cpct_getRandomUniform_u8_f

Returns a pseudo-random byte uniformly distributed using fast method (33*Seed mod 257)

C Definition

u8 cpct_getRandomUniform_u8_f (u8 entropy_byte) __z88dk_fastcall;

Input Parameters (1 byte)

Assembly call (Input parameter on L)

call cpct_getRandomUniform_u8_f_asm

Parameter Restrictions

• entropy_byte this parameter is a byte to be used as sequence mixer.  This byte is XORed with previous seed before calculating next pseudo-random number from the sequence.  There is no restriction for this parameter.  If you use 0 as entropy_byte, the pseudo-random generator will act normally, returning the general (33*Seed mod 257) sequence.  You may give it values coming from entropy sources like number of iterations of a loop until a user presses a key, or location of an object in the screen that depends on user interaction, etc.  That may improve the quality of the random sequence.

Known limitations

• This function will not work from a ROM, as it uses self-modifying code.

Important details

• This function uses a previous seed to calculate next pseudo-random byte, and stores returned byte minus 1 as seed for the next time.  If you want a reproducible sequence you may set the seed using cpct_setRandomSeedUniform_u8 to ensure that all calls to this function (using 0 as parameter) always return the same sequence.  You also may use this as a way to randomize the sequence, by starting with a seed that comes out of an entropy source.

Details

This function returns a pseudo-random byte uniformly distributed.  Each new byte returned is obtained as a result of the function byte = (33*Seed % 257).  Seed is then updated to the value byte - 1, to be used for the next call to the function.  The seed may be manually updated using the function cpct_setRandomSeedUniform_u8 to create predictable sequences (starting always with the same seed) or to randomize the start of a the sequence (by entering a seed coming out of an entropy source).

The parameter entropy_byte is an addition to this function to let the user improve the random quality of the sequence by inputting mixer bytes coming from entropy sources.  The function does an XOR operation between entropy_byte and the previous seed before doing the calculations for the next random byte.  Therefore, if you use *entropy_byte*s coming from good entropy sources, you will be effectively mixing up the sequence and getting better quality random numbers.  This could also be used for other purposes, like creating restricted or repeating sequences, up to the user imagination.

If you have no entropy source or do not want to use the entropy_byte, just input 0 which will have no effect on the sequence.

Destroyed Register values

AF, C, L

Required memory

2 bytes

Time Measures

   Case     | microSecs (us) | CPU Cycles
-----------------------------------------
   Any      |      20        |    80
-----------------------------------------

Credits

This function comes from Fast RND, which can be found published by z80-info.  The original code is reported to be from Spectrum ROM.