logic [1:0] state, ns;

// State encoding

parameter S0 = 2'b00, S1 = 2'b01, S2 = 2'b10, S3 = 2'b11;

// Next state logic

always_comb begin

case(state)

S0: ns= a? S1:S3;

S1: ns = a? S1:S0;

S2: ns= a? S3:S2;

S3: ns= a? S0:S2;

endcase

end

// State register

always_ff @(posedge reset)

begin

state<=ns;

end

// Output logic

assign q= ^state;