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base:ax_tinyrand8 [2021-11-28 03:25]
wil seeding function further explained
base:ax_tinyrand8 [2021-11-29 00:12] (current)
wil updated seeding function and description
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 This version stores the seed as arguments and uses self-modifying code and requires only 15 bytes for the random function. The execution time is constant at 18 cycles and much faster than for example a 789-Xorshift. This version stores the seed as arguments and uses self-modifying code and requires only 15 bytes for the random function. The execution time is constant at 18 cycles and much faster than for example a 789-Xorshift.
  
-The seeding function was tricky, since putting any two values into b1 and c1 comes with a 10% risk of ending up in a cycle with shorter period. Therefore, a seeding function is provided that takes an 8 bit values and generates a seed that is guaranteed to be in cycle with a period of 59748. However, there are only 64 different states reachable after seeding.+The seeding function was tricky, since putting any two values into b1 and c1 comes with a 10% risk of ending up in a cycle with shorter period. Therefore, a seeding function is provided that takes an 8 bit value and generates a seed that is guaranteed to be in cycle with a period of 59748. Thus, there are 256 different states reachable after seeding.
  
 In a test, I plotted the output of 51200 random values, which don't seem to reveal any visible patterns: In a test, I plotted the output of 51200 random values, which don't seem to reveal any visible patterns:
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 rand8:  rand8:
-c1=*+1+b1=*+1
  lda #31  lda #31
  asl  asl
-b1=*+1+a1=*+1
  eor #53  eor #53
- sta c1 
- adc b1 
  sta b1  sta b1
 + adc a1
 + sta a1
  rts  rts
  
 +; sets the seed based on the value in A
 +; always sets a1 and b1 so that a cycle with maximum period is chosen
 +; constants 217 and 21263 have been derived by simulation
 set_seed: set_seed:
  pha  pha
- and #$fa + and #217 
- sta b1+ clc 
 + adc #<21263 
 + sta a1
  pla  pla
- ora #$05 + and #255-217 
- sta c1+ adc #>21263 
 + sta b1
  rts  rts
 </code> </code>
base/ax_tinyrand8.txt · Last modified: 2021-11-29 00:12 by wil