;Recursive Ackermann-Peter-Function in x86 nasm-assembly
;@AUTHOR: Alexander Schneider, Düsseldorf, Germany 
;Ackermann-Peter-Function:
;A(n,m) = m + 1 if n == 0
;A(n,m) = A(n-1, 1) if n > 0 && m == 0
;A(n,m) = A(n-1, A(n, m-1)) if n > 0 && m > 0

%include "asm_io.inc"

segment .data

	prompt1 db "Bitte n (>=0) für die Ackermann-Berechnung eingeben: ",10,0
	prompt2 db "Bitte m (>=0) für die Ackermann-Berechnung eingeben: ",10,0
	prompt3 db "Das Ergebniss ist: ",0
	promptError db "n und m müssen beide >= 0 sein",10,0
	
segment .bss

segment .data
	global asm_main
asm_main:
	ENTER 0,0				;setup Routine
	PUSHA
	MOV eax, prompt1		;ask for n
	CALL print_string
	CALL read_int
	CMP eax, 0				;check n
	JL inputError
	MOV ebx, eax			;save n
	MOV eax, prompt2		;ask for m
	CALL print_string
	CALL read_int
	CMP eax, 0				;check m
	JL inputError
	MOV edx, eax			;save m
	PUSH ebp
	PUSH edx				;push m
	PUSH ebx				;push n
	CALL ackermann
	POP ebx
	POP edx
	POP ebp
	JMP end

ackermann:
	MOV edx, [ebp+8]		;get m from stack
	MOV ebx, [ebp+12]		;get n from stack
	CMP ebx, 0				;check if n is 0
	JE firstcase
	CMP edx, 0				;check if m is 0
	JE secondcase
	DEC ebx
	PUSH ebp
	PUSH edx
	PUSH ebx
	CALL ackermann
	PUSH ebx
	PUSH eax
	CALL ackermann
	POP ebx
	POP edx
	POP ebp
end:
	MOV ebx, eax			;save result
	MOV eax, prompt3
	CALL print_string		;print result
	MOV eax, ebx
	CALL print_int
	CALL print_nl
	POPA
	MOV eax, 0				;return Value for C-driver
	LEAVE
	RET

firstcase:
	MOV eax, [ebp+8]		;Put m+1 in return Register
	INC eax					;
	RET						;return
	
secondcase:
	MOV ebx, [ebp+12]		;get n
	DEC ebx					;n-1
	MOV edx, dword 1		;m= 1			
	PUSH ebp
	PUSH edx				;push new m
	PUSH ebx				;push new n
	CALL ackermann
	RET

inputError:					;n or m was <0
	MOV eax, promptError
	CALL print_string
	JMP end