[Tenhou] is an online Japanese Riichi Mahjong game. As of 2022-10-02, Tenhou has published their algorithm for shuffling the deck / wall of tiles, alongside with validation data for a particular game seed, including intermediate results.
This crate re-implements the published algorithm for reconstructing the full shuffled wall of any game from its RNG seed.
This crate is no_std.
This crate bundles an implementation of [MT19937], copy-pasted from https://crates.io/crates/mt19937. This is mostly
for no_std compatibility.
[u32; 624].u32.Assuming u32 is little-endian (LSByte first) in byte buffers.
Each game begins by seeding an MT19937 RNG for the game. This seed can be retrieved as a base-64 encoding of the seed array.
Each round in the game requires a shuffle. We first derive the randomness as follows:
Generate 9 chunks of 1024 bits ([u32; 288]) from the game-wide RNG. This array is named src
in the original algorithm description. This is implemented as [src_from_mt].
For every 1024-bit chunk ([u32; 32], little-endian), calculate its SHA512 hash. All 9 chunks of 512 bits are
concatenated into [u32; 144]. This array is named rnd in the original algorithm description.
This is implemented as [rnd_from_src].
Then, we shuffle the wall with rnd:
rnd[i] as the random number table, scaled
to i..n as i + rnd[i] % (n - i). This is implemented as [shuffle_with_rnd].