binarytree.pl

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/********************************************************************
    Constraint-based Graphical Programming in B-Prolog
    %
    draw a binary tree
*********************************************************************/
go:-
    binaryTree(Os),
    cgJava(binaryTree,Os).

binaryTree([Root|Os]):-
    createNode(Root),
    binaryTree(4,Root,Os).

createNode(Root):-
    cgCircle(Root),
    Root^width #= 20, 
    Root^color #= red.

binaryTree(N,Root,Os):-N=:=0,!,Os=[].
binaryTree(N,Root,Os):-
    Os=[C1,C2,L1,L2|Os12],
    createNode(C1),
    createNode(C2),
    cgLine(L1),
    cgLine(L2),
    connect(Root,C1,C2,L1,L2),
    N1 is N-1,
    binaryTree(N1,C1,Os1),
    binaryTree(N1,C2,Os2),
    cgLeft(Os1,Os2),
    append(Os1,Os2,Os12).
    
connect(Root,C1,C2,L1,L2):-
    Root^centerX #= (C1^centerX+C2^centerX)//2,
    cgLeft(C1,C2),
    Root^centerY #= C1^centerY-50,
    C1^centerY #= C2^centerY,
    connect(L1,Root,C1),
    connect(L2,Root,C2).
    
connect(L,N1,N2):-
    L^point1 #= N1^centerPoint,
    L^point2 #= N2^centerPoint.