Header Character (16-bit Compression Metadata)

Every JSSC compressed string starts with a single UTF-16 character that stores all compression metadata required for decompression.

This character is called the Header Character.

It encodes the compression mode, flags, and mode-specific parameters entirely inside its 16 bits, allowing the rest of the string to contain only compressed payload data.

Standard Bit Layout

Although each compression mode is free to interpret the 16 bits differently, most modes follow a standard layout.

Instead of viewing the header as four nibbles, JSSC groups bits into blocks:

Name	Type	Meaning / Standart Usage
Code #1	Integer	Compression Mode ID
Code #2	Integer	(Used by compression mode)
Code #3	Integer	Data type / JUSTC used? / BeginID
i?	Boolean	Input RLE used?
o?	Boolean	Output RLE used?
s?	Boolean	Sequences found & compressed?
b?	Boolean	BeginID used? (false = Code #3 is BeginID)

Non-Standard Layouts

The standard layout is not mandatory.

Some compression modes repurpose parts of the header to achieve better compression efficiency.

Whenever a mode deviates from the standard layout, this is explicitly documented on that mode's page.

Compression Mode ID and Code #1

Code #1 does not always represent the actual compression mode ID. JSSC determines the compression mode from the entire Header Character.

For example, when Code #1 is 00 but Code #2 is not 00, this indicates Compression Mode 06, not 00.

In this case, this happens because Compression Mode 06 stores either 06 or 00 in Code #1, depending on the input type.

Therefore, when Code #1 is 00 and Code #2 is also 00, the compression mode ID is 00.