// N-Queen
//
// The original is written as a Lisp program in the following file:
// https://www.istc.kobe-u.ac.jp/activity/mage/m27/27_Lisp.pdf


/** 
 * member
 *   r, done << member(m, list)
 *   return
 *     r: 1 if m occurs in list
 *     done: the same to `list'
 */
// fun member x [] = false
//  | member x (y::ys) = if x=y then true else (member x ys);

member(r, list, int x) >< [] =>
        r~0, list~(*R)[];
member(r, list, int x) >< (int y):ys
| x==y =>
        r~1, list~(*R)(y:ys)
| _ =>
        list~(*R)(y:cnt), member(r, cnt, x)~ys;


/** 
 * len_list (stream version)
 *   len, done << len_list(list)
 *   return
 *     len: length of `list'
 *     done: the same to `list'
 */
 
len_list(len, done) >< [] =>
        len~0, done~(*R)[];
len_list(len, done) >< x:xs => 
        done~(*R)(x:cnt), 
        len_list_(len, cnt, 1)~xs;

len_list_(len, done, int ct) >< [] =>
        len~ct, done~(*R)[];
len_list_(len, done, int ct) >< x:xs => 
        done~(*R)(x:cnt), 
        len_list_(len, cnt, ct+1) ~ xs; 

/** 
 * len (non-stream version)
 *   ret << len(list)
 *   return
 *     ret: length of `list'
 */
 
len(ret) >< [] =>
        ret~0;
len(ret) >< x:xs =>
        len_list(ret, Eraser)~(*R)(x:xs);



/** 
 * threat (stream version)
 * check for (0,q) and (k, qpos[col=k]) where k=1...
 * http://www.nct9.ne.jp/m_hiroi/func/haskell06.html
     0 1 2    --> 
    +-------------
   1| . . . . . .
   2| . . . -3. .  5 - 3 = 2
   3| . . -2. . .  5 - 2 = 3
   4| . -1. . . .  5 - 1 = 4
   5| Q . . . . .  Q position is 5  
   6| . +1. . . .  5 + 1 = 6
   7| . . +2. . .  5 + 2 = 7
   8| . . . +3. .  5 + 2 = 8
    +-------------
 *   r, done << threat(k,m, board)
 *   return
 *        r: 1 if threat conditions are satisfied
 *     done: the same to `board'
 */
// fun threat k m [] = false
//   | threat k m (x::xs) =
//    if (k = x-m) orelse (k = x+m) then true
//    else (threat (k+1) m xs);

threat(r, done, int k, int q)><[] =>
        r~0, done~(*R)[];
threat(r, done, int k, int q)><(int x):xs
| (k == x-q) or (k == q-x) =>
        r~1, done~(*R)(x:xs)
| _ =>
        done~(*R)(x:cnt), threat(r, cnt, k+1, q)~xs;



// fun queen1 b n 0 = []
//   | queen1 b n m =
//     if (member m b) orelse (threat 1 m b) then (queen1 b n (m-1))
//     else if (length b) = (n-1) then (m::b)::(queen1 b n (m-1))
//         else (queen1 (m::b) n n) @ (queen1 b n (m-1));
// fun queen n = queen1 n [] n;

queen(ret) >< (int n) => queen_(ret, [], n)~n;

queen_(r, b, int n) >< (int m)
| m==0 =>
        r~[], (*L)Eraser~b
| _ =>
        mem_ret, done_b << member(m,b), 
        queen__chk_m_in_b(r, done_b, n, m)~mem_ret;

queen__chk_m_in_b(r, b, int n, int m) >< (int mem_ret)
| mem_ret == 1 =>
        (*L)queen_(r, b, n)~(m-1)
| _ =>
        th_ret, done_b << threat(1, m, b), 
	(*L)queen__chk_threat(r, done_b, n, m)~th_ret;

queen__chk_threat(r, b, int n, int m) >< (int threat_ret)
| threat_ret == 1 =>
        (*L)queen_(r, b, n)~(m-1)
| _ =>
        len_ret, done_b << len_list(b), 
	(*L)queen__chk_lastelem(r, done_b, n, m)~len_ret;

queen__chk_lastelem(r, b, int n, int m) >< (int len_ret)
| len_ret == n-1 =>
        b1, b2 << Dup(b), 
        r~(m:b1):cnt, (*L)queen_(cnt, b2, n)~(m-1)
| _ =>
        b1, b2 << Dup(b),
        r << Append(cnt1, cnt2),
        (*L)queen_(cnt1, m:b1, n)~n, queen_(cnt2, b2, n)~(m-1);



// ----------------------------------------------------------
// main
// ----------------------------------------------------------
const N=12;
queen(ret)~N, len(count)~ret;
count;   // count ->14200