USING: assocs io.encodings.utf8 io.files kernel math
math.functions math.parser math.vectors prettyprint sequences
sets splitting ;
IN: aoc.9

: input ( -- lines ) "~/factor/aoc/9/9.in" utf8 file-lines ;
: dir>vec ( dir -- vector )
    { { "U" { 0 1 } }
      { "R" { 1 0 } }
      { "D" { 0 -1 } }
      { "L" { -1 0 } } } at ;
: I ( -- coords ) input [ split-words first2 [ dir>vec ] [ dec> ] bi* swap <repetition> ] map
    concat { 0 0 } [ v+ ] accumulate* { 0 0 } prefix ;

: move-tail ( tail head -- tail' ) 2dup distance 2 >= [ over v- [ signum ] map v+ ] [ drop ] if ;
: (part) ( head rope-length -- pos-num ) 1 - [ dup first [ move-tail ] accumulate* ] times cardinality ;
: part1 ( -- n ) I 2 (part) ;
: part2 ( -- n ) I 10 (part) ;

: solve ( -- ) part1 . part2 . ;