; ============================================================
; Assignment 4 (a) : Concatenation of two strings
; ============================================================

%macro rw 4
    mov rax,%1
    mov rdi,%2
    mov rsi,%3
    mov rdx,%4
    syscall
%endmacro

section .data
    msg1 db "Enter first string  : "
    len1 equ $-msg1
    msg2 db "Enter second string : "
    len2 equ $-msg2
    msg3 db "Concatenated string : "
    len3 equ $-msg3

section .bss
    str1   resb 100
    str2   resb 100
    result resb 200
    n1     resb 1
    n2     resb 1
    ntot   resb 1
    ch     resb 1          ; holds one character at a time

section .text
    global _start

_start:
    ; ---- read first string, one char at a time ----
    rw 1,1,msg1,len1

    mov rdi,str1
    mov bl,0                ; bl = length counter (safe, rw never touches rbx)

read1:
    push rdi                ; save pointer, rw is about to clobber rdi
    rw 0,0,ch,1              ; read exactly one character
    pop rdi

    mov al,[ch]
    cmp al,10                ; newline?
    je done1

    mov [rdi],al
    inc rdi
    inc bl
    cmp bl,99                ; don't overflow the 100-byte buffer
    jae done1
    jmp read1

done1:
    mov [n1],bl

    ; ---- read second string, one char at a time ----
    rw 1,1,msg2,len2

    mov rdi,str2
    mov bl,0

read2:
    push rdi
    rw 0,0,ch,1
    pop rdi

    mov al,[ch]
    cmp al,10
    je done2

    mov [rdi],al
    inc rdi
    inc bl
    cmp bl,99
    jae done2
    jmp read2

done2:
    mov [n2],bl

    ; ---- concatenate ----
    mov rsi,str1
    mov rdi,result
    mov cl,[n1]

copy1:
    cmp cl,0
    je skip1
    mov al,[rsi]
    mov [rdi],al
    inc rsi
    inc rdi
    dec cl
    jmp copy1
skip1:

    mov rsi,str2
    mov cl,[n2]

copy2:
    cmp cl,0
    je skip2
    mov al,[rsi]
    mov [rdi],al
    inc rsi
    inc rdi
    dec cl
    jmp copy2
skip2:

    mov byte [rdi],10

    mov al,[n1]
    add al,[n2]
    inc al
    mov [ntot],al

    rw 1,1,msg3,len3

    movzx rdx,byte [ntot]
    rw 1,1,result,rdx

    rw 60,0,0,0