; Returns:
; rax - read(2) length
; rdi - if line is complete, line length;
;       if line was incomplete, 0;
;       if EOF or read(2) returned an error, undefined
; Errors (in rax):
; -1024 - line too long
; other negative - error from read(2)
; 0 - EOF from read(2)
; Arguments:
; rdi - fd
; rsi - buffer
; rdx - max length
readline:
    push rsi
    push rdx
    mov rax, SYS_READ
    syscall
    pop rdx
    pop rsi
    cmp rax, 0
    jl return
    mov r10, 0

readline__scan:
    cmp r10, rdx
    jge readline__overflow
    cmp r10, rax
    jge readline__incomplete_line
    mov r11b, [rsi + r10]
    add r10, 1
    cmp r11b, 0x0a  ; '\n'
    jne readline__scan
    mov rdi, r10
    ret

readline__incomplete_line:
    mov rdi, 0
    ret

readline__overflow:
    mov rax, -1024
    ret