USING: advent kernel assocs sequences math math.order math.parser math.vectors arrays accessors combinators sets ;
IN: advent.day9

TUPLE: state knots seen ;
: <state> ( n -- state )
    1 + { 0 0 } <array> HS{ { 0 0 } } clone state boa ;

<PRIVATE

: (delta) ( dir -- delta )
    {
        { CHAR: U { 0 1 } }
        { CHAR: D { 0 -1 } }
        { CHAR: L { -1 0 } }
        { CHAR: R { 1 0 } }
    } at ;

: (input) ( -- input )
    "day9.in" get-input
    [ [ first (delta) ] [ 2 tail string>number ] bi 2array ] map
    ;

: (update-pair) ( head tail -- tail' )
    tuck v- dup vabs [ 1 > ] any?
        [ [ sgn ] map v+ ]
        [ drop ] if
    ;

: (update-tails) ( state -- state' )
    [
        unclip
        [ (update-pair) ] accumulate swap suffix
    ] change-knots
    dup
    [ knots>> last ] [ seen>> adjoin ] bi
    ;

: (step) ( state insn -- state' )
    first2 swap
    '[
            [ knots>> [ first _ v+ ] keep set-first ] keep
            (update-tails)
    ]
    times
    ;

PRIVATE>

: day9-part1 ( -- n )
    (input)
    1 <state>
    [ (step) ] reduce
    seen>> cardinality
    ;

: day9-part2 ( -- n )
    (input)
    9 <state>
    [ (step) ] reduce
    seen>> cardinality
    ;