#include <io.h>


// Constants for the RNG
#define LCG_A 25173
#define LCG_C 13849
//#define LCG_M 65536

// register names
#define MulResultLB r18
#define MulResultHB r19
#define ArgLB r24
#define ArgHB r25
#define RetLB r24
#define RetHB r25
#define TmpLB r22
#define TmpHB r23

/**
* Returns an uint16_t (pseudo) random number in R24:R25.
*
* The Method calculates an random number with a linear congruence generator (LCG).
* It uses the formula
*
* 						X(n+1) = (LCG_A*X(n) + LCG_C) modulo m
*
* to generate pseudo random numbers.
*
* Function needs 22 cycles + return (4) (=26 cycles).
* We do not need the modulo operation because we have set m to 2^16.
*
* A more detailed description can be found in:
*
* - Donald E. Knuth, The Art of Computer Programming
* - Dale Grover, Digital Signal Processing and the Microcontroller
*/
.text
.global RandLCG
.type RandLCG,@function
RandLCG:
	; // save used registers
	push r0
	; // do a 16x16 bit multiplication
	; // al * bl , ah * bl, bh * al
	; // (LCG_A * X)
	ldi TmpLB, lo8(LCG_A)
	ldi TmpHB, hi8(LCG_A)
	; // multiply the low bytes
	mul TmpLB, ArgLB
	; // copy the result from r0:r1 to r16:r17
	movw MulResultLB, r0
	; // multiply high byte with low byte
	mul TmpHB, ArgLB
	; // add result to r17
	add MulResultHB, r0
	; // the last multiply
	mul ArgHB, TmpLB
	add MulResultHB, r0
	; // now we have the result in MulResult
	; // now add the constant
	ldi TmpLB, lo8(LCG_C)
	ldi TmpHB, hi8(LCG_C)
	add MulResultLB, TmpLB
	adc MulResultHB, TmpHB
	; // move the result to the return registers so that gcc can use them
	movw RetLB, MulResultLB
	; // restore registers
	pop r0
	clr r1
	; // and return
	ret