====== X ABC Algorithm Random Number Generator ======

I found this algorithm while I was looking for a high quality PRNG that can also handle zeros in the seed bytes. Initial test were promising. I modified the calculation to save a clc instruction over the original code.

I talked to a cryptographic expert about the algorithm and he stated that given the small resources, it is a well-designed algorithm.
It comes with four internal states: X, A, B, C
For an 8 bit number, the value of C (which is also returned in Accu) should be used.
For 16 bit, the value of B should be used as high byte.

The implementation was optimized using self-modifying code, which yields good speed and low resources, but this implementation won't work in ROM therefore.

In a test, I plotted the output of 51200 random values, which don't seem to reveal any visible patterns:

{{ :base:rand_x_abc.png?nolink&400 |}}

<code>
;; X ABC Algorithm Random Number Generator for 8-Bit Devices
;;
;; Algorithm from EternityForest, slight modification by Wil
;; https://www.electro-tech-online.com/threads/ultra-fast-pseudorandom-number-generator-for-8-bit.124249/
;; Implementation and test: Wil
;; This version stores the seed as arguments and uses self-modifying code
;; Routine requires 38 cycles (without the rts) / 28 bytes
;; Return values are in A and, if a 16 bit value is needed also in _rand8_highbyte

.export _rand8:=rand8, _rand8_highbyte:=b1

rand8:	
	inc x1
	clc
x1=*+1
	lda #$00	;x1
c1=*+1
	eor #$c2	;c1
a1=*+1
	eor #$11	;a1
	sta a1
b1=*+1
	adc #$37	;b1
	sta b1
	lsr
	eor a1
	adc c1
	sta c1
	rts
</code>