COMPRESSION.md

Compression notes

The algorithm used is mainly LZSS with the following constants:

package compression

const DataMaxSize = 0x8000

const LiteralBits = 8
// OffsetBits Number of bits for the Window offset. From this the WindowSize will be calculated
// In other LZSS implementations, this is usually set to 12
const OffsetBits = 11
const LengthBits = 4

const WindowSize = 1 << OffsetBits

const MaxUncoded = 2
const MaxCoded = MaxUncoded + (1 << LengthBits)

// WindowInitialIndex The windowIndex needs to be set to this before compression/decompression
// In other LZSS implementations, this is usually set to 0
const WindowInitialIndex = WindowSize - 0x10 - 1

// WindowFillValue Initial value to fill Window[0 : WindowInitialIndex]
// In other LZSS implementations, this is usually set to 0x20, but Window[] is filled entirely
const WindowFillValue = 0x20

When initializing the Window, up to WindowInitialIndex are filled compared to the whole Window as usual.

Additionally, the initial windowIndex needs to also be set to WindowInitialIndex instead of 0.