011s

000000000 10100s

000000000 10101s

000000000 10110s

000000000 10111s

000000000 11000s

000000000 11001s

000000000 11010s

000000000 11011s

000000000 11100s

000000000 11101s

000000000 11110s

000000000 11111s

000000000 010000s

000000000 010001s

000000000 010010s

000000000 010011s

000000000 010100s

000000000 010101s

000000000 010110s

000000000 010111s

000000000 011000s

000000000 011001s

000000000 011010s

000000000 011011s

000000000 011100s

000000000 011101s

000000000 011110s

000000000 011111s

000000000 0010000s

000000000 0010001s

000000000 0010010s

000000000 0010011s

000000000 0010100s

000000000 0010101s

000000000 0010110s

000000000 0010111s

000000000 0011000s

000000000 0011001s

000000000 0011010s

000000000 0011011s

000000000 0011100s

000000000 0011101s

000000000 0011110s

000000000 0011111s

Each group has its own lookup table of 256 entries, and each code will be associated with one or more entries in the lookup table. After stripping off the minimum number of zeros in the group, no entry in the group will have more than 8 bits remaining in the bit code. For codes that have 8 bits remaining, its value identifies the associated table index. For the bit codes that have fewer than 8 bits remaining, you have to walk through every combination of the remaining bits to find all associated indexes.

Example:

Use 0 for sign bit for now: 001100

Strip off first leading 0: 01100

Find all combinations of remaining bits:

` 01100+000 = 96 (table index)`

01100+001 = 97

01100+010 = 98

01100+011 = 99

01100+100 = 100

01100+101 = 101

01100+110 = 102

01100+111 = 103

Thus bit code 00110s will be associated with table indexes 96-103.

Now each table entry needs three values: the inverse discreet cosine transform (IDCT) run of zero-value alternating current (AC) coefficients, the non-zero AC coefficient value, and the length of the bitstream bits that should be skipped.

Once all three tables are constructed, the following pseudo code will parse your bitstream.

Of course the implementation details can vary, but this gives the idea. The Approach 3 I implemented for jPSXdec requires about 8 conditionals to identify a bit code in the worst case. I've found it to be about 10%-15% faster than the Approach 2 I've been using.

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