ERRORC = ^XFFFFFFFF
SPACE  = ^X20
TAB    = ^X9
SPACE  = ^X20

.macro skip_whitespace buffer, ?next, ?incriment, ?end
  next:
    bwhitespace buffer, incriment
    brb end
  incriment:
    incl buffer
    brb next
  end:
.endm

.macro bwhitespace char, branch, ?found, ?end
    bequal (char), #SPACE, found
    bequal (char), #TAB, found
    brb end
  found:
    jmp branch
  end:
.endm
  
.macro bequal one, two, branch, ?end
    cmpb one, two
    bneq end
    jmp branch
  end:
.endm

.macro bdigit char, branch, ?end
    cmpb (char), #^A/0/
    blss end
    cmpb (char), #^A/9/
    bgtr end
    jmp branch
  end:
.endm

.macro toggle_sign sign, ?negative, ?end
    cmpb sign, #^A/-/
    beql negative
    movb #^A/-/, sign
    brb end
  negative:
    movb #^A/+/, sign
  end:
.endm

sign:  .byte ^A/+/

.entry string_to_int, ^m<R2, R3, R4>
.show meb
    movb #^A/+/, sign
    movl 8(AP), R2
    incl R2        ; beginning of the string
    clrl R0        ; for accumulating the int

  begin_number:
    skip_whitespace R2
    bdigit R2, begin_digits
    bequal (R2), #^A/+/, plus
    bequal (R2), #^A/-/, minus
    brw error

  minus:
    toggle_sign sign
  plus:
    incl R2
    brw begin_number

  begin_digits:
  next_digit:
    bdigit R2, get_digit
    brb error

  get_digit:
    clrl R3
    subb3 #^A/0/, (R2)+, R3
    addl  R3, R0
    skip_whitespace R2
    bequal (R2), #0, number_ended
    mull #10, R0
    brb next_digit

  number_ended:
    bequal #^A/+/, sign, end
    mnegl R0, R0
    brb   end

  error:
    movl #ERRORC, R0
  end:
    movl R0, 4(AP)
    ret
.end