module puzzle (grid) is

process MAIN [PLACE:any, PRINT:any] (in var Month:Nat, in var Day:Nat) is
    var Crt_Grid:Grid,
	Pieces:Piece_List,
	Crt_Piece:Piece,
	Crt_Config:Config,
	Fits:Bool,
	J, Line, Coln:Nat,
	G_Line:Grid_Line,
	G_Array:Grid_Array
    in
	if (Month == 0) and (Day == 0) then
	    -- Particular case of the current day

	    Month := Get_Current_Month ();
	    Day := Get_Current_Day ()
	else
	    assert (Month >= 1) and (Month <= 12) and (Day >= 1) and (Day <= 31)
	end if;

	Crt_Grid := Initial_Grid;

	-- Set the grid cells of the desired month and day as occupied

	eval Get_Month_Coordinates (Month, ?Line, ?Coln);
	-- Crt_Grid.A[Line][Coln] := X;
	G_Line := Crt_Grid.A [Line];
	G_Line [Coln] := X;
	G_Array := Crt_Grid.A;
	G_Array [Line] := G_Line;
	Crt_Grid := Crt_Grid.{A -> G_Array};

	eval Get_Day_Coordinates (Day, ?Line, ?Coln);
	-- Crt_Grid.A[Line][Coln] := Y;
	G_Line := Crt_Grid.A [Line];
	G_Line [Coln] := Y;
	G_Array := Crt_Grid.A;
	G_Array [Line] := G_Line;
	Crt_Grid := Crt_Grid.{A -> G_Array};

	Pieces := Puzzle_Pieces;

	-- Enumerate all pieces and attempt to place them on the grid

	while Pieces != nil loop L in
	    Crt_Piece := head (Pieces);

	    -- Choose a piece configuration and a grid cell to place it

	    J := any Nat where J < Crt_Piece.Q; J := J + 1;
	    Line := any Nat where Line < Crt_Grid.M; Line := Line + 1;
	    Coln := any Nat where Coln < Crt_Grid.N; Coln := Coln + 1;

	    Crt_Config := Get_Config (Crt_Piece, J);

	    -- Attempt to place the piece configuration

	    Fits := Check_Placement (Crt_Grid, Crt_Config, Line, Coln);

	    if Fits then
		-- Successful placement: emit event and update the grid

		PLACE (Crt_Piece.P, Line, Coln, Crt_Config);
		Crt_Grid := Update_Grid (Crt_Grid, Crt_Piece.P, Line, Coln, Crt_Config);

		if Valid_Grid (Free_Cells (Crt_Grid)) then
		    -- The updated grid may be valid for placing
		    -- the remaining pieces

		    Pieces := tail (Pieces)
		else
		    -- The updated grid is certainly not valid for placing
		    -- the remaining pieces:
		    -- discard the current grid configuration

		    break L
		end if
	    else
		-- The current piece cannot be correctly placed:
		-- discard the current grid configuration

		break L
	    end if
	end loop;

	if Pieces == nil then
	    -- All pieces have been successfully placed:
	    -- emit the solution event

	    PRINT (Crt_Grid)
	end if
    end var
end process

end module