section	.text
	global	strlen
	global	strcpy
	global	strcat
	global	strcmp

;----- strlen(char* str) -----;
; return value: amount of characters before EOS was found
strlen:
	push	rbp
	mov	rbp, rsp

	xor	rax, rax
	.findEOS:
		cmp	byte [rdi+rax], 0x0
		je	.quit
		inc	rax
		jmp	.findEOS

	.quit:
	leave
	ret
;----- strcpy(char* dest, char* src) -----;
; return value: pointer to dest or NULL if nothing was copied (note that EOS is always copied to dest)
strcpy:
	push	rbp
	mov	rbp, rsp

	mov	r11, rdi
	xor	rax, rax
	xor	rcx, rcx
	.loop:
		cmp	byte [rsi], 0x0
		je	.0f
		mov	r10b, byte [rsi]
		mov	byte [r11], r10b
		inc	rsi
		inc	r11
		inc	rcx
		jmp	.loop

	.0f:
	mov	byte [r11], 0x0
	cmp	rcx, 0
	je	.quit
	mov	rax, rdi

	.quit:
	leave
	ret
;----- strcat(char* dest, char* src) -----;
; return value: pointer to dest
strcat:
	push	rbp
	mov	rbp, rsp

	mov	r10, rdi
	call	strlen
	add	rdi, rax
	call	strcpy
	mov	rax, r10

	leave
	ret
;----- strcmp(char* str1, char* str2) -----;
; return value: 0 if both strings are the same, otherwise index in array str1 where strings are to become different
; returns -1 if str2 is longer than str1 and no difference was found before that happens
strcmp:
	push	rbp
	mov	rbp, rsp

	call	strlen
	mov	rcx, rax
	mov	rax, -1
	.compareLoop:
		inc	rax
		cmp	byte [rdi+rax], 0x0
		je	.eosFound
		mov	r10b, byte [rsi+rax]
		cmp	byte [rdi+rax], r10b
		je	.compareLoop
	jmp	.quit

	.eosFound:
	mov	r10, rax
	mov	rdi, rsi
	call	strlen
	cmp	rax, r10
	jle	.quit
	mov	rax, -1

	.quit:
	leave
	ret