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(*
* References that can be merged
*
* -- Allen
*)
signature UNION_FIND_REF =
sig
type 'a uref
val uref : 'a -> 'a uref
val !! : 'a uref -> 'a
val ::= : 'a uref * 'a -> unit
val == : 'a uref * 'a uref -> bool
val eq : 'a uref * 'a uref -> bool
val find : 'a uref -> 'a uref
val union : ('a * 'a -> 'a) -> 'a uref * 'a uref -> bool
val union' : 'a uref * 'a uref -> bool
end
structure UnionFindRef :> UNION_FIND_REF =
struct
datatype 'a uptree = ROOT of 'a * int
| LINK of 'a uref
withtype 'a uref = 'a uptree ref
fun uref x = ref(ROOT(x,1))
fun eq (x : 'a uref,y : 'a uref) = x = y
fun find r =
let fun look (r as ref(ROOT _)) = r
| look (r' as ref(LINK r)) =
let val r'' = look r
in if r <> r'' then r' := LINK r'' else ();
r''
end
in look r end
fun == (x,y) = find x = find y
fun !! r = let val ROOT(x,_) = !(find r) in x end
fun ::=(r,x) = let val r as ref(ROOT(_,w)) = find r
in r := ROOT(x,w) end
fun union f (x,y) =
let val r as ref(x as ROOT(i,w)) = find x
val r' as ref(y as ROOT(j,w')) = find y
in if r = r' then false
else if w > w' then
(r := ROOT(f(i,j),w+w'); r' := LINK r; true)
else
(r' := ROOT(f(i,j),w+w'); r := LINK r'; true)
end
fun union' (x,y) =
let val r as ref(x as ROOT(i,w)) = find x
val r' as ref(y as ROOT(j,w')) = find y
in if r = r' then false
else if w > w' then
(r := ROOT(i,w+w'); r' := LINK r; true)
else
(r' := ROOT(j,w+w'); r := LINK r'; true)
end
end
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