View source for Random number generator

The GBA Boktai games use a very simple method of generating random numbers:

# There's a list of 1024 pregenerated u16 random numbers stored in EWRAM ("RNG Table")
# The game keeps track of the last list index used in IWRAM ("RNG Index")
# When the game needs a random number, it increments the RNG index (mod 1024)...
# ...and then returns <code>RNG_Table[RNG_Index]</code>.
# When the game needs a random value within a certain range, it instead returns <code>(RNG_Table[RNG_Index] >> 3) % upper_bound</code> (unknown why the game does a right shift/division by 8 here).

These calulations are inlined everywhere, there's no dedicated <code>rand()</code> function. Boktai 1/2 use the same RNG table, Boktai 3 (maybe?) uses a different one, ''TODO''. The RNG table never changes. The quality and period of this RNG is ''very'' bad.

= Bootstrap RNG =
The RNG table is generated by a [[Wikipedia:Linear congruential generator|Linear congruential generator]] on reset. This LCG has the formula <code>seed = seed*0x5d588b65 + 1</code>, with an initial seed of <code>123456</code> (maybe the default <code>rand()</code> implementation of the GBA SDK/libc?).

After generating the RNG table, the initial RNG index is also generated from this LCG.

Because this LCG is only used immediately after booting the game, and never used during active gameplay, it's called the "Bootstrap RNG".

''TODO'': Verify this page for Boktai 3

= See also =
* [[RNG Table]]

[[Category:Documentation]]
[[Category:Boktai 1]] [[Category:Boktai 2]] [[Category:Boktai 3]]