USING: literals sequences.product math.combinatorics math.vectors sets make path-finding ;

<<
CONSTANT: CHESS-INDICES $[
  7 [0..b] dup 2array <product-sequence>
]
>>

CONSTANT: KNIGHT-MOVES $[
  { 1 -1 } 2 all-selections
  [ { 2 1 } v* ] map
  dup [ reverse ] map append
]

<<
: knight-neighbors ( pair -- neighbor-pairs )
  KNIGHT-MOVES [ v+ ] with map
  CHESS-INDICES intersect ;
>>

<<
CONSTANT: ALL-KNIGHT-NEIGHBORS $[
  [
    CHESS-INDICES [ [ knight-neighbors ] keep ,, ] each
  ] H{ } make
]
>>

CONSTANT: KNIGHT-BFS $[
  ALL-KNIGHT-NEIGHBORS <bfs>
]

: pos>row-col ( str -- pair )
  [ last 49 - ] [ first 97 - ] bi 2array ;

: knight-min-moves ( startpos endpos -- n )
  [ pos>row-col ] bi@
  KNIGHT-BFS find-path
  length 1 - ;