source: vis_dev/vis-2.1/examples/rcnum/rcnum.v @ 15

// This Verilog module describes a simple finite state machine for
// the computation of the so-called rollercoaster numbers.
// Given an initial number n[0] > 0, the rollercoaster number
// sequence (n[i]) starting at n[0] is given by these rules:
//   if n[i] is even, n[i+1] = n[i]/2;
//   if n[i] is odd,  n[i+1] = n[i]*3+1.
// The sequences for most (small) numbers eventually reach the
// cycle (4,2,1).
// This finite state machine has a state register storing n[i].
// Since the register is finite, only a few positive integers can
// be represented. Therefore 0 is used as trap state to indicate
// overflow.
//
// This description intentionally avoids the use of '*' and '/'.
//
// Author: Fabio Somenzi <Fabio@Colorado.EDU>


module rollercoasterNumbers(clock,numOut);
    input           clock;
    output [24:0] numOut;
    reg [24:0]    numOut;

    wire [24+2:0] tmp;

    initial begin
        numOut[0] = $ND(0,1);   
        numOut[1] = $ND(0,1);   
        numOut[2] = $ND(0,1);   
        numOut[3] = $ND(0,1);   
        numOut[4] = $ND(0,1);   
        numOut[5] = $ND(0,1);   
        numOut[6] = $ND(0,1);   
        numOut[7] = $ND(0,1);   
        numOut[8] = $ND(0,1);   
        numOut[9] = $ND(0,1);   
        numOut[10] = $ND(0,1);  
        numOut[11] = $ND(0,1);  
        numOut[12] = $ND(0,1);  
        numOut[13] = $ND(0,1);  
        numOut[14] = $ND(0,1);  
        numOut[15] = $ND(0,1);  
        numOut[16] = $ND(0,1);  
        numOut[17] = $ND(0,1);  
        numOut[18] = $ND(0,1);  
        numOut[19] = $ND(0,1);  
        numOut[20] = $ND(0,1);  
        numOut[21] = $ND(0,1);  
        numOut[22] = $ND(0,1);  
        numOut[23] = $ND(0,1);  
        numOut[24] = $ND(0,1);  
    end

    // Compute n[i] * 3 + 1.
    assign tmp = {2'b0,numOut} + {1'b0,numOut,1'b1};

    always @ (posedge clock)
        if (numOut[0]) begin
            // Check overflow.
            numOut = (tmp[24+2] | tmp[24+1]) ? 0 : tmp[24:0];
        end else begin
            // Divide by 2.
            numOut = {1'b0,numOut[24:1]};
        end

endmodule // rollercoasterNumbers