cpct_getRandom_mxor_u16

Gets a high-quality 16-bit pseudo-random number using Marsaglia’s XOR-shift algorithm (using a 32-bits state)

C Definition

u16 cpct_getRandom_mxor_u16 ();

Assembly call

call cpct_getRandom_mxor_u16_asm

Return value (Assembly calls, return HL=random 16-bits)

Known limitations

• This function cannot be used from ROM, as is uses self-modifying code.

Details

This function uses cpct_nextRandom_mxor_u32 to produce a stream of random 32-bits numbers, and then gets 2 16-bits values out of each 32-bits number generated.  Then, it returns each one of the last 2 random values produced before calling cpct_nextRandom_mxor_u32 again.  It uses cpct_mxor32_seed as state storage buffer for the 32-bits number got from cpct_nextRandom_mxor_u32.

As cpct_nextRandom_mxor_u32 produces (2^32)-1 32-bits numbers without repetition, this function will produce (2^33)-1 16-bits values without repetition.

It is important to know that this function will have 2 different behaviors

• [available] 1 out of every 2 calls, it will only return the next random 16-bits value from the buffer (cpct_mxor32_seed).
• [production] 1 out of every 2 calls, it will also call cpct_nextRandom_mxor_u32 to produce a new 32-bits number (2 new 16-bits values).  This behaviour is reflected in the time measures.

This function uses Marsaglia’s XOR-shift standard algorithm with a concrete shift tuple.  Check cpct_nextRandom_mxor_u32 to know details on how this is produced.

Destroyed Register values

AF, BC, DE, HL

Required memory

73 bytes divided in,

• 34 bytes (this function’s code)
• +35 bytes (from cpct_nextRandom_mxor_u32 function’s code)
• +4 bytes (from cpct_mxor32_seed)

Time Measures

   Case     | microSecs (us) | CPU Cycles
-----------------------------------------
 available  |      16        |     64
 production |      76        |    304
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 average-2  |      46        |    184
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