! day ten
! from day 4 solution:
! : iterate-through-everything ( ... grid quot: ( ... x y grid -- ... ) -- ... ) over first length [ -rot over length [ -roll call ] 3with each-integer ] 2with each-integer  ; inline
! : at-coord ( x y grid -- val ) swapd nth nth ;
! : valid-grid-index ( x y grid -- x y grid ? ) [ tuck [ length [ < ] [ drop 0 >= ] 2bi and ] [ first length [ < ] [ drop 0 >= ] 2bi and ] 2bi* and ] 3check ;
! from day 8 solution:
! : by-cols ( grid -- cols ) [ first length ] keep [ [ nth ] with map ] curry map-integers ;

: trailhead-coords ( grid -- coords ) [ [ 2dup ] dip at-coord 0 = [ 2array ] [ 2drop f ] if ] collector [ iterate-through-everything ] dip sift >array ;

: surrounding-squares ( x y grid -- squares ) [ {
            [ [ 1 + ] 2dip valid-grid-index [ drop 2array ] [ 3drop f ] if ]
            [ [ 1 - ] 2dip valid-grid-index [ drop 2array ] [ 3drop f ] if ]
            [ [ 1 + ] dip valid-grid-index [ drop 2array ] [ 3drop f ] if ]
            [ [  1 - ] dip valid-grid-index [ drop 2array ] [ 3drop f ] if ] } 3cleave ] output>array sift ;
! x y grid square
: surrounding-valid-squares ( x y grid -- squares ) 3dup surrounding-squares [ over [ first2 ] dip at-coord 1 - [ at-coord ] dip = ] 2with with filter ;

: valid-squares? ( x y grid -- ? ) surrounding-valid-squares length 0 > ;
: not-on-nine? ( x y grid -- ? ) at-coord 9 < ;

: (do-trail) ( x y grid -- final-coords )
    3dup [ valid-squares? ] [ not-on-nine? ] 3bi and [ [ surrounding-valid-squares ] keep [ [ first2 ] dip (do-trail) ] curry map concat ] [ [ 2dup ] dip not-on-nine? [ 2drop { } ] [ 2array 1array ] if ] if ;

: do-trail ( x y grid -- final-coords ) (do-trail) { } [ over [ index ] keepd swap [ drop ] [ suffix ] if ] reduce ; 

: parse-day-ten-input ( string -- grid ) split-lines harvest by-cols [ [ 1string dec> ] map ] map ;

: solve-day-ten ( grid -- total-trailhead-count ) [ trailhead-coords ] keep [ [ first2 ] dip do-trail ] curry map concat length ;

: solve-day-ten-part-two ( grid -- total-trailhead-count ) [ trailhead-coords ] keep [ [ first2 ] dip (do-trail) ] curry map concat length ;