math.combinatorics sequences ;
IN: rosetta-code.two-sum

DEFER: (two-sum)
TUPLE: helper
     sum seq index hash ;

: <two-sum-helper> ( sum seq -- helper )
    \ helper new 
        swap >>seq 
        swap >>sum
        0 >>index 
        H{ } clone >>hash ;

: no-sum ( helper -- empty ) 
    drop { } ;

: in-bounds? ( helper -- ? )
    [      index>> ] 
    [ seq>> length ] bi < ;

: next-sum ( helper -- pair )
    dup in-bounds? [ (two-sum) ] 
                   [    no-sum ] if ;

: next-index ( helper -- helper )
    [ 1 + ] change-index ;

: remember-item ( helper -- helper )
    dup [      hash>> ]
        [ index>> dup ] 
        [       seq>> ] tri nth swap set-of drop ;

: result ( helper index -- helper )
    swap index>> 2array ;

: find-compliment-index ( helper -- helper index/f )
    dup [       sum>> ] 
        [     index>> ] 
        [ seq>> nth - ] 
        [             ] quad hash>> at ;

: (two-sum) ( helper -- pair )
    remember-item find-compliment-index 
        [ result ] [ next-index next-sum ] if* ;

: two-sum ( sum seq -- pair )