(*
	Queue:  person on the cho or wait queue
	New:                 --> Queue
	inc: Queue x string  --> Queue
	del: Queue x string  --> Queue
	size: Queue          --> int
*)

datatype Queue = 
    New
    | inc of Queue * string;

fun del( New, s ) = New
    | del( inc( Q, i ), s ) = if i=s
                              then Q
                              else inc( del( Q, s ), i );

fun front( New ) = ""
    | front( inc( Q, i ) ) = if front(Q)=""
                             then i
                             else front(Q);

fun size( New ) = 0
    | size( inc( Q, i ) ) = size(Q) + 1;



(* 
	LIB: library
	new:                       --> LIB
	add:   LIB x BOOK          --> LIB
	rem:   LIB x BOOK          --> LIB
	cko:   LIB x BOOK x PERSON --> LIB
	cki:   LIB x BOOK x PERSON --> LIB
	wait:  LIB x BOOK x PERSON --> LIB
	off:   LIB x BOOK x PERSON --> LIB
	has:   LIB x BOOK          --> boolean
	here:  LIB x BOOK          --> int
	num:   LIB x BOOK          --> int
*)

datatype ('E) LIB = 
    new 
    | entry of ('E) LIB * 'E;

fun add( new, b ) = entry( new, (b, New, New, 1) )
    | add( entry( S, (i, j, k, c) ), b ) = if i=b
                                             then entry( S, (i, j, k, c+1) )
                                             else entry( add( S, b ), (i, j, k, c) );

exception nobookError
fun rem( new, b ) = raise nobookError
    | rem( entry( S, (i, j, k, c) ), b ) = if i=b
                                             then if c > 1
                                                  then entry( S, (i, j, k, c-1) )
                                                  else S
                                             else entry( rem(S, b), (i, j, k, c) );

exception nobookAvailable
fun cko( new, b, p ) = raise nobookError
    | cko( entry( S, (i, j, k, c) ), b, p ) = if i=b
                                                then if c > size(j)
                                                     then entry( S, (i, inc(j, p), k, c) )
                                                     else raise nobookAvailable
                                                else entry( cko( S, b, p ), (i, j, k, c) );

fun cki( new, b, p ) = raise nobookError
    | cki( entry( S, (i, j, k, c) ), b, p ) = if i=b
                                                then if size(k) >= 1
                                                     then cko( entry( S, (i, del(j, p), del(k, front(k)), c) ), b, front(k) )
                                                     else entry( S, (i, del(j, p), k, c) ) 
                                                else entry( cki( S, b, p ), (i, j, k, c) );

fun wait( new, b, p ) = raise nobookError
    | wait( entry( S, (i, j, k, c) ), b, p ) = if i=b
                                                 then entry( S, (i, j, inc(k, p), c) )
                                                 else entry( wait(S, b, p), (i, j, k, c) );

fun off( new, b, p ) = raise nobookError
    | off( entry( S, (i, j, k, c) ), b, p ) = if i=b
                                                then entry( S, (i, j, del(k, p), c) )
                                                else entry( off(S, b, p), (i, j, k, c) );

fun has( new, b ) = false
    | has( entry( S, (i, j, k, c) ), b ) = if i=b
                                             then true
                                             else has(S, b);

fun here( new, b ) = 0
    | here( entry( S, (i, j, k, c) ), b ) = if i=b
                                              then c-size(j)
                                              else here( S, b );

fun num( new, b ) = 0 
    | num( entry( S, (i, j, k, c) ), b ) = if i=b
                                             then c
                                             else num( S, b );

val lib = new;
val lib = add( lib, 0 );
val lib = add( lib, 1 );
val lib = add( lib, 0 );
val lib = cko( lib, 0, "Tom" );
val lib = cko( lib, 1, "Jerry" );
val lib = wait( lib, 0, "Amy" );
val lib = cki( lib, 0, "Tom" );
val lib = cki( lib, 1, "Jerry" );
has( lib, 1 );
here( lib, 1 );
num( lib, 1 );