DAY4_PT2.asm

# define registers
$A: @end+0
$B: $A+1
$C: $B+1
$D: $C+1
$E: $D+1
$F: $E+1
$G: $F+1
$H: $G+1

$RET: $H+1
$RESULT: $RET+1

$START: $RESULT+1
$END: $START+6
$TOTAL: $END+6

# from
IN $A
ADD $A 0 $START+0
IN $A
ADD $A 0 $START+1
IN $A
ADD $A 0 $START+2
IN $A
ADD $A 0 $START+3
IN $A
ADD $A 0 $START+4
IN $A
ADD $A 0 $START+5

# to
IN $A
ADD $A 0 $END+0
IN $A
ADD $A 0 $END+1
IN $A
ADD $A 0 $END+2
IN $A
ADD $A 0 $END+3
IN $A
ADD $A 0 $END+4
IN $A
ADD $A 0 $END+5


# SUBROUTINE - MAIN - entry point
main: 
    # reduce $start by one to check
    ADD 0 this+5 $RET
    JEZ 0 subtract_one

    # compare $start to $end, halt if equal
    ADD 0 this+5 $RET
    JEZ 0 check_equal
    JNZ $RESULT halt

    # check if current $start is valid, inc $total if true
    ADD 0 this+5 $RET
    JEZ 0 check_valid
    ADD $TOTAL $RESULT $TOTAL

    # loop
    JEZ 0 main


# SUBROUTINE - SUBTRACT ONE - subtract one from $start
subtract_one:
    # Remove one from 6th digit, return if >=0
    ADD $START+5 -1 $START+5
    IFL $START+5 0 $A
    JEZ $A $RET
    # if <0 set to 9
    ADD 0 9 $START+5

    # Remove one from 5th, return if >=0
    ADD $START+4 -1 $START+4
    IFL $START+4 0 $A
    JEZ $A $RET
    # if <0 set to 9
    ADD 0 9 $START+4
    
    # Remove one from 4th, return if >=0
    ADD $START+3 -1 $START+3
    IFL $START+3 0 $A
    JEZ $A $RET
    # if <0 set to 9
    ADD 0 9 $START+3
    
    # Remove one from 3rd, return if >=0
    ADD $START+2 -1 $START+2
    IFL $START+2 0 $A
    JEZ $A $RET
    # if <0 set to 9
    ADD 0 9 $START+2
    
    # Remove one from 2nd, return if >=0
    ADD $START+1 -1 $START+1
    IFL $START+1 0 $A
    JEZ $A $RET
    # if <0 set to 9
    ADD 0 9 $START+1
    
    # Remove one from 1st, return if >=0
    ADD $START+0 -1 $START+0
    JEZ 0 $RET


# SUBROUTINE - CHECK VALID - check if $start is a valid password
check_valid:
    ADD 0 $START+0 $A
    ADD 0 1 $F
    ADD 0 0 $G

    second:
        # compare 2nd to 1st, inc $F if equal
        IFE $START+1 $A $B
        ADD $F $B $F
        # jump to next op if equal
        JNZ $B third
        # else if less, failure
        IFL $START+1 $A $C
        JNZ $C check_valid_failure
        # else check if has created a valid pair, if so inc $G, set $A to current value and $F to 1
        IFE $F 2 $D
        ADD $G $D $G
        ADD 0 $START+1 $A
        ADD 0 1 $F

    third:
        # compare 3rd to 2nd, inc $F if equal
        IFE $START+2 $A $B
        ADD $F $B $F
        # jump to next op if equal
        JNZ $B fourth
        # else if less, failure
        IFL $START+2 $A $C
        JNZ $C check_valid_failure
        # else check if has created a valid pair, if so inc $G, set $A to current value and $F to 1
        IFE $F 2 $D
        ADD $G $D $G
        ADD 0 $START+2 $A
        ADD 0 1 $F

    fourth:
        # compare 4th to 3rd, inc $F if equal
        IFE $START+3 $A $B
        ADD $F $B $F
        # jump to next op if equal
        JNZ $B fifth
        # else if less, failure
        IFL $START+3 $A $C
        JNZ $C check_valid_failure
        # else check if has created a valid pair, if so inc $G, set $A to current value and $F to 1
        IFE $F 2 $D
        ADD $G $D $G
        ADD 0 $START+3 $A
        ADD 0 1 $F

    fifth:
        # compare 5th to 4th, inc $F if equal
        IFE $START+4 $A $B
        ADD $F $B $F
        # jump to next op if equal
        JNZ $B sixth
        # else if less, failure
        IFL $START+4 $A $C
        JNZ $C check_valid_failure
        # else check if has created a valid pair, if so inc $G, set $A to current value and $F to 1
        IFE $F 2 $D
        ADD $G $D $G
        ADD 0 $START+4 $A
        ADD 0 1 $F

    sixth:
        # compare 6th to 5th, inc $F if equal
        IFE $START+5 $A $B
        ADD $F $B $F
        # jump to final if equal
        JNZ $B final
        # else if less, failure
        IFL $START+5 $A $C
        JNZ $C check_valid_failure
        # else check if has created a valid pair, if so inc $G, set $A to current value and $F to 1
        IFE $F 2 $D
        ADD $G $D $G
        ADD 0 $START+5 $A
        ADD 0 1 $F

    final:
        # check if 6th created valid pair
        IFE $F 2 $D
        ADD $G $D $G

        # if $G > 0 then valid
        JEZ $G check_valid_failure

    # success
    check_valid_success:
        ADD 0 1 $RESULT
        JEZ 0 $RET

    # failure
    check_valid_failure:
        ADD 0 0 $RESULT
        JEZ 0 $RET


# SUBROUTINE - CHECK EQUAL - check if $start equals $end
check_equal:
    # Step 1 - compare all digits
    IFE $START+0 $END+0 $A
    IFE $START+1 $END+1 $B
    IFE $START+2 $END+2 $C
    IFE $START+3 $END+3 $D
    IFE $START+4 $END+4 $E
    IFE $START+5 $END+5 $F

    # Step 2 - sum comparisons
    ADD $A $B $A
    ADD $A $C $A
    ADD $A $D $A
    ADD $A $E $A
    ADD $A $F $A

    # Step 3 - compare to 6
    ADD $A -6 $A
    JEZ $A check_equal_success

    # failure
    check_equal_failure:
        ADD 0 0 $RESULT
        JEZ 0 $RET

    # success
    check_equal_success:
        ADD 0 1 $RESULT
        JEZ 0 $RET
    

# SUBROUTINE - HALT - process output and halt
halt:
    OUT $TOTAL
    HALT