module arp_cache
(
 input                clk ,
 input                rst_n ,
 
 input                arp_found,
 input  [31:0]        arp_rec_source_ip_addr,
 input  [47:0]        arp_rec_source_mac_addr,
 
 input  [31:0]        destination_ip_addr,
 output reg [47:0]    destination_mac_addr,
 
 output reg           mac_not_exist
) ;
       
reg [79:0]  arp_cache ;

//init arp cache
always @(posedge clk or negedge rst_n)
  begin
    if (~rst_n)
      arp_cache  <= 80'h00_00_00_00_ff_ff_ff_ff_ff_ff ;
    else if (arp_found)
      arp_cache  <= {arp_rec_source_ip_addr, arp_rec_source_mac_addr} ;
    else
      arp_cache  <= arp_cache ;
  end
  
always @(posedge clk or negedge rst_n)
  begin
    if (~rst_n)
      destination_mac_addr  <= 48'hff_ff_ff_ff_ff_ff ;
    else if (destination_ip_addr == arp_cache[79:48])
      destination_mac_addr  <= arp_cache[47:0] ;
    else
      destination_mac_addr  <= 48'hff_ff_ff_ff_ff_ff ;
  end
  
always @(posedge clk or negedge rst_n)
  begin
    if (~rst_n)
      mac_not_exist  <= 1'b0 ;
    else if (destination_ip_addr != arp_cache[79:48])
      mac_not_exist  <= 1'b1 ;
    else if (destination_ip_addr == arp_cache[79:48] && arp_cache[47:0] == 48'hff_ff_ff_ff_ff_ff)
      mac_not_exist  <= 1'b1 ;
    else
      mac_not_exist  <= 1'b0 ;
  end
  
endmodule