global _start

section .data
  str: db "Please give me a string: " ; \n = 0xA
  strSize: equ $ - str

  stdin: equ 0  ; POSIX
  stdout: equ 1 ; POSIX

section .bss
  strBuffer: resb 64    ; reserve 64 bytes
  strBufferSize: equ 64 ; same as above

section .text
_start:
  mov rax, 1 ; Print str to screen
  mov rdi, stdout
  mov rsi, str
  mov rdx, strSize
  syscall

  xor rax, rax ; Read user input
  mov rdi, stdin
  mov rsi, strBuffer
  mov rdx, strBufferSize
  syscall

  mov rbx, rax ; store bytes read
  dec rbx ; subtract the newline
  xor rcx, rcx ; make register C our loop var
start_loop:
  cmp rcx, rbx 
  jge print

  ; Calculate the position
  mov rdx, strBuffer
  add rdx, rcx
  ; Load the char at the position
  xor rax, rax
  mov al, byte [rdx]
  mov r8, rdx
  ; Check if we're upper or lower case
  mov rdx, 95
  cmp rax, rdx
  jge lower_case
upper_case:
  mov r9, 91 ; 90 is the highest valid upper case value
  jmp algorithm
lower_case:
  mov r9, 123 ; 122 is the highest lower case value
algorithm:
  ; Shift it 13 positions
  mov rdx, 13
  add rax, rdx
  ; Check if we should wrap around
  cmp rax, r9
  jl loop_end
  jmp ascii_wrap
ascii_wrap:
  ; Wrap around
  mov rdx, 26
  sub rax, rdx
loop_end:
  ; Store the result
  mov byte [r8], al
  inc rcx
  jmp start_loop

print:
  inc rbx ; Add the newline back to it
  mov rax, 1 ; Print str to screen
  mov rdi, stdout
  mov rsi, strBuffer
  mov rdx, rbx
  syscall

  mov rax, 60
  xor rdi, rdi
  syscall