A level defines a finite alphabet and a bijection between valid words over that alphabet and symbols of the next level. SUL uses two distinct kinds of levels: literal and hash.

The first three levels use an exact integer bijection. Alphabet sizes grow via tetration starting from n = 2:

Level 4 would have 2^136 + 1 symbols — far too large for integer storage.

See literal/README.md for encoding details, tables, and streaming API.

Beyond level 3, each symbol is a 256-bit content-addressed Id. Any pair of identifiers merges into a new identifier via a SHA2-based compress, enabling unbounded nesting at a fixed size. The hash level array grows dynamically as encoding proceeds.

See hash/README.md for the encoding algorithm and streaming API.

All levels share the same word structure. A valid word has the form [s0, s1, ..., sk, t] where:

• s0 > s1 > ... > sk — a strictly decreasing prefix
• sk <= t — a terminating symbol no less than the last prefix element

See the SUL overview for full details.