wc.asm

  global  main
  extern  printf

  section .text
main:
  sub     rdi, 1  ; argc includes program name, so substract 1 to get arg count
  cmp     rdi, 1  ; check if we got exactly 1 arg
  jne     bad_arg_count ; if not, jump to bad_arg_count
  mov     r15, [rsi+8] ; main's argv[1]
  mov     r14, 0 ; index (also byte count)
  mov     r13, 0 ; line count
  mov     r12, 0 ; word count
  mov     r11, 0 ; wasSpace

loop:
  cmp     byte [r15+r14], 32 ; is space
  je      space
  cmp     byte [r15+r14], 10 ; is newline
  je      newline
  jmp     other_char
newline:
  add     r13, 1
space:
  cmp     r11, 0
  jne     continue
  mov     r11, 1 ; wasSpace = true
  add     r12, 1 ; inc word count
  jmp     continue
other_char:
  mov     r11, 0
continue:
  add     r14, 1 ; inc byte count
  cmp     byte [r15+r14], 0
  jne     loop
finalize:
  cmp     r11, 1 ; was last char a space?
  je      print_result ; if yes, print_result
  add     r12, 1 ; otherwise increment word count by 1
print_result:
  mov     rdi, format; arg1
  mov     rsi, r13 ; arg2 = line count
  mov     rdx, r12 ; arg3 = word count
  mov     rcx, r14 ; arg4 = byte count
  mov     rax, 0 ; 0 floating point params to variadic printf function
  call    printf

  mov     rax, 0      ; exit code 0
  ret

; print error message
bad_arg_count:
  mov     rsi, rdi       ; arg 2
  mov     rdi, args_str  ; arg1
  mov     rax, 0        ; 0 floating point params to variadic printf function
  call    printf
  mov     rax, 1      ; exit code 1
  ret

format:
  db  "%d %d %d", 10, 0 ; world, line, byte count
args_str:
  db  "expected 1 argument, got %d", 10, 0